Specification of electrical works

Specification of electrical works
Cubicle panels
Main and distribution switchboards as listed in the schedules shall be purpose-made and
shall consist of a sheet-steel cubicle designed for access from front only or from front and
rear according to site position, containing distribution busbars, all cable terminations and
interconnections. All items. of switch and fusegear are to be flush-mounted on the front
of the cubicle which is to be finished grey stoved enamel.
Switchboards shall be suitable for controlling reduced voltage supplies and shall be
comprehensively tested before dispatch from the manufacturer’s works to ensure
satisfactory operation of all component parts. The tests shall include continuity and 2kV
flash tests. Busbar systems in switchboard shall be tested at 50kA for 1s.
Full protection shall be provided by means of mechanical interlocks with the covers
and operating levers, The switchboard shall have an earthing terminal and earthing bar.
External connections shall be provided to allow all outgoing cables to terminate at
either top or bottom of the switchboard.
Busbars
Busbar panels as shown on the drawings and listed in the schedules are to be of the
ratings indicated and are to be of high-conductivity copper mounted on robust vitreous
porcelain insulators complete with all necessary clamps. The busbars are to be enclosed
in a stove-enamelled sheet-steel casing with cast-iron frame members and detachable top,
bottom and side plates. The covers are to be of the screw-on type. The panels are to be
provided with all necessary holes and bushes for incoming and outgoing cables.
Incoming and outgoing switch fuses of the types and ratings shown on the drawings
and schedules shall be provided and installed adjacent to the busbar chamber. All
necessary interconnections between switch fuses and busbars shall be made and all the
equipment shall be fixed on a common angle-iron frame which is to be supplied as part of
the electrical contract. The frame is to be painted one coat primer and two coats grey
finish.
Where necessary, trunking shall be supplied and installed from busbars to meter
positions.
Fuse switches
Fuse switches shall be heavy-duty pattern fitted with HRC fuse links. They shall have
heavy-gauge steel enclosures with cast-iron frame members, rust-protected and finished
grey stoved enamel. Front access doors shall be fitted with dust-excluding gaskets and
shall be interlocked so that they cannot be opened when the switch is ‘on’. Operating
handles shall be lockable in both the on and off positions. The top and bottom endplates
shall be removable.
Each fuse switch shall be supplied complete with the correct HRC fuse links.
Each fuse switch shall have flag on-off indication.
Design example 291


Fuse switches shall be 500V rating and shall be clearly marked with their current
rating, and the items of equipment that they serve.
Switch fuses
Switch fuses shall be industrial pattern dust-proof type with HRC fuse links. They shall
have enclosures fabricated from sheet steel finished grey stoved enamel with removable
top and bottom endplates and shall have doors fitted with dust-proof gaskets. They shall
have front-operated handles with visible on-off indication.
The interiors shall have vitreous porcelain bases fitted with plated non-ferrous
conducting components. Switches shall be of the quick make-and-break type and have
removable shields over the fixed contacts and removable moving contact bars.
Each switch fuse shall be supplied complete with the correct HRC fuse links.
Switch fuses shall be 500V rating and shall be clearly marked with their current rating,
and the items of equipment that they serve.
Isolators
Isolators shall be heavy-duty pattern with steel enclosures having cast-iron frame
members, rust-protected and finished grey stoved enamel. Front access doors shall be
fitted with dust-excluding gaskets and shall be interlocked so that they cannot be opened
when the switch is ‘on’. Operating handles shall be lockable in both the on and off
positions and shall have visible on-off indication.
Isolators shall be 500V rating and shall be clearly marked with their current rating.
The moving contact assemblies are to be removable for inspection and maintenance.
Distribution boards
All distribution boards shall be single-pole or triple-pole with neutral bar, and with
fuseways/CBs 20 or 30A or above as specified.
All distribution boards shall be of the surface pattern in heavy sheet-steel cases of the
500V range with HRC fuse carriers.
On all triple-pole and neutral distribution boards the number of neutral terminals to be
provided shall be the same as the total number of fuse-ways/CBs in the board. This
information must be given to the manufacturers when ordering, together with information
with regard to composite boards having a multiplicity of fuse ratings as specified.
In the case of flush installations, the distribution boards shall be mounted over flush
adaptable iron boxes into which the conduits and wiring of the system will terminate. The
boxes shall be of a size to be agreed on site with the consulting engineer.
Doors of all distribution boards shall be lockable either by means of a barrel-type lock
with detachable key or by means of a modified door-fixing screw, bracket and padlock.
Ample clearance shall be provided between ‘live’ parts and the sheet-steel protection
to allow cables to be brought to their respective terminals in a neat and workmanlike
manner.
To separate opposite poles a fillet of hard incombustible insulating material shall be
provided of sufficient depth to reach the inside of the door.
Design of electrical services for buildings 292



In each distribution board spare fuse carriers shall be provided and held in place by a
suitable clip so that the carrier cannot be inadvertently dislodged.
Distribution boards shall be fixed at a height to give easy access, and be provided with
a schedule complying with 514–09–01 of BS 7671.
Starters
A starter shall be provided for each motor as indicated on the drawings and schedules.
The starters shall be surface-mounted with a sheet-steel case containing a triple-pole
contactor with vertical double break per pole having silver-faced contacts. They are to
have continuously rated operating coils with inherent undervoltage release. Operating
coils are to be supplied from phase to neutral. The starters shall have magnetic-type
overload relays with adjustable oil dashpot time lags and stop/reset push-buttons in the
front cover. The cover is also to contain the start push-button.
Starters are to have single-pole auxiliary switches and shall incorporate single-phase
protection.
Star-delta starters shall have a time-delay device complete with all main and control
wiring and terminal block for incoming and outgoing cables. The time-delay device shall
be of the pneumatic pattern with an instantaneous reset allowing restarting immediately
after a star-delta switching operation. The star and delta contactors are to be mechanically
and electrically interlocked.
The overload relays are to be correctly set to ensure adequate protection without
nuisance tripping.
Steel conduit
All steel Class ‘B’ conduits and conduit fittings throughout the whole of this installation
shall comply in all respects with British Standard Specification BS 31:1940. All PVC
insulated cables, other than flexibles, shall be protected throughout their length with
heavy-gauge screwed welded conduit (enamelled or galvanized as required) with the
necessary malleable iron loop-in, draw-in, angle and outlet boxes. No type of ‘elbow’ or
‘tee’ will be allowed on works under this Specification.
Where adaptable boxes are used they shall be of cast iron or heavy-gauge sheet steel
of not less than 12-gauge.
No conduit of less than 20mm diameter shall be used.
A solid coupling shall be inserted in every flush conduit run at the point where it
leaves a ceiling, wall or floor for ease of dismantling if required.
Except where otherwise stated conduit is to be finished black enamel.
No conduit shall be installed with more than two right-angle bends without draw-in
boxes and draw-in boxes shall not be more than 8m apart.
All conduits, except where otherwise specified, shall drop not rise to the respective
points. In no circumstances shall the conduit be erected in such a manner as to form a U
without outlet, or in any other way that would provide a trap for condensed moisture.
Provision shall be made for draining all conduits or fixtures by a method approved by
the consulting engineer.
Design example 293



No ceiling looping-in point box shall be used as a draw-in box for any other circuit
than that for which such point box is intended.
Ceiling point boxes are to be of medium pattern malleable iron, with fixing holes at
50mm centres and conforming to BS Specification.
Flush ceiling point boxes which do not finish flush with the finished surface of the
ceiling, etc., shall be fitted with malleable iron extension rings.
Horizontal or diagonal runs of flush conduit on structural or partition walls will not be
permitted. All flush conduits shall drop or rise vertically to their respective points.
Connections between conduits and trunking and conduit and steel boxes, or between
conduit and steel cases of distribution gear or equipment, shall be made by means of a
flanged coupling and brass smooth-bore entry bush. The lead washer shall be fitted on the
inside of the trunking or box, etc.
All lids for draw-in boxes, etc., whether of the BS or adaptable type, shall be of heavy
cast-iron or 12-gauge sheet steel, and shall be fixed (overlapping for flush work) by
means of two or four M6 round-headed brass screws as required.
Conduits set through walls will not be permitted. When change of direction is required
after passing through a wall an appropriate back outlet box is to be fitted.
All joints between lengths of conduit, or between conduit and fittings, etc., are to be
threaded home and butted.
Sets and bends are to be made without indentation, and the bore must be full and free
throughout. All screw-cutting oil must be carefully wiped off before joining up.
Conduit runs, as far as possible, are to be symmetrical and equally spaced.
The electrical contractor must take all precautions in situations likely to be damp to
see that all conduits and boxes in the vicinity are rendered watertight.
During the progress of the work all exposed ends of conduits shall be fitted with
suitable plastic or metal plugs. Plugs of wood, paper and the like will not be acceptable as
sufficient protection.
Lighting, heating, power and any other types of circuit shall be run in separate
conduits and no circuit of any one system shall be installed in any conduit or box of any
other system.
The proposed runs shall be submitted to the consulting engineer for approval before
work is commenced.
Conduit fittings
All conduit fittings shall be of malleable iron which shall conform to the British
Standards Specification BS 31:1940.
All fittings shall be of the screwed pattern, and no solid or inspection elbows, tees or
bends shall be installed. Generally, all conduit fittings shall be stove-enamelled black or
other approved finish inside and out, but where galvanized conduit is installed, all fittings
shall be galvanized by the hot process both inside and out. Such fittings shall be of Class
B pattern.
All conduit fittings not carrying lighting or other fittings shall be supplied with
suitable cast-iron covers with round-head brass screws. Where flush boxes are installed
the covers shall be of the overlapping rustproof pattern.
Design of electrical services for buildings 294



All ceiling point boxes, except in the case of surface conduits, shall finish flush with
the underside of the ceiling, extension rings being used where necessary.
Every flush ceiling point box to which a luminaire is to be attached shall be fitted with
a break-joint ring of approved type.
Where surface conduit is used in conjunction with distance saddles, special boxes shall
be used, to obviate the setting of conduit when it enters or leaves the boxes.
All conduit boxes, including boxes on and in which fittings, switches and socket
outlets are mounted, shall be securely fixed to the walls and ceilings by means of not less
than two countersunk screws, correctly spaced, and the fixing holes shall be countersunk,
so that the screw heads do not project into the box.
Flexible conduit
Connections to individual motors and heating equipment run in conduit shall be made
using a minimum of 300mm of watertight flexible conduit. The conduit shall be Kopex
LS/2.
Flexible conduit connecting to heating equipment shall employ butyl rubber insulated
CSP sheathed cables and suitable terminal blocks shall be used in all boxes where a
change in cable type is involved.
Earth continuity of all flexible conduits shall be maintained by 4mm2 minimum copper
conductors forming one of the cores of the cable.
Flexible conduits shall be terminated with the Kopex couplings and connectors
specially made for the purpose.
Cable trunking
Cable trunking shall be supplied and installed complete with fittings and accessories and
shall be of an approved manufacture. It shall be manufactured from zinc or lead-coated
sheet steel finished stove enamelled grey or galvanized and shall be of 18swg for sizes up
to and including 75mm×75mm section and 16swg for sizes above.
All bends, tees, reducers, couplings, etc., shall be of standard pattern: where it is
necessary for a special fitment to be used, it is to be fabricated by the manufacturers.
Where it is necessary to provide additional trunking over fixings, these shall be
supplied by the manufacturer and shall be applied with the manufacturer’s special tools.
Where holes or apertures are formed in the trunking for cable entry, they shall be
bushed with brass smooth-bore entry bushes, or PVC grommet strips. Cable supports are
to be inserted in vertical runs of trunking and cables are to be laced thereto in their
respective groups.
Fire barriers of hard insulating material shall be provided in vertical runs of trunking
where they pass through floors.
Where more than one service is involved multi-compartment trunking shall be
employed to separate the services.
Cable tray
Design example 295


Cable tray is to be made of 16-gauge perforated mild-steel sheet and is to be complete
with all coupling pieces and bends, offsets and fixing brackets, to enable the tray to fit the
structure accurately.
Where cables are taken over the edge of the tray they shall be protected by rubber
grommets.
MICC cable
Mineral-insulated metal-sheathed cables shall be high-conducting copper conductors
embedded in magnesium oxide and sheathed with copper with an overall covering of
PVC.
All cable terminations shall be protected and sealed with ring-type glands with screwon
pot-type seals utilizing cold plastic compound and neoprene sleeving all of an
approved pattern, and applied with the special tools recommended by the cable
manufacturers.
Cold screw-on pot-type seals shall be used except where the ambient temperature in
which the cable will operate will exceed 170°F, where the hot-type seal shall be used.
Four-core MICC cables shall not be used for ring circuits.
Vibration-absorbing loops shall be formed in MICC cables connected to motors and
other vibrating equipment.
Connections and joints in MICC cables shall only be made at the terminals of
switches, ceiling roses, or at connector blocks housed in outlet boxes. Connector blocks
shall have a minimum of two screws per conductor.
Where entry is made into equipment which does not have a spouted entry, the cable
shall be made off by means of coupling, male bush, and compression washer.
MICC cables shall be neatly installed and shall be clipped by means of copper saddles
secured by two brass screws. Where two, three or four cables are run together multiple
saddles shall be used.
MICC cables shall be delivered to the site with the manufacturers’ seals and
identification labels intact and shall be installed in accordance with the manufacturers’
recommendations and using the specialized tools recommended by the manufacturer.
They shall be tested when installed before being sealed and again at the end of the
contract.
They shall be sealed against the ingress of moisture at all times during the contract.
PVC cable
Cable PVC-insulated only, and PVC-insulated PVC-sheathed cable shall be 600/1000V
grade to BS 6004:1969.
The cable shall be delivered to site on reels, with seals and labels intact and shall be of
one manufacturer throughout the installation.
The cable shall be installed direct from the reels and any cable which has become
kinked, twisted or damaged in any way shall be rejected. The installation shall be cabled
on the loop-in system, i.e. wiring shall terminate at definite points (switch positions,
lighting points, etc.) and no intermediate connections or joints will be permitted. Cables
shall not pass through or terminate in lighting fittings.
Design of electrical services for buildings 296


Where it is necessary to make direct connection between the hard wiring and flexible
cord, this shall be done by means of porcelain-shielded connectors with twin screws. No
luminaire shall be connected directly to the hard wiring.
The terminations shall be suitable for the type of terminal provided and shall be either
sweated lugs of appropriate size, or eyelet or crimped type cable terminations, all of
reputable manufacture. Shakeproof washers shall be used where electric motors are
connected.
Where cable cores are larger than terminal holes, the cables shall be fitted with
thimbles. For all single connections, they shall be doubled or twisted back on themselves
and pinching screws shall not be permitted to cut the conductors. Cables shall be firmly
twisted together before the connection is made.
In no circumstances shall cables be trapped under plain washers as a termination.
Cables shall be coloured in accordance with IEE Regulations BS 7671.
Only two cables shall generally be bunched together at one terminal. In exceptional
cases three cables may be bunched together at one terminal with the authority of the
engineer given on site.
Flexible cords
All flexible cords shall comply with British Standard Specification BS 6500:1969, and
shall consist of high-conductivity tinned copper conductors of the required crosssectional
area insulated and sheathed as detailed hereunder:
Lighting pendants
Two-core 0.75mm2 heat-resisting circular flexible cord EP rubber-insulated CSPsheathed.
Colour of sheath, white.
Heating apparatus and equipment requiring flexible cable connection
Heat-resisting circular flexible cable. EP rubber-insulated CSP-sheathed having the
number of cores with cross-sectional areas as specified.
Apparatus and equipment, other than heating, requiring flexible cable connection
Circular flexible cable PVC-insulated PVC-sheathed having the number of cores with
cross-sectional areas specified.
The cores of all flexible cords shall be coloured throughout their length and colourcoded
to comply with the British Standard Specification.
PVC SWA cable
PVC-insulated single-cable armoured cables shall be 500/1000V grade and shall comply
with BS 6346:1969.
The cable shall comprise round or shaped conductors, of equal cross-sectional area,
composed of high-conductivity plain annealed copper cable insulated with PVC, coloured
for identification. The cores to be laid up circular and sheathed with PVC. The cable shall
be served with one layer of steel-cable armour and sheathed overall with PVC.
Design example 297


The cable shall be manufactured and supplied in one length on a suitable drum. No
through joints will be allowed. All cables shall be of one manufacture.
Where individual cables are run on the surface suitable supports shall be fitted to give
a minimum clearance of 15mm between cables and face of structure. Where cables are
installed vertically, the cable shall be gripped firmly by clamps of an approved pattern.
Where cables are grouped and run on the surface they shall be carried on wrought-iron
brackets or purpose-made clips of approved design, fixed at not more than 600mm
centres.
All PVC SWA cables run on the surface shall be adequately protected to a height of
2m from the ground.
Where PVC-insulated SWA cables are laid in the ground they shall be laid on not less
than 75mm of sand, covered by a further 75mm of sand and protected by means of
continuous interlocking warning tiles of approved pattern.
All cable trenches shall be excavated to a depth of 0.5m in unmade ground and 0.7m
where crossing roadways and backfilled by the builder who will also provide and install
all necessary cable ducts and earthenware pipes for cable entry into buildings, but the
electrical contractor shall be responsible for correctly marking out all cable routes,
supplying and installing waring tiles and marker posts, and generally supervising all work
in connection with the cable-laying requirements.
Cable joints
All cable runs between one definite terminal point and another throughout the whole of
the installation shall be installed without intermediate joints.
Luminaires
All luminaires shown and listed on the drawings and schedules shall be provided and
installed. Luminaires with non-standard suspension lengths shall be ordered to the correct
lengths to suit mounting height as indicated on the drawings and schedules. The
installation of luminaires shall include all necessary assembling, wiring and erection.
Terminations to non-pendant luminaires shall be in heat-resisting flexible cord with
porcelain-insulated terminal block connectors for connection to PVC-insulated cable or
PVC-insulated PVC-sheathed cable.
Fluorescent luminaires shall be mounted either directly or on suspensions from two
BS conduit boxes installed at the spacing required to suit the fitting.
Ceiling roses
Ceiling roses shall be white of reputable manufacture in accordance with BS 67. They
shall be of porcelain, or of plastic with porcelain interiors and shall be fitted with plastic
backplates or plastic mounting blocks semi-recessed where necessary to comply with the
IEE Regulations.
Where they are of the three-plate type the phase terminal shall be shrouded so as to
prevent accidental contact if the cover is removed.
Design of electrical services for buildings 298


Lamp holders
Lamp holders shall be of the bayonet-cap type for tungsten lamps up to and including
150W, and of the Edison screw type for larger lamps.
Where they are integral with lighting luminaires, they shall be brass with porcelain
interiors. For use with flexible pendants, they shall be of white plastic with compression
glands. Where batten lamp holders are installed the lamp holders shall be of white plastic.
In damp situations they shall be fitted with Home Office skirts.
Lamp holders for fluorescent tubes shall be of the heavy pattern bi-pin type of white
plastic construction.
All lampholders shall be lubricated with molybdenum disulphide to ensure easy
removal of threaded rings and lamps.
Lamps
Lamps shall be supplied and fitted to all points and luminaires shown and listed on the
drawings and schedules.
Tungsten filament lamps of 40–100W (inclusive) shall be of the coiled coil type.
Generally they shall be energy saving types. Lamps shall be pearl-finished when fitted in
open shades or in globes which are unobscured and shall be of the clear type when fitted
in closed units of opalescent glassware or any other type of luminaire elected where the
filament is not under direct vision.
The colour of all fluorescent lamps shall be the new white, 3500K.
Lighting circuits
Lighting circuits shall be installed on the loop-in system with three terminal-type ceiling
roses with shrouded live terminal, integral backplate, earth terminal and break-joint ring.
Looping shall not be carried out at switch positions.
No luminaire shall be connected directly to the hard wiring or have circuit wiring
passing through it.
Cables on one circuit are not to run through the BS boxes behind ceiling roses or
luminaires on other circuits.
Light switches
Flush switches shall be rocker-operated with white flush plastic plates of the singleswitch
or grid-switch type.
Surface switches shall be heavy-gauge steel with conduit entries and shall have
rocker-operated mechanisms. They shall have steel front plates of the single-switch or
grid-switch type.
Switches outdoors or otherwise exposed to damp conditions shall be of industrial
pattern watertight type with galvanized steel, or thermoplastic boxes and waterproof
gaskets.
Socket outlets
Design example 299


All socket outlets shall be of the switched type with rocker-operated switch mechanisms.
Flush socket outlets shall be of the insulated pattern with white or ivory finish.
Surface socket outlets shall be metal clad type with steel front plate.
Connections to space heaters
The circuit to each unit heater, fan convector and other similiar piece of heating
equipment shall terminate in a double-pole isolator from which the final connection shall
be made in heat-resisting rubber insulated cable in flexible conduit. The casing of the
heater shall be bonded to the earth continuity conductor.
Connections to motors and machinery
The circuit to each machine shall terminate in an isolator as near the machine as possible.
Where a motor starter is required it shall be placed adjacent to and immediately after the
isolator. The final connection from the isolator or starter to the machine shall be in PVCinsulated
cable in conduit. The rigid conduit shall terminate in a box approximately
300mm from the machine terminals and the final section from this point to the machine
shall be in flexible conduit. The metalwork of the machine shall be bonded to the earth
continuity conductor.
Regulations
The installation shall comply with Electricity at Work Regulations, Electricity Safety
Quality, and Continuity Regulations 2002 and any other applicable statutory regulations.
It shall conform with the Institution of Electrical Engineers’ Regulations for the Electrical
Equipment of Buildings BS 7671.
The installation and all material used shall comply with all relevant British Standards
and Codes of Practice.
Clearance from other services
All electric conduit and equipment shall be installed at least 150mm clear of any other
metalwork, and in particular of any water, gas, steam or chemical pipes.
Bonding and earthing
All conduit connections, boards, fittings, trunking, etc., shall be properly screwed
together so as to ensure proper mechanical and electrical continuity throughout.
Great care is to be taken in bonding and earthing the installation and tests are to be
carried out as the work progresses to check the electrical continuity of all metalwork,
conduits, etc., and earth continuity conductors. This is particularly important where work
is built into the fabric of the building.
For the purpose of estimating the electrical contractor may assume that he can earth to
the supply authority’s earthing terminal.
Design of electrical services for buildings 300


The electrical contractor shall contact the supply authority at an early stage in the
works to ensure that a suitable earthing terminal will be provided.
The electrical contractor shall be responsible for the bonding and earthing of all
exposed metalwork, structural or otherwise, and of the metalwork of any gas or water
service, to the earthing termination at the intake position, in accordance with IEE
Regulations BS 7671.
No earth continuity conductor shall be less than 1.5mm2 copper cable insulated and
coloured green and yellow.
The steel cable armouring of the sub-main cables shall be efficiently bonded together
and to the respective switchboard, distribution board, sealing chamber and conduits at
which they terminate and to all adjacent metalwork.
The frames of all electric motors and starting panels, etc., are to be efficiently earthed.
Where flexible metallic conduit is used, a stranded insulated and coloured green and
yellow copper cable of not less than 6mm2 is to be run from the terminal box through the
flexible metallic conduit to terminate in the first cast metal box in the conduit run.
The circuit protective conductor shall be attached at each end by means of a crimped
socket, brass screw and spring washer.
Pipelines, tanks, vessels and all other equipment associated with the piping or storage
of highly inflammable materials shall be statically bonded to an effective earth continuity
conductor by means of 30mm×10mm hard drawn tinned copper tape, secured by means
of the flange bolts. The earth continuity conductor shall be taken to an earth electrode and
bonded to it.
Earth electrode
Where an earth electrode is required it shall take the form of extensible copper earth rods
driven into the ground at suitable spacing. The number of rods and the depth to which
they are driven shall be determined according to the soil resistivity at the site to give an
earth resistance not exceeding 0.5ohms.
Concrete inspection covers shall be provided over every earth electrode and a means
shall be provided for disconnecting the bonding cable from every earth electrode.
Connection to and between electrodes shall be carried out in insulated stranded cable.
Testing
Continuity and insulation tests shall be carried out during installation.
At completion polarity, bonding, earth loop impedance, continuity and insulation tests
shall be carried out on the entire installation and in each part of it. The tests shall be
witnessed by the consulting engineer and shall be carried out in accordance with the
requirements of the Institution of Electrical Engineers’ Regulations BS 7671. A
completion certificate schedule of inspection, and schedule of test results as prescribed by
BS 7671 shall be provided.
Circuit lists and labels
Design example 301

At each distribution board a circuit schedule complying with BS 7671 IEE Wiring
Regulations shall be supplied and fixed on the inside of the distribution board door.
The schedule shall state clearly the position, number and wattage of lamps, socket
outlets, etc., which the fuseways/CBs control. A sample circuit schedule shall be
submitted for approval before installation.
On the cover of each distribution board, fuse switch, switch fuse, isolating switch and
starter a 45mm×20mm traffolyte label (white-black-white) shall be fixed and engraved in
5mm characters, giving details of the service position and phase, etc. In addition,
traffolyte (white-red-white) labels engraved in 8mm characters ‘400V’ shall be fixed to
all TPN distribution boards.
All labels shall be fixed by means of four 6mm round-headed brass screws.
Identification of cables
All power, instrument, control and indication cables shall be provided with indestructable
cable marking collars which shall bear the cable number. The marking collars shall be
fitted at every cable termination.
The individual cores of cables shall be numbered to indicate which terminal they are
connected to.
Fuses
HCR fuse links of the current rating shall be supplied and installed in all fuse carriers.
Design of electrical services for buildings 302

Design example

Chapter 18
Design example
In order to illustrate the practical application of the principles discussed in previous
chapters we shall, in this chapter, describe a typical industrial design. The example
chosen is taken from a scheme handled in the previous author’s office some years ago. It
does embody the criteria in use today, although the design package would yield closer
limits. It is appreciated that a building services engineer will now use software design
packages. It is useful to see how the design values were arrived at. The buildings of a
disused factory were taken over by a chemical manufacturing company which proposed
to adapt them as a new works. Electric services were needed for lighting and power to
machinery.
The general plan of the buildings is shown in Figures 18.1–18.3, which also show the
main part of the lighting layout. As the design of lighting has been excluded from the
subject matter of this book it is not proposed to reproduce the lighting calculations here,
but it should be noted that after the number of lights needed in each area had been
calculated they were positioned with regard to the layout of the machinery as well as to
the need to maintain reasonable uniform levels of illumination.
The factory consists of an east building of two storeys with a basement and a threestorey
west building with a covered yard between them extending the full height of the
east building. There is a walled car park adjacent to the buildings and a new boiler-house
was to be built in this area. Since the existing buildings provided more space than was
needed for the new works, part of the west building was to left unoccupied: no services
were to be installed in this part but the installation as a whole was to be capable of
extension into this area.
The bulk of the lighting consisted of twin-tube 1500mm fluorescent luminaires with
some single-tube luminaires in passages and areas requiring lower illumination. A few
incandescent luminaires were provided in toilets and on stairs (not all of which are shown
in Figures 18.1–18.2). The covered way between the occupied and unoccupied sections
of the west building in which materials would be hoisted to the upper levels was lit by
three wall-mounted mercury lamps at ground-floor level and three at second-floor level.


Figure 18.1 Factory ground-floor
lighting layout
One end of the west building contained tall machinery on the ground floor and the firstfloor
slab was not carried across this. An area of double the normal height was thus left
and this was lit by wall-mounted mercury lamps at the lower level and high-bay industrial
mercury luminaires under the first-floor ceiling. The covered yard was lit by wallmounted
high-pressure sodium floodlights at the level of the first-floor ceilings. Four
street-lighting lanterns were provided for the car park, three of them being mounted on
columns on the roadway from the building and one on a bracket on the wall of the
building.
The first stage in the design was to arrange the lights in circuits and to arrange the
circuits in convenient groups to be served from several distribution boards. The lighting
would have to be divided in a suitable manner between the three phases to give as nearly
as possible the same loading on all three phases and this had to be borne in mind when
the lights were
Design of electrical services for buildings 266


Table 18.1 Types of lighting
Ref. Type Current (amps)
A 1500mm twin fluorescent 0.92
B 1500mm single fluorescent 0.46
C Wall-mounted 125W MBF 1.15
D Tungsten bulkhead 0.42
E High-bay industrial mercury with 250W MBF lamp 2.15
F Wall-mounted area floodlight with 250W SON lamp 3.0
G Bulkhead luminaire with 50W MBF/U lamp 0.6
H Side-entry street-lighting lantern with 35W SOX lamp 0.6
arranged into circuits. For convenience, the different types of luminaire used were listed,
as shown in Table 18.1.
It was decided that in this type of factory the lighting could be run in 2.5mm2 cable
fused at 15A. To allow a margin for safety and small alterations the circuits would be
designed to carry not more than 12A each. Although it was intended to use three different
sizes of mercury lamp it was felt that there was a possibility that at some time in the
future a works manager might change the luminaires without checking the capacity of the
wiring and it was therefore decided to design all the circuits serving luminaires with
mercury lamps to be capable of taking 250W lamps. Similarly, circuits serving singletube
fluorescent luminaires would, where appropriate, be designed to take twin-tube
luminaires so that the luminaires could at any time be replaced without alterations to the
wiring. Hence, a maximum number of luminaires on a circuit would be:
It was clearly going to be desirable to control more than this number of lights from one
switch and it was decided to do so by switching the lights through contactors. One switch
would operate a multi-pole contactor controlling several lighting circuits.
At this stage a check was made on the voltage drop in the lighting circuits. Probable
positions of distribution boards were guessed and from the drawings the average length
of a lighting circuit was estimated as 35m. At the time when this design was produced,
the maximum volt drop allowed on 230V was 9V, it was decided to build in a factor of
safety and allow a maximum of 6V, and it seemed reasonable to allow half of this in the
sub-mains and half in the final circuits, that is to say 3V. The single phase volt drop of
2.5mm2 cable is 18mV per ampere per metre.
Design of electrical services for buildings 268


This is a pessimistic value since the current in the circuit will reduce as current is
‘dropped off’ at each luminaire, and the design current Ib will not flow in the whole
length of the circuit. The designer may decide to average. However we took this value as
true.
Clearly the need to reduce voltage drop was more critical than the current rating of the
cable and the number of luminaires per circuit would have to be reduced. Acceptable
figures would be six fluorescent luminaires or two sodium (slightly over the design
value) or two mercury luminaires per circuit.
The loadings were now estimated for each area in a convenient tabulated form as
shown in Table 18.2.
This formed a preliminary guide. The number of circuits in each area was decided by
referring to the maximum number of luminaires per circuit as determined above and also
with an eye to convenient switching arrangements. At the same time, some margins were
allowed to make it possible to adjust the circuit arrangements later without major
modifications to the distribution scheme. It will be noted for example that the car-park
lights are not included in the table. This was because the design had to proceed before the
client had taken final decisions on all his requirements. The fact that last-minute
alterations would certainly be made had therefore to be kept constantly in mind.
The total load from Table 18.2 was 460.94A. It should therefore be distributed to give
about 150A per phase. An ideally equal distribution could not be hoped for but each
phase should carry between 140 and 160A and at the same time each phase should be
contained within a reasonably clear zone of the building. As a first step towards
achieving this the loads for each area were summarized from Table 18.2, as shown in
Table 18.3. They were then arranged in three groups for the three phases. After two
attempts the results shown in Table 18.4 were obtained.
This was not as good as had been hoped for. However, the process of manipulating the
figures had given the designer a feel for them and he realized that he was not likely to
achieve any further improvement at this stage. It would be possible to make some
adjustment after the distribution boards were scheduled and this was done next.
Table 18.2 Loading estimates for each area
Area Luminaire
Ref. No.
off
Amps
each
Amps
total
No. of circuits needed
Gate-house A 1 0.92 0.92
B 2 0.46 0.92
D 3 0.42 1.26
3.10 1
Design example 269



Pump house A 8 0.92 7.36 2
Boiler house A 7 0.92 6.44 2
Covered way C 6 2.15 12.9 3 controlled by 1 contactor
W bldg Grd flr A 24 0.92 21.00 3:1 switched directly, 2 controlled by
1 contactor
G 5 2.15 4.30 2
E bldg Stores A 48 0.92 44.2 8 controlled by 4 contactors
Ovens A 33 0.92 30.36 6 controlled by 4 contactors
Toilet area A 1 0.92 0.92
B 3 0.46 1.38
D 1 0.42 0.42
2.72 1
Maintenance A 8 0.92 7.36 2
Lockers A 8 0.92 7.36 2
Stairs D 1 0.42 0.42 1
1st Floor Covered
yard
F 34 3.0 102.0 16 controlled by 8 contactors
Side yard F 20 3.0 60.0 10 controlled by 4 contactors
Lockers B 12 0.46 5.52 1
W bldg 1st flr A 20 0.92 22.08 6 controlled by 2 contactors
E 12 2.15 25.80 6 controlled by 2 contactors
E bldg 1st flr A 48 0.92 44.2 8 controlled by 4 contactors
1st flr Maintenance A 7 0.92 6.44 2
1st flr Pump house A 7 0.92 6.44 2
E bldg Cellar A 12 0.92 11.04 2
W bldg 2nd flr A 30 0.92 27.6 6 controlled by 2 contactors
W bldg 3rd flr B 5 0.46 2.30 1
Table 18.3 Load summaries
Area Load in amps
W bldg grd flr 25.30
Pump house 7.36
Gate-house 3.10
Design of electrical services for buildings 270


Covered way 12.90
48.66
Boiler house 6.44
W bldg 1st flr 47.88
Covered yard (main area) 102.00
Covered yard (side area) 60.00
W bldg 2nd flr 27.6
W bldg 3rd flr 2.3
E bldg grd flr 44.20
E bldg grd flr 30.36
E bldg Toilet area 2.72
Maintenance 7.36
Lockers 7.36
Stairs 0.42
92.42
E bldg 1st flr 44.20
E bldg Maintenance 6.44
E bldg Pump house 6.44
Lockers 5.52
62.60
E bldg cellar 11.04
Table 18.4 Distribution of load across phases
(provisional)
Amps
Brown (Red) phase
W bldg 2nd flr 27.6
W bldg 3rd flr 2.3
Covered yard main area 102.0
Boiler house 6.4
138.3
Black (Yellow) phase
W bldg grd flr 48.66
Design example 271

W bldg 1st flr 47.88
East cellar 11.04
Covered yard site area 60.00
167.58
Grey (Blue) phase
E bldg grd flr 92.42
E bldg lst flr 62.60
155.02
The lights and switching were shown on drawings. In each area, the luminaires were
grouped into circuits in accordance with the maximum number of luminaires per circuit
previously determined. Clearly the luminaires on any one circuit must be in a reasonably
compact group. Also, although the luminaires on one circuit can be controlled by more
than one switch, the converse is not true: one switch cannot control luminaires on several
circuits unless a multi-pole contactor is used. The most practicable way of settling these
matters is to mark the circuits and switching groups on drawings of an adequately large
scale.
Standard distribution boards are available with 12 and 16 ways. Suitable positions
were chosen on the drawings for distribution boards to serve groups of 7 to 12 circuits to
allow a reasonable number of spare ways on each board. The positions were chosen to
keep the final circuits reasonably short and so that as far as possible each board would be
in the ‘load centre’ of the area it was serving. It became evident in. this process that the
second and third floors of the west building should be served from a single board, that the
gate-house would need its own board, that three distribution boards would conveniently
handle both parts of the covered yard, that the ground and first floors of the east building
would each need two distribution boards and that the cellar of the east building would be
most conveniently served from the gate-house. The information from the drawings was
then summarized in distribution-board schedules which are reproduced in Table 18.5. A
further table was then made in order to decide on which phase each of these boards
should be and this is given in Table 18.6.
The figures in the three right-hand columns were entered in pencil, rubbed out and
moved from column to column until by a process of trial and error quite a good balance
over the phases was obtained. The first attempt
Table 18.5 Lighting distribution boards
Board no. 1 E bldg maintenance area 1st flr Phase Black (Yellow) sub-main 35mm2
Circuit no. No. and location of lights Fuse (A) Cable (mm2)
1 4 Pump house and changing rooms grd flr 15 2.5
2 4 Changing rooms grd flr 15 2.5
3 5 1st flr maintenance area 15 2.5
Design of electrical services for buildings 272


4 5 1st flr maintenance and sub-station 15 2.5
5 6 Pump house 15 2.5
6 6 Changing rooms 1st flr 15 2.5
7 5 Changing rooms 1st flr and stairs 15 2.5
8 4 Car park lights 15 2.5
9
10
11
12
Board no. 2 W bldg grd flr Phase Black (Yellow) sub-main 35mm2
Circuit no. No. and location of lights Fuse (A) Cable (mm2)
1 5 Bulkheads on wall 15 2.5
2 3 Production area 15 2.5
3 3 Production area 15 2.5
4 4 Production area 15 2.5
5 3 Production area 15 2.5
6 4 Production area 15 2.5
7 4 Production area 15 2.5
8 3 Hoist yard, low level 15 2.5
9 3 Hoist yard, high level 15 2.5
10 5 Rear entrance 15 2.5
11 4 Stairs 15 2.5
12
13
14
15
16
Board no. 3 W bldg 1st flr Phase Black (Yellow) sub-main 35mm2
Circuit no. No. and location of lights Fuse (A) Cable (mm2)
1 2 Mercury over vats 15 2.5
2 2 Mercury over vats 15 2.5
3 2 Mercury over vats 15 2.5
Design example 273


4 2 Mercury over vats 15 2.5
5 2 Mercury over vats 15 2.5
6 2 Mercury over vats 15 2.5
7 3 Fluorescent production area 15 2.5
8 4 Fluorescent production area 15 2.5
9 4 Fluorescent production area 15 2.5
10 3 Fluorescent production area 15 2.5
11 4 Fluorescent production area 15 2.5
12 4 Fluorescent production area 15 2.5
13
14
15
16
Board no. 4 Phase Brown (Red) sub-main 35mm2
Circuit no. No. and location of lights Fuse (A) Cable (mm2)
1 4 Production area 15 2.5
2 5 Production area 15 2.5
3 5 Production area 15 2.5
4 4 Production area 15 2.5
5 5 Production area 15 2.5
6 5 Production area 15 2.5
7 3 Laboratory and landing 15 2.5
8 5 Third floor 15 2.5
9
10
11
12
Board no. 5 E bldg grd flr stores Phase Grey (Blue) sub-main 35mm2
Circuit no. No. and location of lights Fuse (A) Cable (mm2)
1 4 Stores 15 2.5
2 4 Stores 15 2.5
3 4 Stores 15 2.5
Design of electrical services for buildings 274


4 4 Stores 15 2.5
5 4 Stores 15 2.5
6 4 Stores 15 2.5
7 4 Stores 15 2.5
8 4 Stores 15 2.5
9 4 Stores 15 2.5
10 4 Stores 15 2.5
11 4 Stores 15 2.5
12 4 Stores 15 2.5
13
14
15
16
Board no. 6 E bldg grd flr oven area Phase Grey (Blue) sub-main 35 mm2
Circuit no. No. and location of lights Fuse (A) Cable (mm2)
1 5 Circulation area 15 2.5
2 4 Ovens 15 2.5
3 4 Ovens 15 2.5
4 4 Ovens 15 2.5
5 4 Ovens 15 2.5
6 6 Ovens 15 2.5
7 6 Ovens 15 2.5
8 5 Maintenance area 15 2.5
9 4 Maintenance area 15 2.5
10 5 Toilets and stairs 15 2.5
11
12
13
14
15
16
Board no. 7 E bldg 1st flr stores Phase Grey (Blue) sub-main 35mm2
Design example 275


Circuit no. No. and location of lights Fuse (A) Cable (mm2)
1 4 Stores 15 2.5
2 4 Stores 15 2.5
3 4 Stores 15 2.5
4 4 Stores 15 2.5
5 4 Stores 15 2.5
6 4 Stores 15 2.5
7 4 Stores 15 2.5
8 4 Stores 15 2.5
9 4 Stores 15 2.5
10 4 Stores 15 2.5
11 4 Stores 15 2.5
12 4 Stores 15 2.5
13
14
15
16
Board no. 8 Gate-house Phase Black (Yellow) sub-main 16mm2
Circuit no. No. and location of lights Fuse (A) Cable (mm2)
1 6 Lodge and toilets 15 2.5
2 6 E bldg cellar 15 2.5
3 6 E bldg cellar 15 2.5
4
Board no.9 Covered yard Phase Brown (Red) sub-main 35mm2
Circuit no. No. and location of lights Fuse (A) Cable (mm2)
1 2 Covered yard 15 2.5
2 2 Covered yard 15 2.5
3 1 Covered yard 15 2.5
4 2 Covered yard 15 2.5
5 2 Covered yard 15 2.5
6 1 Covered yard 15 2.5
7 2 Covered yard 15 2.5
Design of electrical services for buildings 276


8 2 Covered yard 15 2.5
9 2 Covered yard 15 2.5
10
11
12
Board no. 10 Covered yard Phase Brown (Red) sub-main 35mm2
Circuit no. No. and location of lights Fuse (A) Cable (mm2)
1 2 Covered yard 15 2.5
2 2 Covered yard 15 2.5
3 2 Covered yard 15 2.5
4 2 Covered yard 15 2.5
5 2 Covered yard 15 2.5
6 2 Covered yard 15 2.5
7 2 Covered yard 15 2.5
8 2 Covered yard 15 2.5
9 3 Covered yard 15 2.5
10 3 Covered yard 15 2.5
11
12
Board no. 11 Covered yard Phase Black (Yellow) sub-main 35 mm2
Circuit no. No. and location of lights Fuse (A) Cable (mm2)
1 2 Covered yard 15 2.5
2 2 Covered yard 15 2.5
3 2 Covered yard 15 2.5
4 2 Covered yard 15 2.5
5 2 Covered yard 15 2.5
6 2 Covered yard 15 2.5
7 2 Covered yard 15 2.5
8 2 Covered yard 15 2.5
9
10
11
Design example 277



12
Table 18.6 Distribution of load across phases (final)
Phase
Board no. Area Amps Brown (Red) Black (Yellow) Grey (Blue)
1 E and W bldgs grd flr 24 24
2 W grd flr 32 32
3 W 1st flr 30 30
4 W 2nd and 3rd flr 31 31
5 E grd flr 48 48
6 E grd flr 45 45
7 E 1st flr 48 45
8 Gate-house 15 15
9 Covered yard 48 48
10 Covered yard 66 66
11 Covered yard 48 48
Total 145 149 138
was made on the basis of the provisional phasing decided on before the distribution
boards had been scheduled.
The size of sub-main necessary to serve these boards was next calculated. The
necessary current rating was evident from Table 18.6 but it was also necessary to
calculate the size of cable needed to give an acceptable voltage drop. The distance from
the intake to the furthest board was measured on the drawings and found to be 98m. This
was rounded off to 100m for the purpose of calculation. The current taken by the most
distant board was 31A and for the calculation this was rounded off to 30A. It had
previously been assumed that 3V would be lost in the final circuits and it was now
decided to allow a 2V drop in the sub-main. This would make the total well less than the
permissible maximum but there is no restriction on how low the voltage drop is and it
seemed prudent to allow a margin for future extensions and also for possible alterations
in the final positions of distribution boards and routes of cables.
2V=2000mV
Permissible drop is given by
Design of electrical services for buildings 278


35mm2 cable has a voltage drop of 1.25mV/A/m and is rated at 145A, when run singly.
For a 30A current we could suffer a correction factor of 30/145=0.2. We could run 12 or
more multicore PVC steel wire armoured cable on cable tray single-layer clipped
touching each other, and the correction factor would be 0.7. A larger sub-main would
seem unreasonable for the loads involved. Although the volt drop of 35mm2 cable is
higher than the calculated figure, the calculation was on the safe side and was carried out
only for the longest sub-main. The next size of cable is 50mm2 which is considerably
harder to handle and therefore more expensive to install. It would be rather unusual to use
such large cable for lighting distribution and it was therefore decided that 35mm2 cable
would be acceptable. Each of these cables would be served from a 60A switch fuse. An
exception was made for board no. 8 which would carry only 15A. By inspection and
without any calculation, it was decided that a 16mm2 cable rated at 85A clipped direct or
94A clipped to cable tray, run singly, with a volt drop of 2.8mV/A/m would be adequate
for this. It would be served from a 30A switch fuse.
Attention was now turned to the design of the power distribution. A list of the
machinery to be installed was obtained from the client and written out as shown in Table
18.7. The locations of the equipment were also obtained and are shown in Figures 18.4–
18.5. It should be noted that all power equipment was to be three-phase except for FHP
motors on rotary valves.
Figure 18.4 Factory ground-floor
equipment layout
Design example 279


Table 18.7 Machinery, assumed current demand
Running current (amps per phase)
Item No. of kW each Each Total Allow for diversity
‘A’ Agitator 4 3.73 8 32 16
Type 1 mill 1 37.3 70 70 35
Type 2 mill 1 18.65 36 36 –
‘A’ Mixers 2 3.73 8 16 8
Shakers 3 2.24 5 15 10
Extractor 13 3.73 8 104 52
‘B’ Mixers 2 11.19 22 44 22
‘G’ Mixers 10 14.92 30 300 200
‘G’ Mills 10 7.83 16 160
Rotary valves 20 0.19 3 60 15
Single-phase motors
‘A’ Pumps 1 5.60 11 11 –
‘B’ Pumps 1 3.73 8 8 8
‘C’ Pumps 3 2.98 6 18 9
‘D’ Pumps 1 3.73 8 8 8
‘E’ Pumps 3 5.60 11 33 11
‘F’ Pumps 3 5.60 11 33 11
Ovens 18 3.73 8 144 96
Hoist 1 3.73 8 8 –
‘A’ Fans 3 5.60 11 33 22
Conveyors 1 7.46 15 15 15
Dissolver 5 5.60 11 55 33
Coupling tanks 3 11.19 22 66 44
Lift 1 0.75 2 2 –
‘B’ Agitators 1 1.49 3 3 3
‘G’ Pump 1 2.24 5 5 –
‘B’ Fans 1 22.38 40 40 –
Boiler burner 1 7.46 15 15 15
‘H’ Pumps 1 11.19 22 22 22
Design of electrical services for buildings 280


‘C’ Fans 11 5.60 11 11 11
Burner auxiliary motor 1 2.24 5 5 5
‘J’ Pumps 1 3.73 5 5 5
Total amps per phase 1369 676
KvA over 3 phases 982 486
Most of it was accounted for by motors driving pumps, agitators and other mechanical
equipment: the running currents per phase were taken from standard motor performance
tables.
The allowance for diversity was based on the designer’s previous industrial experience
and his assessment of what equipment might normally be in use simultaneously. The
lighting load on the most heavily loaded phase was 149A and in view of the nature of the
building it seemed reasonable to apply
Figure 18.5 Factory first-floor
equipment layout
a diversity factor of 0.6 to this, giving an after-diversity lighting load of 90A. Addition of
this to the power load gave a total after-diversity load of 766A per phase which is
570kVA over all three phases. This could conveniently be catered for by 800A busbars at
the main intake.
A difficulty arose over this figure. The supply to the existing board came from a
315kVA transformer. If the electricity company were to be asked for a bigger supply they
Design example 281


would make a substantial charge which the factory owner wished to avoid. The client
also thought the calculated load was high but could not dispute the total installed load. He
told the designer that at an older but similar works belonging to the same company
measurements showed that the actual maximum demand was 27 per cent of the total
installed load. If the same figure were applied to the new factory the maximum demand
would be 0.27×(982kVA power load+120kVA lighting load)=298kVA which would be
within the capacity of the existing supply. The client therefore wanted this figure to be
used. Whilst unable to challenge the client’s measurements the designer felt that a
diversity factor of 27 per cent was surprisingly low. He pointed out that if the distribution
was designed on this figure and it turned out to be low it would be very difficult and
expensive subsequently to increase the capacity of the installation. He was reluctant to
work on this basis. After discussion it was agreed that 800A
Figure 18.6 Factory second-floor
equipment layout
busbars would be installed at the main intake but would be served through a 400A switch
fuse from the existing 315kVA (equivalent to 440A per phase) supply. This would make
it possible to cater for a larger load if the need arose without expensive alterations but
would not increase the initial cost very much. It therefore satisfied both points of view.
A description such as this inevitably makes the design process seem very precise
whereas in practice at each stage there are many unknown facts for which the designer
has to make a guessed allowance. In the present case the plant design was proceeding at
the same time as the electrical design and neither the ratings nor the positions of all the
equipment were finally settled. Table 18.7 is in fact based on the third attempt to draw up
such a list; it would be an unnecessary waste of space to reproduce the earlier tables
which differed only in detail. However, the element of uncertainty led to two important
decisions about the general scheme.
Design of electrical services for buildings 282


First, it was decided to use busbars with separately mounted switch fuses rather than a
cubicle-type switchboard. This would give excellent flexibility for future extensions and
also for changes and additions which might become necessary before the installation was
completed. It seemed quite likely that this would be necessary because of the uncertainty
of the final plant layout.
Second, it was decided that the design of the power installation would go only as far as
the final distribution boards. The final circuits from these to the various motors would be
settled on site after the machines were installed. In areas where there was to be a lot of
equipment horizontal busbars could be run along the building walls with tap-off boxes
spaced as required.
With these considerations in mind the load was listed again but this time area by area.
The load was summed for each area and a decision made on the size and rating of the
distribution board to serve that area, This list is
Table 18.8 Distribution of load across phases
Running current (amps per phase)
Area Item No. of Each Total After diversity
1 ‘A’ Agitators 2 8 16
E bldg N end Type 1 mills 1 70 70
‘B’ Pumps 1 8 8
‘C’ Pumps 1 6 6
Shaker 1 5 5
6 115 100
2 ‘A’ Agitator 2 8 16 16
E bldg N end ‘B’ Mixer 2 22 44 44
Type 2 mill 1 36 36
‘A’ mixer 2 2 16
‘C’ Pump 2 6 12
Shaker 2 5 10
Extractor 1 8 8
41
12 142 101
3 Per cubicle
E bldg Grinding ‘C’ Mixer 1 30
cubicles ‘C’ Mill 1 16
Rotary valve 2 6
Extractor 1 8
5 60
Design example 283



4 Ovens 18 8 144
E bldg Ovens Extractors 2 8 16
160
5 Hoist 1 8 8
W bldg 3rd flr ‘A’ Fans 3 11 33
Conveyors 1 15 15
‘D’ Pump 1 8 8
6 64 45
6 Dissolver 5 11 55 33
W bldg 2nd flr
7 Coupling tanks 3 22 66 22
W bldg 1st flr
8 ‘E’ Pump 3 11 33
W bldg Grd flr ‘F’ Pump 3 11 33
Lift 1 2 2
‘B’ Agitator 1 3 3
‘A’ Pump 1 11 11
‘G’ Pump 1 5 5
10 87 40
9 ‘B’ Fan 1 40
Boiler room Burner 1 15
‘M’ Pump 1 22
‘C’ Fan 1 11
Auxiliary motor 1 5
Pump 1 5
6 98
shown in Table 18.8, It will be noticed that some additional items not listed in the
previous table were added at this stage.
The load in areas 1 and 2 could be catered for by a 24-way 300A TPN distribution
board served from a 200A switch fuse.
A check with manufacturers’ catalogues showed that it would not be possible to get a
standard board with outgoing fuse-ways in the wide range of sizes needed, that is to say
from 5A to the 100A needed for the mills. It would therefore be necessary to use two
separate boards, one with fuse-ways from 2 to 30A and one with fuse-ways from 20 to
Design of electrical services for buildings 284

100A. The former would have fourteen items with an installed load of 97A giving about
58A after diversity and the latter would serve four items with an installed load of 150A
giving about 90A after diversity. A 20-way 60A TPN board and a 6-way 100A TPN
board would meet these requirements.
In area 3, the equipment in one cubicle only is listed. The mixer and the mill do not
run at the same time. The rotary valves run intermittently, therefore the maximum
simultaneous demand can be assessed as 30+3+8 = 41A. There are ten such cubicles
making the installed load 10×60=600A. After diversity this will be say 400A.
In this area there is a total of 10×5=50 pieces of equipment. To allow spare capacity
65 ways on a distribution board are needed.
Each bay is approximately six metres long. A busbar would have not more than six
tap-off points along this length but each bay contains two cubicles with ten pieces of
equipment. Therefore a busbar is not the most practicable method and distribution boards
should be used.
The distribution boards will be mounted on a wall. As there is a central gang-way
midway between the two facing walls the arrangement will have to be symmetrical so
that either two or four distribution boards will have to be used. This gives the possibility
of either 2 of 36-way 300A TPN boards from 200A switch fuses or 4 of 18-way 200A
TPN boards from 150 switch fuses.
At first sight the first alternative appeared cheaper but on checking with the
manufacturers it was revealed that standard boards are not made as large as this so the
second alternative had to be adopted.
Area 4 can be conveniently served by 200A TPN busbars fed by a 200A isolator at the
busbars which can in turn be served from a 150A switch fuse on the main panel.
At this stage it had been decided that the east building would require six distribution
boards and one set of busbars. All this could conveniently come from a subsidiary
distribution centre in the building. The sum of the after-diversity loads calculated for this
was 741A but allowing for diversity between the boards the maximum load on the
busbars would be less. To allow for adequate short-circuit strength and also for future
extensions it was decided to use 800A busbars for the subsidiary centre. It had already
been decided, as explained above, that the main intake would have 800A busbars with a
400A incoming switch fuse. To give discrimination, the outgoing switch fuse could not
be larger than 300A. The after-diversity load was probably still being over-estimated but
whereas switch fuses and if necessary cables can be changed later it would be very
expensive to replace the busbars. Therefore the local busbars can still be 800A but should
have an incoming isolator of a lower rating. There is a fuse at the outgoing end of the
cable from the main intake and there is no need for another fuse at its other end. As the
fuse is 300A the isolator which is protected by the fuse should have a higher rating, say
400A.
Reference was made in the last paragraph to short-circuit strength. In fact no separate
calculation was made for this design but the results of calculations on other projects were
made use of.
The busbars are rectangular copper bars supported at regular intervals. The dimensions
of the bars and the spacing of supports are given in the manufacturer’s catalogue. If the
length between two successive supports is treated as a simply-supported beam the
Design example 285


maximum permissible bending moment can be calculated from the bending stress
formula:
where
M=bending moment (Nm)
I=second moment of area (m4)
p=stress (Nm−2)
y=distance from neutral axis to outermost fibre (m).
I and y are easily calculated for a rectangular section, p is the maximum allowable
working stress of pure copper, and M is the moment to be calculated.
The bending moment for a simply-supported beam with uniform loading is
where
M=bending moment (Nm)
w=load per unit length (Nm−1)
L=distance between supports (m).
As M has been established and L is known, this enables w to be calculated to give the
maximum permissible uniform load on each bar.
When a current flows in two parallel rectangular bars the resulting mechanical force
between them is given by
where
w=force per unit length (Nm−1)
i=current (A)
s=spacing between bars (m).
If the force per unit length is taken as the maximum permissible uniform load which
has just been calculated, and the spacing between the bars is known from the
manufacturer’s catalogue, this formula allows the maximum allowable value of the
current to be calculated. This value is then the maximum current which the bars will be
just strong enough to withstand and they should not be exposed to a possible short-circuit
current higher than this.
Perhaps this seems a lengthy and somewhat circuitous piece of reasoning. It is,
however, a typical example of the way in which the various requirements for a
distribution system have to be fitted together. It has not been written as a description of
the final scheme but rather to show the process by which the scheme was arrived at.
The subsequent distribution centre in the east building could also serve a distribution
board in the maintenance area and the unit heaters for the space heating. No information
was available at this stage of the equipment which would be installed in the maintenance
Design of electrical services for buildings 286


workshop but a 12-way 60A TPN board would certainly be adequate. The area would
have ten heaters which could be served from a 12-way SPN board.
At this stage there was still some uncertainty about the exact positions of the
equipment in the lower part of the west building and indeed about how much of the
equipment planned would be installed initially and how much left for the future.
Partly for this reason and partly to give the greatest possible flexibility for the final
connections, it was decided after discussion with the client to provide a busbar under the
first floor ceiling of the west building to serve the first and ground floors. It will be
remembered that part of the first-floor slab was omitted to give a two-storey height to
part of the ground floor. This made it possible to serve the ground floor from a busbar at
high level on the first floor. Indeed in view of the lack of information available to the
electrical designer about the height of the motors on the machinery to be installed this
seemed the only reasonable thing to do.
From Table 18.8 the total amps per phase on the ground and first floors were 153
installed and 62 after diversity. Some 200A busbars served by a
Table 18.9 Summary for W building
Running current (amps per phase)
Floor Total installed After diversity
Grd 87 40
1st 66 22
2nd 55 33
3rd 64 45
272 140
200A switch fuse would be ample for this load (this is the lowest standard rating for
busbars).
The second floor needed an 8-way 60A TPN distribution board served from a 60A
switch fuse.
The third floor needed an 8-way 60A TPN distribution board served from a 60amp
switch fuse.
The loading for the various areas of the west building was then summarized as shown
in Table 18.9. It became evident that the whole of this could conveniently come from a
subsidiary distribution centre within the west building. Adequate margins and provision
for future additions suggested that a suitable size would be 400A busbars served from a
200A switch fuse.
The space heating of the west building was to be provided by four unit heaters on the
second floor and four on the first floor which all required a supply for fans and
thermostats. Much of this equipment could be conveniently served from a six-way SPN
distribution board on its own floor and these two boards could also be served from the
subsidiary distribution centre.
Design example 287


The only remaining area to be dealt with was the boiler room. This is Item 9 in Table
18.8. There would be little diversity here and it was therefore decided to provide a 150A
TPN distribution board served from a 150A switch fuse.
To save a multiplicity of sub-mains cables, it was decided that in the west building the
lighting distribution boards would be served from the subsidiary distribution centre. The
east building was nearer the main intake so that it would not be so cumbersome and
expensive to run several cables between them. Also the number of switch fuses required
on the east building distribution centre for the power boards alone was already quite high.
It was therefore decided that the east building lighting boards would be served directly
from the main intake.
The distribution scheme was now sketched, as shown in Figure 18.7. This is the most
convenient method of summarizing the decisions taken so far and checking for any
inconsistencies or omissions. In its final form it is also the
Figure 18.7 Factory distribution
diagram
clearest way of explaining the scheme to be installed to contractors and suppliers.
It now remained to decide the sizes of the various sub-main distribution cables which,
at this stage, had not been written into the scheme of Figure 18.7. The necessary current
ratings were clear from the switch fuse ratings needed on the scheme but the cables also
had to be calculated for voltage drop. It is only necessary to make sure that the voltage
drop is not excessive and one of a limited number of standard-size cables must be chosen.
The calculation was therefore simplified by taking 100m as the longest run of a sub-main
cable and using the same length in calculating all of them. It had earlier been assumed
Design of electrical services for buildings 288

 that, of the total permissible voltage drop, half would be in the final circuit and half in the
sub-mains. To keep to this assumption the sub-mains had to be calculated for a drop of
4.6V (4 per cent of the Nominal Voltage/2), to take into account future extensions the
volts drop would be restricted to 3V.
In view of the number of sub-mains involved the calculation was again set out in a
tabular form as shown in Table 18.10. Column 2 gives the current rating for reference
method 11 single layer clipped to cable tray. In column 3 the drop in millivolts per amp
metre to give a drop of 1V over the assumed total length of 100m has been calculated.
This figure was then multiplied by
Table 18.10 Sizing of sub-main distribution cables
1 2 3 4 5 6 7
mV/A/m for 1 volt drop
=(1000) Cable
Board no. Amps amps×100 mV/A/m for 3 volt
drop
mm2 Amps mV/A/m
21 60 0.17 0.50 50 125 0.81
22 100 0.10 0.30 70 155 0.57
23 150 0.07 0.20 95 190 0.43
24 150 0.07 0.20 95 190 0.43
25 150 0.07 0.20 95 190 0.43
26 150 0.07 0.20 95 190 0.43
27 60 0.17 0.50 50 125 0.81
28 30 0.33 1.00 35 72 1.1
29 45 0.22 0.67 35 72 1.1
30 25 0.40 1.20 35 72 1.1
31 12 0.83 2.50 25 62 1.5
32 12 0.83 2.50 25 62 1.5
33 100 0.10 0.30 70 155 0.57
W. busbars 100 0.10 0.30 70 155 0.57
Oven
busbars
150 0.07 0.20 95 190 0.43
E. subcentre
300 0.03 0.10 300 390 0.185
W. subcentre
120 0.08 0.25 150 250 0.35
Design example 289

three and rounded off in column 4 to give the millivolts per amp metre for a 3V drop.
Columns 2 and 4 thus showed the minimum requirements of the cable: the cable chosen
to match this was then entered in columns 5, 6 and 7. The cable sizes were then entered
on the scheme which is reproduced in Figure 18.7 and this completed the design.
In this example the designer’s freedom was restricted by the limitations of an existing
building. In principle, where a new building is being designed, the designer of the
electrical services can be called in early enough to suggest arrangements which would
result in a more economical services installation. The effect of voltage drop or cable sizes
makes it desirable to have the spaces allocated for intake panels and distribution boards
as near as possible to the centre of the area being served, load centres. The provision of
false ceilings, and horizontal and vertical ducts influences the type of wiring system to be
employed. The thickness of plaster may determine whether or not cables can be buried
within it. If walls are built of a single thickness of brick it will not be practicable to chase
them for cables or conduits, and the electrical installation may have to be run on the
surface.
All these matters can be discussed by the electrical services designer and the architect
at a very early stage and should in theory influence the building design. In practice it
seems that since it is always possible to adapt the electrical installation to any building,
purely architectural considerations always override the engineering ones. Many architects
have no objection to conduit on the surface of walls even in a completely new building,
and if this is accepted the type of construction no longer matters.
In the early stages of design the architect’s ideas are very fluid and it is difficult for the
electrical designer to make suggestions which are more than vague generalities. By the
time he receives drawings on which he can start design work of his own the shape and
style of the building have been settled and can no longer be altered to accommodate or
simplify the services.
Thus the engineering designer’s influence on the overall design of the building tends
to exist more in theory than in practice.
When the work is to be put out to competitive tender it is necessary to draw up a
specification describing the quality and standard of the equipment to be used and the
standard of workmanship expected. The specification which was used for the scheme
described in this chapter is reproduced in the following pages.
It is often prudent to include in a specification descriptions of equipment which may
not be needed for the scheme as designed. Variations are frequently made during
construction, and they could introduce a piece of equipment not originally needed. If it
has not been described in the specification a short variation instruction can give rise to
different interpretations which could result in a contractual dispute.
For example, the original scheme may not require any isolators as opposed to fuse
switches and switch fuses. If an isolator is subsequently required and there is no clause in
the specification covering isolators, the variation instruction must include a complete
description or there is the possibility that an unsuitable type will be supplied. Since there
is invariably less time available for drafting variations than for the original specification,
it is better to have a few extra clauses in the specification than to risk contractual
difficulties later on.
Design of electrical services for buildings 290

Regulations

Chapter 17
Regulations
In most countries the supply of electricity is governed by legislation and we ought not to
conclude this book without an account of the rules which apply in the UK. We have to
refer to both the Electricity Safety Quality, and Continuity Regulations 2002 and the
Electricity at Work Regulations 1989.
The Electricity Safety Quality, and Continuity Regulations 2002 place an obligation
on the Distribution Companys to provide a supply of electricity to everyone in their area
who asks for it. They naturally charge for the electricity and may make a charge for
making the connection to their distribution system. This will depend on how much they
have to extend that distribution network in order to reach the new consumer’s premises.
The Acts also confer power on certain government departments to make further
regulations to control the supply of electricity.
These deal chiefly with the standards of service and safety to be met by the
distribution companies, and are not of direct concern to the designer of services in a
building who is concerned with what happens on the consumer’s side of the connection
and not with what goes on in the road outside. The Regulations do, however, give the
distribution company some powers of supervision over the consumer’s installation. The
chief of these is that the Company may not connect a consumer’s installation to its supply
if it is not satisfied that the insulation meets a prescribed value and that the installation
has adequate protective devices. If a consumer does not comply with the regulations, the
company may refuse to connect a supply, or if it has already been connected may
disconnect it.
These Regulations are somewhat general and it is conceivable that there could be
doubt about their precise interpretation. In practice, difficulties hardly ever arise, and
there are probably two reasons for this. First, distribution companies do not in practice
inspect installations and are content with the installer’s certificate of completion showing
the insulation resistance. Second, and more importantly, installations complying with the
Institution of Electrical Engineers’ Regulations for the Electrical Equipment of Buildings
BS7671 are deemed to comply with the Electricity Safety Quality, and Continuity
Regulations 2002. This brings us to consideration of the IEE Regulations, and we can
note a curious, and perhaps typically British, feature about them. They are the most
important regulations which in practice have to be observed and yet they are published as
a National Standard and have no legal standing of their own. They may however be used


in a court of law to claim compliance with a legal requirement. This comes about in the
following way.
There is no legal need for an installation to comply with the IEE Regulations. If an
installation satisfies the Electricity Safety Quality, and Continuity Regulations 2002 the
law does not care whether or not it also satisfies the IEE Regulations. But the law also
says that if it happens to satisfy BS 7671 it will be deemed to satisfy the Electricity
Safety Quality, and Continuity Regulations 2002, and in practice this is the easiest way of
showing that the Electricity Safety Quality, and Continuity Regulations 2002 have been
satisfied. As a result everyone in the industry is familiar with the IEE Regulations, but
very few people are aware of the curiously roundabout legal sanction behind them.
The sixteenth edition of BS 7671 was issued in 1991 with subsequent amendments to
BS7671:2004. References to the Regulations in this book are to this edition.
Part 1 of the Regulations sets out the scope, object, and fundamental principles; Part 2
contains definitions of terms; Part 3, assessment of general characteristics, lists the main
features of an installation which have to be taken into account in applying the subsequent
parts; Part 4 describes the measures to be taken for protection against the dangers that
may arise from the use of electricity and Part 5 deals with the selection of equipment and
accessories and with the details of construction and installation. Part 6 deals with special
locations. Part 7 is concerned with inspection and testing and there are six appendices
giving further need to know information.
Further information is given in a set of Guidance notes published by the IEE,
Guidance Note 1 often being referred to in this book, which gives further information on
means by which the regulations are complied with.
Chapter 13 of Part 1 of the IEE Regulations states Fundamental Principles. If this
chapter is not complied with, it may be taken that the distribution company would be
justified in disconnecting the supply. Parts 3 to 6 of the Regulations set out methods and
practices which are considered to meet the requirements of Chapter 13. A departure from
these parts of the Regulations does not necessarily involve a breach of Chapter 13 but
should be given special consideration. In fact the Regulations make clear that they are not
intended to discourage invention and that departure from them may be made if it is the
subject of a written specification by a competent body or person and results in a degree of
safety not less than that obtained by adherence to the Regulations.
If a manufacturer wishes to introduce a new technique which is not envisaged in the
current edition of the Regulations, application can be made for a certificate from a
qualifying body.
Much of what has been said in previous chapters is based on the methods and practices
described in BS 7671 and there seems little point in attempting either an abridgement or a
gloss on the Regulations here. The Regulations do not contain instruction in the basic
engineering principles on which they are based; they are regulations and not a textbook.
Engineers who have followed a suitable course and understood the principles of electrical
services should be able to read and understand the regulations without the interposition of
a detailed commentary. Designers who follow the principles we have tried to explain in
this book ought to find that their schemes almost inevitably comply with the regulations.
The other main source of legislation we have to refer to are the Electricity at Work
Regulations 1989. These are concerned with safety in all places of work, but do not
themselves contain detailed rules for the use of electricity. Compliance with BS 7671 will
Regulations 263

almost inevitably satisfy all the requirements of the Electricity at Work Regulations 1989,
and the design principles explained in this book take into account the requirements of the
Regulations.

Lifts, escalators and paternosters

Chapter 16
Lifts, escalators and paternosters
Introduction
The general design of lifts is very well established, and in this country at least, nearly all
the reputable lift manufacturers will design and supply a satisfactory lift as a matter of
routine if given the details and the size of building. Nevertheless, the designer of the
building electrical services must be able to advise the client about the lifts, to negotiate
with the lift suppliers and to compare competing tenders. The designer must, therefore,
know something about the technical details of lifts and we shall accordingly devote this
chapter to a brief outline of the subject.
First, we can note that there are three categories of lifts. Passenger lifts are designed
primarily for passenger use; goods lifts are mainly for goods but can on occasion carry
passengers; and service lifts are for goods only and are of such a size that passengers
cannot enter into the car. Lift speeds are determined by the number of floors served and
the quality of service required. They vary from 0.5ms−1 to 10ms−1 in high office blocks.
In deciding the size of car one can allow 0.2m2 for each passenger, and when
determining the load the average weight of a passenger can be taken as 75kg. It must,
however, be remembered that in many buildings the lift will be used for moving in
furniture and the car must be big enough for the bulkiest piece of furniture likely to be
needed. The author has made measurements of domestic furniture and has concluded that
the most awkward item to manoeuvre is a double bed, which can be up to 1670mm wide
by 1900mm long and 360mm high. In flats it is unfortunately also necessary to make sure
that stretchers and coffins can be carried in the lift. To accommodate these, a depth of
2.5m is required. The whole car can be made this depth or it can be shallower but have a
collapsible extension which can be opened out at the back when the need arises. The lift
well must, of course, be deep enough to allow the extension to be opened. In hospitals
some of the lifts must take stretchers on trolleys and also hospital beds and these lifts
must be the full depth of a complete bed.
Grade of service
The quality of service is a measure of the speed with which passengers can be taken to
their destination. It is the sum of the time which the average passenger has to wait for a

lift and of the travelling time once in the lift. The maximum time a person may have to
wait is called the waiting time (WI) and is the interval between the arrival of successive
cars. It depends on the round trip time (RTT) of each lift and on the number of lifts.
The average time a person has to wait is WI/2. The average time travelling is RTT/4.
The sum of these, WI/2=RTT/4, is called the grade of service. If there are N lifts, then
WI=RTT/N, and grade of service becomes WI(2+N)/4. It is usual to classify the grade of
service as excellent if WI(2+N)/4 is less than 45s, good if it is between 45 and 55s, fair if
it is between 55 and 65s and casual if it is more than 65s.
The use of a building will often enable a designer to estimate the probable number of
stops during each trip. If this is difficult, then a formula can be developed by probability
theory, and is:
where
Sn=probable number of stops
n=number of floors served above ground floor
N=number of passengers entering lift at ground floor on each trip
P=total population on all floors
Pa, Pb,…pn=population on 1st, 2nd…nth floor.
Three to four seconds must be allowed for opening and closing the doors at each stop.
A further 1 to 11.5s have to be allowed for each passenger to enter the lift and 1 to 2s for
each passenger to leave.
The travelling time is made up of periods of acceleration, constant speed and
retardation. Figure 16.1 gives the time versus distance curves for the acceleration
normally associated with various lift speeds. On each curve, the point marked X indicates
the end of acceleration and start of constant velocity. The retardation is generally taken to
be equal in magnitude to the acceleration. Providing the distance between stops is long
enough for the lift to reach steady speed before starting to slow, the total travelling time
of a round trip is given by:
Design of electrical services for buildings 236


Figure 16.1 Acceleration curves for
lifts
where
t=total travelling time
d=distance during which acceleration takes place
D=distance between ground and top floors
Sn=number of stops between ground and top floors
V=lift speed.
The best way of showing how all this data is used to assess the grade of service is by
means of an example. Let us assume we are dealing with an office block with eight
floors. The heaviest traffic will occur in the morning when people are arriving at work,
and we shall assume that we know enough about the occupancy of the building to have
been able to estimate that 75 per cent of the work force will arrive in one particular half
hour. For estimating the probable number of stops, traffic to the first floor can be ignored
and we can set out the number of people requiring service as follows:
Lifts, escalators and paternosters 237


Table 16.1 Lift service comparison
A B C D
Load persons 10 20 10 15
Speed ms−1 1.5 1.5 2.0 2.0
Probable no. of stops per trip (Sn) 5.23 6.35 5.23 5.99
Accelerating distance (d) m 2.60 2.60 2.20 2.20
d×Sn 13.60 16.50 11.50 13.20
Distance between ground and top floors (D) m 25.00 25.00 25.00 25.00
(dSn+D+d) 41.20 44.10 38.70 40.40
55.00 59.00 38.00 40.00
Door opening time (s) 21.00 28.00 21.00 24.00
Passengers entering and leaving (s) 25.00 50.00 25.00 37.00
Total travelling time 101 137 84 101
10% margin 10 13 8 10
RTT(s) 111 150 92 111
No. of lifts 4 3 4 3
No. of trips per lift in 30min 16 12 19 16
No. of persons per lift in 30min 160 240 190 240
Total no. of persons carried in 30min 640 720 760 720
WI(s) 28 50 23 37
42 62 35 46
Grade of service Excellent Fair Excellent Good
Calculation of Sn P=662
n=7
i Pi P−Pi
2 36 626 0.95 0.60 0.46 0.36
3 93 569 0.86 0.22 0.10 0.05
4 160 502 0.76 0.06 0.01 0.00
5 85 577 0.58 0.22 0.10 0.05
Design of electrical services for buildings 238


6 120 542 0.82 0.12 0.04 0.02
7 105 557 0.84 0.18 0.08 0.03
8 63 599 0.905 0.37 0.22 0.14
Σ 1.77 1.01 0.65
Sn=7−Σ 5.23 5.99 6.35
Floor 2 3 4 5 6 7 8 Total
No. of persons requiring service 36 93 160 85 120 105 63 662
The figures in the second line are 75 per cent of the floor populations, which we assume
we have either been given or can guess. The distance between the ground and eighth floor
is 25m.
The round trip time can be calculated and hence it is possible to calculate the number
of lifts needed to carry 662 people in 30mm. From this, the grade of service can be
obtained. If the calculation is set out in tabular form, different combinations can be easily
compared. This has been done in Table 16.1. A 10 per cent margin is added to the
calculated total time to allow for irregularities in the time interval between different lifts
in the bank of lifts.
It can be seen that in this example the most satisfactory arrangement is four lifts each
taking 10 persons at a speed of 1.5ms−1. A speed of 2.0ms−1 would be unnecessarily
extravagant.
It will be found that where the service is not so concentrated lower speeds are
sufficient. For this reason, it should not be necessary to use speeds of more than 0.75 or
1.0 ms−1 in blocks of flats.
Accommodation
The machine room for the lifting gear is normally at the top of the lift shaft or well. It can
be at the bottom or even beside the well, and in the latter case it can be at any height, but
from these positions the ropes must pass over more pulleys so that the overall
arrangement becomes more complicated. It is, therefore, better to provide space for the
machine room at the top of the well. Room must also be left for buffers and for inspection
at the bottom, or pit, of the well. The sizes of the machine and pit rooms must ultimately
be agreed with the lift manufacturers, but for preliminary planning before a manufacturer
has submitted a quotation the dimensions in Table 16.2 may be taken as a guide.
Drive
Nearly all lifts use a traction drive. In this, the ropes pass from the lift car around a cast
iron or steel grooved sheave and then to the counterweight. The sheave is secured to a
steel shaft which is turned by the driving motor. The drive from the motor to the shaft is
usually through a worm gear. The force needed to raise or lower the lift car is provided
Lifts, escalators and paternosters 239


by the friction between the ropes and the sheave grooves. The main advantage of the
traction drive is that if either the car or counterweight comes into contact with the buffers
the drive ceases and there is no danger of the car being wound into the overhead
structure. Other advantages are cheapness and simplicity.
Table 16.2 Lift dimensions
Passenger lifts
Well Machine room
Load
persons
Speed
(ms−1)
Width
(m)
Depth
(m)
Width
(m)
Length
(m)
Height
(m)
Top landing to
MIC room
floor (m)
Pit
depth
(m)
General purpose passenger lifts
8 1.0 1.80 1.90 3.10 4.80 2.60 4.00 1.60
10 0.75 2.00 1.90 3.10 5.00 2.60 4.00 1.60
10 1.0 2.00 1.90 3.10 5.00 2.60 4.00 1.70
10 1.5 2.00 1.90 3.10 5.00 2.60 4.20 1.70
16 0.75 2.60 2.20 3.50 5.30 2.70 4.10 1.70
16 1.00 2.60 2.20 3.50 5.30 2.70 4.20 1.90
16 1.50 2.60 2.20 3.50 5.30 2.70 4.30 1.90
20 0.75 2.60 2.50 3.50 5.60 2.70 4.10 1.70
20 1.00 2.60 2.50 3.50 5.60 2.70 4.20 1.90
20 1.50 2.60 2.50 3.50 5.60 2.70 4.30 1.90
High speed passenger lifts
12 2.5 2.20 2.20 3.20 7.50 2.70 6.80 2.80
16 2.5 2.60 2.30 3.20 8.00 2.70 6.80 2.80
16 3.5 2.60 2.30 3.20 8.00 3.50 6.90 3.40
20 2.5 2.60 2.60 3.20 8.30 3.50 6.20 2.80
20 3.5 2.60 2.60 3.20 8.30 3.50 7.10 3.40
20 5.0 2.60 2.60 3.20 8.30 3.50 8.20 5.10
Goods lifts
Well Machine room
Load
(kg)
Speed
(ms−1)
Width
(m)
Depth
(m)
Width
(m)
Length
(m)
Height
(m)
Top landing to
M/C room
floor (m)
Pit
depth
(m)
General purpose goods lifts
Design of electrical services for buildings 240


500 0.5 1.80 1.50 2.00 3.70 2.40 3.80 1.40
1000 0.25 2.10 2.10 2.10 4.30 2.40 3.80 1.50
1000 0.50 2.10 2.10 2.10 4.30 2.40 3.80 1.50
1000 0.75 2.10 2.10 2.10 4.30 2.40 3.80 1.50
1500 0.25 2.50 2.30 2.50 4.50 2.70 4.00 1.50
1500 0.75 2.50 2.30 2.50 4.50 2.70 4.20 1.80
1500 1.00 2.50 2.30 2.50 4.50 2.70 4.20 1.80
2000 0.25 2.80 2.40 2.80 4.70 2.90 4.10 1.50
2000 0.75 2.80 2.40 2.80 4.70 2.90 4.50 1.80
2000 1.00 2.80 2.40 2.80 4.70 2.90 4.50 1.80
3000 0.25 3.50 2.70 3.50 5.00 2.90 4.20 1.50
3000 0.50 3.50 2.70 3.50 5.00 2.90 4.40 1.70
3000 0.75 3.50 2.70 3.50 5.00 2.90 4.50 1.80
Well Machine room
Load
(kg)
Speed
(ms–1)
Width
(m)
Depth
(m)
Width
(m)
Length
(m)
Height
(m)
Top landing to
M/C room floor
(m)
Pit
depth
(m)
Heavy duty goods lift
1500 0.50 2.60 2.40 2.60 4.80 2.70 4.80 1.70
1500 0.75 2.60 2.40 2.60 4.80 2.70 4.80 1.80
1500 1.00 2.60 2.40 2.60 4.80 2.70 4.80 1.80
2000 0.50 2.90 2.50 2.90 5.00 2.90 4.80 1.70
2000 0.75 2.90 2.50 2.90 5.00 2.90 4.80 1.80
2000 1.00 2.90 2.50 2.90 5.00 2.90 4.80 1.80
3000 0.50 3.50 2.80 3.50 5.30 2.90 4.80 1.70
3000 0.75 3.50 2.80 3.50 5.30 2.90 4.80 1.80
3000 1.00 3.50 2.80 3.50 5.30 2.90 4.80 1.80
4000 0.50 3.50 3.40 4.00 6.20 2.90 5.20 1.70
4000 0.75 3.50 3.40 4.00 6.20 2.90 5.20 1.80
5000 0.50 3.60 4.00 4.00 6.80 2.90 5.20 1.70
5000 0.75 3.60 4.00 4.00 6.80 2.90 5.20 1.80
Figure 16.2 shows a geared traction unit has a drive motor permanently coupled to the
gear by a vee rope drive. There is a brake working on a brake disc on the end of the worm
Lifts, escalators and paternosters 241

shaft and the grooved sheave is easily discernible at the side of the machine. The control
panel is in the foreground
The only other kind of drive is the drum drive. In this case, one end of the car ropes
and one end of the counterweight ropes are securely fastened by
Figure 16.2 Geared drive unit
(Courtesy of Schindler Lifts Ltd)
clamps on the inside of a cast iron or steel drum, the other ends being fastened to the car
and counterweight respectively. One set of ropes is wrapped clockwise around the drum
and the other set anti-clockwise, so that one set is winding up as the other set is
unwinding from the drum. As the car travels, the ropes move along the drum in spiral
grooves on its periphery. The drum drive suffers from the disadvantage that as the height
of travel increases the drum becomes large and unwieldy. It has been almost entirely
superseded by the traction drive.
Figure 16.3 shows various arrangements of ropes for traction drives. Figure 16.3b
shows a double wrap drive, in which each rope passes over the sheave
Design of electrical services for buildings 242


Figure 16.4 Compensating ropes
twice. The increased length of contact between rope and sheave increases the maximum
available lifting force, or alternatively permits a lower coefficient of friction for the same
lifting force. Figure 16.3c shows a double wrap two-to-one system in which the speed of
the car is half the peripheral speed of the sheave. Figures 16.3d, e and f correspond to
Figures 16.3a, b and c but with the winding machine at the bottom of the well.
Compensating ropes are sometimes fitted on long travel lifts in order to make the load
on the motor constant by eliminating the effect of the weight of the ropes. A simple
method of doing this is shown in Figure 16.4.
Motors
A lift motor should have a starting torque equal to at least twice the full load torque; it
should be quiet and it should have a low kinetic energy. The last requirement is necessary
for rapid acceleration and deceleration and also for low wear in the brakes. The
theoretical power needed can be calculated from the lifting speed and the greatest
difference between the weights of car plus load and counterweight. The actual power will
depend on the mechanical efficiency of the drive which can be anything from 30 per cent
to 60 per cent. Suitable motor sizes for various lifts are given in Table 16.3. The
acceleration is settled by the torque-speed characteristic of the motor and the ratio of
motor speed to lift speed.
Design of electrical services for buildings 244


Types of motor
In most cases, in the UK at least, a three phase a.c. supply is available in a building which
is to have a lift installation. For lift speeds up to about 0.5ms−1 a single-speed squirrel
cage motor is suitable, although it has a high starting
Table 16.3 Approximate lift motor ratings (motor
ratings in kW)
Low efficiency geared lifts
Contract load (kg)
Car speed (ms−1) 250 500 750 1000 1500 2000
0.25 2 3.5 5 5 8 10
0.50 3 5.0 8 10 15 20
0.75 4 8.0 10 15 20 25
1.00 5 10 12 18 25 35
1.25 6 12 15 20 30 45
1.50 7 15 20 25 40 50
High efficiency geared goods lifts
Contract load (kg)
Car speed (ms–1) 250 500 750 1000 1500 2000 3000
0.25 1 2.0 3 4.0 6.0 8 10
0.50 3 4.0 6 7.5 10.0 14 20
0.75 5.0 8 10.0 12.5 20 30
1.00 7.5 10 14.0 20.0 25
High efficiency geared passenger lifts
No. of passengers
Car speed (ms−1) 4 6 8 10 15 20
0.50 3 3 4.0 5.0
0.75 5 7.5 7.5 12 15
1.0 10 10 15 18
1.5 12 17.5 22
current and tends to overheat on duties requiring more than 100 starts an hour.
Lifts, escalators and paternosters 245


For speeds between about 0.5ms−1 and 1.25ms−1, it becomes necessary to use a twospeed
motor in order to have a low landing speed. The squirrel cage motor can be wound
to give two combinations of poles, thus giving two speeds. A two-speed system is not a
variable-speed drive; it is variable in the sense that it has two speeds. These systems were
used widely in the 1970s and 1980s. They effectively employed direct on-line starting
and thus needed flywheels attached to the motor to smooth the movement and reduce
jerk. The use of a flywheel makes it inefficient
DC silicon-controlled rectifier (SCR)
Using power electronics, a DC motor can be used to vary the speed of the lift by varying
the armature voltage. This method is the most widely used in DC drives, in the form of a
controlled three-phase rectifier. This can be implemented in two forms. One form is a
fully controlled bridge rectifier, which allows two-quadrant operation.
The other form uses two bridges in parallel, each connected to drive the motor in the
opposite direction of the other. By using both bridges, the motor can be operated in both
driving and braking modes, in forward and reverse directions (i.e. four-quadrant
operation).
AC variable voltage (ACVV)
These systems were widely used in the mid-1980s and early 1990s. They are very simple
in the method of operation. They rely on three pairs of back-to-back thyristors for varying
the stator AC voltage on a double-cage squirrel cage motor. By varying the firing angle,
the stator voltage is varied and a new speed torque curve results.
Variable voltage variable frequency (VVVF)
The most widely used system today is the VVVF system, usually referred to as an
inverter drive. The principle of operation relies on a rectifier to produce DC into the socalled
DC link and an inverter, which produces sinusoidal current into the windings. By
changing the frequency of the inverted signal, the synchronous frequency and hence the
speed torque curve is moved to the desired profile. The supply is fed to a servo motor.
Servo motors can be programmed to give the required S curve profile for travel
between floors.
For speeds above 1.5ms–1 the same types of motor could be used as for the lower
speeds, but in fact they very rarely are. At the higher speeds, it is possible to design a d.c.
motor to run at a speed which makes reduction gearing to the drive unnecessary. The a.c.
supply is therefore used to drive a variable voltage motor generator set which supplies the
d.c. to the lift driving motor.
If only a single phase supply is available, a repulsion-induction motor can be used for
speeds up to 0.5ms−1. For higher speeds it is better to use the a.c. to drive a motor
generator and have a d.c. lift machine.
When d.c. is used for speeds less than 0.5ms−1, a single speed shunt or compound
wound motor is employed. When the lift is decelerating the machine runs as a generator,
and in the case of a compound wound motor this makes special arrangements necessary.
Design of electrical services for buildings 246


For speeds between 0.5ms−1 and 1.25ms−1 two-speed shunt motors are used, so that a
lower speed is available
Figure 16.5 Gearless drive unit
(Courtesy of Schindler Lifts Ltd)
for good levelling at the landings. The increase from low to high speed is obtained by the
insertion of resistance in the shunt field. For speeds of 1.5ms–1 and above, a d.c. shunt
wound motor running at between 50 and 120 rpm is employed. At these speeds the motor
can be coupled directly to the driving sheave without any gearing. Because of the size of
the motor it is not possible to vary the speed by more than 1.5 to 1 by field control, and
speed was usually controlled by the Ward Leonard method. The absence of gearing
increases the overall efficiency, improves acceleration and results in smoother travelling.
A picture of a gearless machine is shown in Figure 16.5.
External set value systems
The more modern approach has been to integrate a logic controller and the drive system.
This has the advantage that the precise speed can be selected at any point in the journey,
levelling speed can be eliminated, and different speeds can be selected depending on the
expected length of journey.
This integration has taken the form of feeding the required speed value directly into
the variable speed drive from the logic controller. Thus, the logic controller, which is
monitoring the position of the lift car and its destination and its exact position with
respect to the floor and landings, generates the required speed profile and sends it either
as an analogue signal or a digital signal to the drive. The analogue signal is usually sent
to the drive via an isolation amplifier, in order to eliminate any interference or faults in
Lifts, escalators and paternosters 247


one system affecting the other. If sent in a digital format, an opto-coupler is usually used
to isolate the two systems.
The set value generator in this case will reside inside the lift logic controller, which is
invariably implemented using a microprocessor-based system. The logic controller would
receive pulses from the shaft encoder or the motor indicating the exact position of the lift
car in the shaft. Based on this information, a value in a table is provided, giving the
required value of speed at that point. This value is sent to the variable-speed drive, which
then controls the speed accordingly. These systems are always position-dependent
systems, as the speed always follows the position, regardless of time
Brakes
Lift brakes are usually electromagnetic. In the majority of cases, they are placed between
the motor and the gearbox; in a gearless machine the brake is keyed to the sheave. The
shoes are cperated by springs and released by an electromagnet the armature of which
acts either directly or through a system of links. A typical brake is shown in Figure 16.6.
The brake is used only when the car is parked. To slow the car down, several methods are
employed. Plugging is reversing the phase sequence as the motor is running; the
synchronous magnetic field reverses direction, causing the motor to slow
Figure 16.6 Lift brake
Design of electrical services for buildings 248


down rapidly. Eddy current braking uses an aluminium disc on the end of the drive motor
shaft. A magnetic field is applied to the disc and currents are induced in the disc which
act to slow the shaft down. The braking effect is proportional to the speed of rotation of
the shaft. Another method is to inject a DC current into the motor winding. Injecting DC
in a motor winding will tend to try to stall the rotor. This is because the magnetic field set
up inside the motor is a stationary constant field always pointing in one direction.
Lift cars
Passenger cars should be at least 2.00m high, and preferably 2.15m or more. They can be
made to almost any specification, but most manufacturers have certain standard finishes
from which the client should choose.
Lift cars consist of two separate units, namely the sling and the car proper. The sling is
constructed of steel angles or channels and the car is held within the sling. The sling also
carries the guide shoes and the safety gear. The car is sometimes insulated from the sling
frame by anti-vibration mountings. Goods cars are of rougher construction than passenger
cars but otherwise follow the same principles.
Except for very small installations it is now almost universal practice to have an
emergency telephone in the car. It can be connected either as a direct line to the public
telephone network or as an extension of the private branch exchange in the building. It is
generally fitted in a recess in the wall of the car with a hinged door over it.
All electrical connections to a car are made through a multi-core hanging flexible
cable. One end of this is connected to a terminal box under the car, and the other end to a
terminal box on the wall of the well approximately half-way down. A separate hanging
cable may be needed for the telephone.
Counterweights
A counterweight is provided to balance the load being carried. As the load carried varies,
the counterweight cannot always balance it exactly; it is usual for the counterweight to
balance the weight of the car plus 50 per cent of the maximum load to be taken in the car.
A typical counterweight frame is shown in Figure 16.7. It contains cast-iron sections held
in the steel framework and rigidly bolted together by tie rods. The lifting ropes are
attached to eye bolts which pass through the top piece of the frame.
Guides
Both the car and the counterweight must be guided in the well so that they do not swing
about as they travel up and down. Continuous vertical guides are provided for this
purpose. They are most commonly made of steel tees,
Lifts, escalators and paternosters 249


Figure 16.7 Counterweight frame
and there are standard tees made especially for use as lift guides. The guides are fastened
to steel plates by iron clamps at intervals of about 2m and these plates are secured to the
sides of the well. They may be secured by bolts passing through the wall of the well and
held by back plates on the other side or by being attached to angle irons or channels
which are in turn built into the wall. The latter is the usual practice with concrete building
construction.
Guide shoes are fitted on the car and on the counterweight and run smoothly on the
guides. Figure 16.8 shows a shoe on a guide. For smooth
Design of electrical services for buildings 250

Figure 16.8 Shoe of lift guide
running, the guides must be lubricated and various types of automatic lubricators have
been designed for lift guides. The commonest kind makes use of travelling lubricators
mounted on the car and counterweight. More recently unlubricated guides have been used
with shoes lined with carbon or PTFE.
Doors
Solid doors have now entirely superseded collapsible mesh gates. They are quieter,
stronger and safer. It is usual now for the car and landing doors to be operated together. If
the entrance to the car is not to be much narrower than the car itself then in the open
position the door must overlap the car. To accommodate this, the well must be wider than
the car. This will be clear from the plan of a typical lift installation shown in Figure 16.9.
Doors can be opened and closed manually, but it is more usual to have them power
operated. In order not to injure passengers caught by closing doors, the drive has to be
arranged to slip or reverse if the doors meet an obstruction. Every lift car door must have
an interlock which cuts off the supply to the lift controller when the door is open. This
can be a contactor which is pushed closed by the door and falls open by gravity or spring
action when the door opens.
The landing door must be locked so that it cannot be opened unless the car is in line
with the landing. The most usual way of doing this is by means of a lock which combines
a mechanical lock and an electrical interlock. The electrical interlock ensures that there is
no supply to the controller unless the gate is locked. The mechanical part can be unlocked
only when a cam on the car presses a roller arm on the lock; thus the landing door can
only be opened when the car is at the landing. The controls withdraw the cam when the
car is in motion and return it only as the car approaches a floor at which it is to stop. This
makes it impossible for anyone to open a landing door as the car passes the landing if the
car is not stopping there.
Lifts, escalators and paternosters 251

 Figure 16.9 Plan of typical lift
arrangement
Indicators
Indicators are available for showing when the car is in motion, the direction of travel and
the position of the car in the well. A position indicator may be installed in the car, and in
many cases also at each landing. Direction indicators are provided at the landings, and a
common arrangement is to have a position an direction indicator in the car and at the
ground floor with direction indicators at the other landings.
The car positional controllers previously described may be interfaced with an
electronic system which then sends the car positional information to LCD displays at
each floor level and in the car itself. It would be beyond the scope of this book to
describe the circuitry required to do this.
Safety devices
Every lift car must have a safety gear which will stop it if its speed increases above a safe
level. The motor and brake circuits should be opened at the same time as the safety gear
operates. If the lift travel is more than about 9m the safety gear should be operated by an
overspeed governor in the machine room.
Design of electrical services for buildings 252

For speeds up to 0.8ms−1 instantaneous gear is generally used. This consists of a pair
of cams just clear of each guide, one on each side of the guide. The cams have serrated
edges and are held away from the guide by springs. A safety flyrope passes from the
safety gear over a top idler wheel to the counterweight. Tension on the safety rope causes
the cams to come into instantaneous contact with the guides and they then clamp the car
to the guides. The safety rope comes into tension if the lifting ropes break. Alternatively,
it can be connected to an overspeed governor. One type of governor is shown in Figure
16.10. It has a pulley driven from the car by a steel rope. Flyweights are mounted on the
pulley and linked together to ensure that they move simultaneously and equally. The
flyweights move against a spring which can be adjusted to give the required tripping
speed. As the speed increases, the weights move out against the spring force and at the
tripping speed they cause a jaw to grip the rope, which produces the tension necessary to
operate the cams.
For lifts at higher speeds, gradual wedge clamp safety gear is used. This also works by
clamping the car to the guides, but the clamps are forced against the guides gradually and
so bring the car to rest more smoothly. The clamps can be brought into play by screw
motion or by a spring.
Figure 16.10 Lift governor
Lifts, escalators and paternosters 253



Figure 16.11 Flexible guide clamp
Another type of safety gear used on high speed lifts is the flexible guide clamp, an
example of which is illustrated in Figure 16.11. It consists of two jaw assemblies, one for
each guide, mounted on a common channel under the car. The jaw assembly has a pair of
jaws with a gib in each jaw. Tension on the governor rope resulting from operation of the
governor pulls the operating lever and causes the gibs to move up the jaws. The
consequent wedging action of the gibs between the jaws and the guides compresses the
jaw spring to produce a gripping force on the guides which gives a constant retardation.
A lift must also have upper and lower terminal switches to stop the car if it overruns
either the top or bottom floor. These can take the form of a switch on the car worked by a
ramp in the well, or they may consist of a switch in the well worked by a ramp on the car.
There should be a normal stopping switch and a fixed stopping switch at each end of the
travel.
Clearance must be allowed for the car at the top and bottom of the well to give it room
to stop if the normal terminal switch fails and is passed and the terminal switch operates.
The clearances can be as given in Table 16.5. The bottom clearance given in the table
includes the buffer compression.
The final safety device consists of buffers in the well under the car and under the
counterweight. For low speed lifts they can be made as volute or helical springs, but for
high speed lifts oil buffers are used.
Design of electrical services for buildings 254


Table 16.4 Lift clearances
Lift speed (ms−1) Bottom clearance (m) Top clearance (m)
0−0.5 0.330 0.455
0.5−1.0 0.410 0.610
1.0−1.5 0.510 0.760
Landing
As it stops, the car must be brought to the exact level of the landing. With an automatic
lift, this depends on the accuracy with which the slowing and stopping devices cut off the
motor current and apply the brake. Levelling is affected by the load being carried; a full
load travels further than a light load when coming to rest from a given speed on the
downward trip and less far on the upward trip. To overcome this, it is desirable that the
car should travel faster when carrying a full load up than when travelling up empty. A
motor with a rising characteristic would be unstable, but the desired effect can be easily
achieved with variable voltage control. The rising characteristic is needed only at the
levelling speed, which is from about 1/6 to 1/20 of the maximum speed.
Type of control
An automatic control system has a single call button at each landing and a button for each
floor in the car. A passenger presses the car button for the desired floor and the lift
automatically travels there. Calls made from landings while the car is in motion are
stored in the controller memory. With Automatic Collective Control, each landing has
both an UP and a DOWN button, and there is a set of floor buttons in the car. Every
button pressed registers a call, and up and down calls are answered during up and down
journeys respectively, in the order in which the floors are reached. The order in which the
buttons are pressed does not affect the sequence in which the car stops at the various
floors, and all calls made are stored in the system until they have been answered. Down
calls made while the lift is travelling up are kept until after the up journey is finished, and
up calls made while the lift is moving down are similarly kept until that trip is finished.
The system can be modified to work as a collective system in the down direction and
as a simple automatic system in the up direction. It is then known as Down Collective.
This version is sometimes used in blocks of flats and is based on the assumption that
occupants and their visitors travelling up like to go straight to their own floors, but that
everyone going down wants to get off at the ground floor. Thus upward travellers should
be able to go straight to their own floor without interference, while downward travellers
are less likely to be irritated by intermediate stops to pick up other passengers going to
the same destination. This reasoning ignores milkmen, postmen and other delivery
workers, and the author of this book finds it unconvincing. Nevertheless, it appears to be
popular with many authorities.
Lifts, escalators and paternosters 255

Duplex control is used when two lifts are installed in adjacent wells. The landing
buttons serve both lifts. Landing calls are stored and allotted to the cars one at a time as
the cars finish journeys already in progress. For three lifts working together Triplex
Control is used.
It is also possible to arrange lifts with dual control so that they can be used either with
or without an attendant.
Electronic systems have made more sophisticated forms of control possible and these
have proved particularly valuable in tall office buildings with a group of three or more
lifts in each service core. The term ‘home floor’ is used to designate the floor to which a
car returns when it is not in use. It is not necessary for all the cars in a group to have the
same home floor. Programmed control of groups of high speed lifts is a system in which
cars are despatched from their home floor in a predetermined sequence so that even if
they are not in use they are already moving through the building when the first call is
made. Providing the programme is suitable for the pattern of traffic, this will ensure that
when a call is made there is a car nearer than the home floor to the floor at which the call
was made.
In a ‘Balanced’ programme cars are dispatched at set intervals from both terminal
floors and each car makes a complete round trip. An ‘Up Peak’ programme dispatches
cars from the lower terminal only and reverses each car when it is empty. A ‘Down Peak’
programme despatches them from the upper terminal at set intervals and returns each car
from the lower terminal as soon as it is empty. In an ‘Intermittent’ programme cars can
be taken out of service in a pre-arranged sequence.
When there is traffic in both directions but more going up than down a ‘Heavier Up’
programme will dispatch cars from both terminals at intervals which are automatically
adjusted so that the cars are equally spaced. In the reverse situation cars stop more
frequently when going down and a ‘Heavier Down’ programme adjusts the dispatch
times accordingly.
A traffic analyser measures the rate at which calls are made and answered and in doing
so distinguishes between up and down calls. It can be used to change operation from one
control scheme to another. This is valuable in office blocks where the pattern of traffic
changes throughout the day. ‘Up Peak’ and ‘Down Peak’ programmes are useful at the
beginning and end of the working day with the ‘Heavier Up’ and ‘Heavier Down’
programmes making a contribution during the lunch period.
Weighing contacts under the car floor can detect the load and this can be used to make
a car which is already fully loaded pass a landing without stopping even though there is a
call waiting at that landing in the direction in which the car is travelling.
Every lift manufacturing company has its own particular system. This means that the
specifier must discuss the scheme with manufacturers and if it is desired to put the lift
installation out to competitive tender the description of the controls must be loose enough
to allow some variation.
Controllers
The conventional lift motors are started, stopped and reversed by contactors. The
operating coils of the contactors are energized at the appropriate times by relays which
Design of electrical services for buildings 256


are connected in a circuit to give the required scheme of operation. The assembly of
contactors, relays and associated wiring forms the controller, which is usually placed in
the motor room, which may be the physical size of the panel shown in Figure 16.5.
Sufficient space must be left round the controller for maintenance, and it must also be
placed so that a maintenance technician working at it cannot accidentally touch a moving
part of the lift machine.
A controller circuit is necessarily complex because it contains many interconnected
relays; but each relay performs only one function and they are arranged to energize each
other in a logical sequence to achieve the operation required, an also interlocked to
prevent up and down functions to be selected at the same time. The resulting wiring
diagram may perhaps be described as involved but not complicated. In a book which
deals with electrical services as a whole and devotes only one chapter to lifts, it is not
practicable to give a full description of circuits for all the possible modes of control, and
we shall do no more than explain the working of a simple automatic system. It will have
to serve as a model illustrating principles which can be extended and adapted for other
systems.
Servo motors are controlled by electronic servo drives, and do not require relay. The
motor commands are supplied to the motor from the servo drive. The motor position thus
the car position can be controlled to a fraction of a degree of rotation, by utilising the
digital encoder fitted to the motor shaft.
Escalators
An electrical services engineer should also know something about escalators. These are
moving staircases. They consist essentially of pivoted steps linked to each other and
pulled along by an endless chain. The steps have guide pins which move in tracks on
either side of the tread arranged so that the steps come out of the concealed section
horizontally, are then pulled or pivoted into the shape of a staircase and finally return to
the horizontal before returning back into the concealed section. The steps complete their
circuit within the concealed section, and on this part of their travel they are flat. This is
illustrated in Figure 16.12.
The surface of each tread has grooves parallel to the direction of motion. The stationary
platforms at the top and bottom of the escalator have fixed combs which mesh with the
grooves in the treads in order to ensure a smooth run in and out of the treads under the
fixed floor. The stationary platform is actually a floor plate over the recess under the
moving staircase and covers the working mechanism at the top and bottom landings. It
has an extension known as a combplate and the combplate carries the projecting comb
teeth.
An escalator also has a balustrade with a handrail, and the handrail moves on an
endless chain in step with the stairs. There are separate chains for the handrail and the
steps but they are both driven through a gearbox from the same motor. The usual speed of
an escalator is 0.5ms−1 and it ought never to exceed 0.75ms−1. The inclination varies
between 27° and 35° to the horizontal.
Lifts, escalators and paternosters 257

Figure 16.12 Escalator
The drive and transmission have to carry the total load on the escalator. Since people
do not stand at even and regular intervals on the whole staircase the load averaged over
the whole length of the escalator is less than the maximum load on individual treads. The
peak load on each tread is of concern to the structural designer but the electrical engineer
concerned with the power requirements can use the average passenger load taken over the
total area of exposed treads. This average can be taken as 290kgm−2.
An escalator must have a brake which has to fail safe if there is an interruption to the
electrical supply. The brake is therefore applied by a spring or a hydraulic force and is
held off against the mechanical force by an electrically energized solenoid. As is the case
with lifts, there is also provision for releasing the brake manually and handwinding the
escalator.
Since an escalator is in continuous operation there are no passenger controls, but there
must be an on/off switch which can be worked by a responsible person in charge of the
premises. It would be dangerous for the escalator to be started or stopped by someone
who could not see the people on it and the switch must be in a place from which the
escalator can be seen. Such a place is almost inevitably in reach of the public using the
escalator, who ought not to be able to work the switch, and so the switch must be a key
operated one. The British Standard on Escalators (BS 2655) requires a key operated
starting switch to be provided at both ends of the escalator.
Emergency stop switches are provided in the machinery spaces under the escalator and
also in positions accessible to the public at the top and bottom of the escalator. The
operation of any of these switches disconnects the electrical supply from both the driving
machine and the brake. The removal of the supply from the brake allows the mechanical
force to apply the brake and bring the escalator to a halt. Some escalators are fitted with a
speed governor which similarly disconnects the electrical supply from the drive and
Design of electrical services for buildings 258


brake. There are further safety devices to disconnect the supply if one of the driving
chains breaks.
Most escalators are reversible. The driving motor is a squirrel cage induction motor
and the drive is reversed by contactors which change the phase sequence of the supply to
the motor.
A travellator differs from an escalator in being either horizontal or having a very small
slope not exceeding 12°. This makes it unnecessary for it to form steps and passengers
are conveyed on a continuous platform. The upper surface of the platform must have
grooves parallel to the direction of motion which mesh with the combplates. In other
respects a travellator or moving walkway is designed in exactly the same way as an
escalator.
Paternosters
Whilst paternosters are a type of lift they also have similarities with escalators and it is
more convenient to discuss them after the latter. A paternoster is a lift which has a series
of. small cars running continuously in a closed loop. It is difficult to explain this clearly
in words but it should be clear from Figure 16.13. The cars are open at the front and
move slowly enough for people to step in and out of them whilst they are in motion, just
as they step on and off an escalator. In fact a paternoster can perhaps be thought of as a
vertical escalator. To make it safe for people to get on and off whilst the cars are in
motion the speed must be less than 0.4ms−1.
The cars are constructed in the same way as ordinary lift cars but do not have doors and
are not large enough to take more than one person each. In practice this means that the
cars are less than 1.0m×1.0m in plan. They must of course be of normal height. The front
of the floor of each car is made as a hinged flap. This ensures that if a person has one foot
in the car and one on the landing he will not be thrown off balance as the car moves up.
Since the cars move in a continuous loop they provide their own counterweight and no
additional counterweight is needed. Rigid guides are provided for the cars which have
shoes similar to those of ordinary lift cars.
In the space between cars there is a protective screen level with the front of the cars.
This prevents people stepping into the shaft in between cars. It is still necessary to make
sure that the landings and entrances are well illuminated. The cars are carried on a
continuous steel link chain. The driving machinery is similar to that of an escalator and is
always placed above the well. It includes a brake which is applied mechanically and held
off electrically, so that the paternoster is braked if the electrical supply fails. As in the
case of both lifts and escalators there is provision for handwinding.
A paternoster is started by a key-operated switch, either at the ground floor or at the
main floor if this is other than the ground floor. There are emergency stop buttons at each
floor, in the pit and in the machinery space.
Lifts, escalators and paternosters 259


Figure 16.13 Paternoster
Although they have advantages, paternosters are not used very often. They take up
rather less space than escalators but have a lower carrying capacity. We can show this by
considering an escalator with a slope of 350 and a vertical rise between treads of 230mm.
The distance along the slope between succeeding steps is 230/sin 35°=400mm or 0.4m. If
the speed of the escalator is 0.5ms−1, the steps follow each other at intervals of
0.4/0.5=0.8s. As each step can take one passenger, the carrying capacity is one person in
every 0.8s, or 75 persons per minute.
The vertical speed of a paternoster is at most 0.4s−1’ and the car height cannot be less
than 2.2m. Neglecting any gap between the cars we see that the cars follow each other at
intervals of 2.2/0.4=5.5s, and each car can take only one passenger. The carrying capacity
is thus one person every 5.5s, or 11 persons per minute.
However well constructed and carefully operated it is, a paternoster cannot help being
more of a hazard than an escalator to the elderly, the infirm and above all to small
children. This practically restricts its applications to industrial premises not needing a
high carrying capacity.
Standards relevant to this chapter are:
Design of electrical services for buildings 260