Distribution Electricity

Chapter 6
Distribution
Electricity is supplied to a building by a supply authority; in the UK this is an area
electricity company, while in other countries it may be an electricity supply company or
public body. The supply is provided by a cable brought from outside into a suitable point
in the building which is referred to as the main intake, and from this the electricity has to
be distributed to all outlets which use it. The incoming cable may be a 120 or 150mm2
PVC insulated cable and the current flowing along it must be divided between a number
of smaller cables to be taken to the various final destinations throughout the building.
This division is the function of the distributing system.
In Chapter 5, we described the final circuits which serve the final outlets. Each final
outlet takes a comparatively small current, and it would be impracticable to serve it with
a large cable. As we have seen, the final circuits are most commonly cabled in 1.5mm2,
2.5mm2, 4mm2 and 6mm2 cables. The cable size in turn limits the number of outlets on
each circuit, and in a building of any size a large number of circuits is needed. It would
be very expensive to run all the final circuits from the main intake point. Also, voltage
drop in cables of this size over a long distance would be excessive. It is more economic
and more practical to divide the supply first over a few large cables and then into the final
small cables in a second step. The normal method is to distribute current from the main
intake to a number of distribution or fuseboards, each of which splits it further among a
number of final circuits. A typical scheme is shown diagrammatically in Figure 6.1.
The cable from the main intake to a distribution board is known as a sub-main, and it
must be rated to carry the maximum simultaneous current (after diversity has been taken
into account) taken by all the final circuits on that board. Once this current is known, the
size of the cable can be determined by current-carrying capacity and voltage drop, as
explained in Chapter 4.
The protection devices in the distribution board protect the final circuits, but the submain
cable also needs protection against short circuits and overloads, and there must be a
fuse or other protective device at the main intake. We can note the principle that every
cable must be protected for short

circuit at its feeding end, for overload protection it is permissible to protect the cable
along its run. A convenient device for protecting a sub-main cable is a switch fuse. A
switch fuse is illustrated in Figure 6.2, and it can be seen that it consists simply of an
isolating switch and a fuse carrier housed together in a substantial casing. The one
illustrated is a three-phase switch fuse and, therefore, has three poles on the switch and
three fuses. The fuse protects the cable leaving the switch fuse and the switch is useful
for isolating the sub-main from the rest of the electrical system when this is required for
maintenance or alteration work. The switch is designed to make and break the rated


current of the switch fuse, and units are made in a series of standard ratings. A typical
model, for example, is manufactured in increasing sizes rated at 30, 60, 100, 160, 200,
400, 600 and 800A.
A switch fuse also includes terminals which enable the earth cables on the incoming
and outgoing sides to be connected together. Under no circumstances must there be a
break in this circuit as it would destroy the safety of the system. The neutral cable on the
other hand can be taken through the switch fuse in one of two ways.
The more usual way is for the switch to include terminals for connecting the incoming
and outgoing neutrals in the same way as the earth cables. The alternative is for it to have
a switch blade in the neutral line as well as in the phase lines, thus making it a 4-pole
device. In this case there is a solid link instead of a fuse in the neutral line.
A fuse switch, illustrated in Figure 6.3, is similar to a switch fuse, but in this case the
fuse carriers are mounted on the moving blades of the switch.
The whole of the current going into the sub-main passes through the switch fuse which
carries no current for any other part of the system.
The total incoming current must be divided to go to several switch fuses, and the
simplest device for distributing current from one incoming cable to a number of outgoing
ones is a busbar chamber. This consists of a number of copper bars held on insulating
spacers inside a steel case. It is shown in Figure 6.4. Cables can be connected to the bars
anywhere by means of cable clamps which are usually bolted to the bars. The incoming
cable can be connected to the bars at one end or at some convenient point along them.
Connecting the incoming cable to the centre of the busbar enables 300A
Figure 6.3 Fuse switch (Courtesy of
Eaton Electric Ltd)
Design of electrical services for buildings 88


Figure 6.4 Busbars
busbars to supply a total load of 600A with 300A flowing each way from the connection.
Care must be taken that the loads are distributed properly. Therefore outgoing cables are
connected at suitable intervals along them. The switch fuses are usually mounted
immediately above and below the busbar chamber so that the connections, or tails, from
the busbars to the fuses are kept as short as is reasonably possible. These tails are of the
same size as the cables leaving the switch fuse but are protected only by the fuse on the
main intake. It is not normally permissible to protect a cable with a fuse rated at more
than the maximum current carrying capacity of the cable. It may, however, be done for
short tails between busbars and switch fuses. Provided that its length is no greater than
3m, the cable is installed such that short circuits are unlikely to occur, and installed to
reduce the risk of fire or danger to persons. A short circuit or overload on the load side of
the switch fuse will blow the switch fuse and thus stop current through the tail, and a
short circuit on the tail will produce such a heavy overcurrent that the main intake fuse
will blow. For the fuse protecting the mains cable to protect the tails, the formula as
explained in Chapter 4 is employed, t=K2 S2/I2 where t is the time in seconds for the cable
to reach its limit temperature under fault conditions, K is the constant for the cable, S is
the cross-sectional area of the cable, and I is the prospective short-circuit current.
Another matter which requires attention in the intake room is metering. The supply
authority will want to meter the supply afforded and will want to install a meter at the
intake position. If it is a small enough supply, the whole of it can be taken through the
meter. The arrangement then is that the incoming cable goes first to a fuse which is
supplied, fixed and sealed by the electricity company, and from there to the meter, which
is also supplied and fixed by the electricity company. The fuse is the electricity
company’s service cut out and it is sealed so that only they have access to it. From the
meter, the cable is connected to the busbar chamber. It is normal for the installer to
provide the last piece of cable from the meter to the busbars but to leave the meter end of
it loose for the electricity company to connect to the meter.
If the building takes a very large current, a different arrangement is used. It is not
practicable to take a large current directly through a meter and large supplies are metered
with the help of current transformers. The arrangement is similar to that described in the
last paragraph, but the place previously occupied by a meter is taken by the primary coil
of a current transformer. The output from the secondary coil of the transformer is taken to
Distribution 89


the meter, which can be made to give a direct reading of the current in the primary coil by
suitable calibration. Some current transformers are made to slip over the busbars, or the
mains cable cores if the busbar is not supplied from one end, and use the bars, or cable
conductors as a primary coil. The method to be used should be agreed with the electricity
company before installation commences even if this is one or two years before the
building is to be finished and a supply of electricity will be needed. It is very
embarrassing if, when the electricity company come to connect the supply, they find that
there is not enough space for their equipment and it is much better to agree everything
well in advance.
The size of busbars is determined by the current they are to carry which is normally
the whole current of the building if the busbar is supplied from one end. The current
carrying capacity is governed by temperature rise and has been tabulated in published
data in the same way as the current carrying capacity of cables. The spacing between the
bars is determined by the voltage at which they are to operate, since the air gap between
adjacent bars and between bars and case is the only insulation provided. Busbars must
also be capable of taking short-circuit currents for the time it takes for fuses or circuit
breakers to operate. In a short time, the bars will not overheat and the short-circuit
capacity is a measure of the mechanical forces which the bars will withstand. A heavy
current gives rise to large electro-mechanical forces and the bar supports have to be
capable of withstanding these. Busbars are obtained from specialist manufacturers and
the electrical services engineer does not usually design his own. Further details on the
methods of calculation would, therefore, be beyond the scope of this book.
A busbar chamber with a large number of switch fuses takes up a lot of wall space. It
can also look untidy. Both these disadvantages can be overcome by the use of a cubicle
switchboard. Such a board contains the busbars and the switch fuses all housed together
in one large panel, a typical example being shown in Figure 6.5. It works in exactly the
same way as the busbar chamber with separate switch fuses, but is made in the
manufacturer’s factory instead of being assembled on site, and all the interconnecting
wiring is inside the casing of the switchboard. This makes it possible to arrange the
switch fuses in a more compact way and to fit all the equipment into a much smaller
space.
The meters can also be included within the composite switchboard. It is usual to have an
incoming isolator or switch fuse which both protects the board against short circuits and
makes it possible to isolate the board for maintenance.
Such a switchboard does, however, have disadvantages. First, although it requires less
space than a site assembly of individual pieces of equipment, it is likely to cost more.
Second, it is not easy to add further switch fuses to it once it has been made. A main
intake consisting of separate pieces of equipment can very easily be extended; it is a
simple matter to make an extra connection to the busbars and to take another pair of
cables out through a short length of conduit to a new switch fuse fixed to any free wall
space in the intake room. This is often necessary during the life of a building, and
sometimes even before a new building is completed, since building owners are apt to
change their minds and want equipment installed which they had not thought of when
building operations started. Unless blank spaces have been left in a cubicle switchboard it
Design of electrical services for buildings 90


cannot be extended to take more switch fuses, and even if spaces have been left they
often turn out to be not quite large enough for an unforeseen and unexpected extension.
Figure 6.5 Cubicle switchboard
(Courtesy of Eaton Electric Ltd)
In general each sub-main goes from the main intake to a distribution board. This
consists of a case inside which is a frame holding a number of fuse carriers. Behind the
frame, or sometimes alongside it or above it, is a busbar to which the incoming sub-main
is connected. From the bar, there is a connection to one side of each fuseway or circuit
breaker provided. Each final circuit is then connected by the installer to the outgoing
terminal of one of the fuses/CBs. The circuit is completed when a fuse carrier with a fuse
is pushed into the fuseholder, or the CB is closed. A second busbar is provided to which
the incoming neutral and the neutrals of all outgoing circuits are connected. A typical
distribution board is shown in Figure 6.6. The only difference between a distribution
board and a fuseboard is the name.
Standard distribution boards usually have either 4, 6, 8, 12, 16, 18 or 24
fuseways/CBs. Both single and three-phase boards are available, the latter having three
fuseways/CBs for each outgoing circuit. It is not necessary to utilize all the available
fuseways/CBs on a board, and in fact it is very desirable to leave several spare ways on
each board for future extensions, although the sizing of the cable supplying the board
must be capable of supplying the additional load. Trunkings and conduits will also need
to be sized with future extensions in mind. These are often required before a building is
even finished, and are almost certain to be wanted during the life of an installation which
may last 40–60 years. A label must be provided inside the cover of every distribution
board stating which fuse serves which outlets.
The position of distribution boards within a building must obviously depend on the
plan of the building. Apart from architectural considerations, it is a matter of balancing
Distribution 91


the lengths of sub-mains and the lengths of final circuits to find the most economic way
of keeping the total voltage drop between intake and final outlet to a minimum. It is
possibly better to keep sub-mains long and final circuits short, but it is also desirable to
keep the number of distribution boards down by having a reasonably high number of final
circuits on each board. To achieve this without excessively long final circuits, one must
have the board fairly central for all the circuits it is serving.
In some cases, it is convenient to have a subsidiary control centre between the main
intake and the distribution boards. When this is done a main cable runs from the main
intake to the subsidiary control centre, sometimes referred to as a load centre, which is
itself similar in construction to the main intake. The main cable is supplied through a
switch fuse, fuse switch, or circuit
Figure 6.6 Distribution board
(Courtesy of Eaton Electric Ltd)
breaker at the main intake end and leads to busbars at the subsidiary centre. The
arrangement is shown schematically in Figure 6.7. Such a scheme would be adopted only
in a large building, but is very useful when several distribution boards have to be placed a
long way from the main intake. It is more economical to keep the voltage drop down with
one large main cable for the greater part of the distance to be covered than with several
sub-mains running next to each other along the same route. It also requires less space for
the cables. It is particularly useful when the premises being served consist of several
different buildings. Normally, the electricity company will provide only one incoming
service to one set of premises and if there are several buildings, the distribution to them
must take place on the consumer’s side of the meters. It is very convenient to run one
main from the intake to a centre in each building and then distribute in each building
from its own centre. Colleges and hostels are examples of consumers who may have
several buildings on one site all forming part of the same premises.
The subsidiary control centre is also a good solution to the problem of extending
existing buildings. A great deal of new building in fact consists of extensions to existing
premises, sometimes by actual enlargement of an existing building or erection of a new
Design of electrical services for buildings 92


building on spare ground within the site of the existing one. It is comparatively easy and
does not need much room to add one switch fuse at the main intake. From this, it is again
a comparatively simple matter to run one new cable to the new building. Here it is
possible to plan a subsidiary distribution centre with as many switch fuses and sub-mains
to as many distribution boards as are necessary. In this way, the amount of alterations in
the existing building is kept to a minimum and the new building is treated as an entity in
itself.
When an extension is made it may be necessary for the electricity company to increase
their service cable. This will depend on the existing load, the new load and the margin by
which the capacity of the existing service cable exceeds the existing load. Whenever
extensions are planned the capacity of the service cable must be checked with the supply
authority, but replacing an incoming service with a larger one is not a difficult operation
and does not add greatly to the work which has to be done in the existing part of the
building.
In the UK, the electricity company’s final distribution network to their customers is at
400V three-phase four-wire, but all domestic and nearly all commercial equipment
requires a single-phase 230V supply. Motors above 2.5kW are now very often three
phase and they can be found in boiler-rooms, kitchen ventilation plants, air conditioning
plant rooms, lift motor rooms and similar places in buildings such as office blocks,
schools and colleges, hospitals and blocks of flats. Any equipment of this nature will
usually need a three-phase supply, but distribution to lights and to ordinary socket outlets
for power purposes must be at 230V single phase. Whenever there is three-phase
equipment in a building the supply authority must obviously be asked
Figure 6.7 Distribution through
subsidiary centres
to bring in a three-phase supply. Where there is no three-phase equipment in a building
single-phase supply would be enough, but there is another consideration which has to be
taken into account. The supply authority wants its load to be evenly spread over the three
Distribution 93


phases of its network and this may need the cooperation of the consumers. For individual
dwellings, whether flats, maisonettes or houses, the supply authority will bring in only a
single-phase cable, but it will arrange the supplies to adjacent or nearby dwellings so that
the total supply is balanced over the three phases. Thus in a new development either each
dwelling will be on a different phase from its immediate neighbour or small groups of
dwellings will be on different phases from adjacent groups. For larger buildings, the
electricity company will demand the cooperation of the consumer, or more properly of
the engineer who plans the installation. The company will insist that the consumer
accepts a three-phase supply even if there is no three-phase equipment to be connected,
and it will further insist that the demand is as nearly as possible balanced over the three
phases. The designer must take some care about how he achieves this balance and he has
to fulfil other requirements at the same time.
Most people are used to an electric supply at 230V in their homes and expect the same
in offices and public buildings. This is quite high enough to be dangerous, and it is
largely for reasons of safety that the USA and some other countries have standardized
domestic supplies at 110V. It would be most undesirable to expose people to even
accidental contact with 400V and it is preferable to keep the three phases away from each
other. In other words, the three-phase supply comes into the main intake and if necessary
to subsidiary control centres but sub-mains and distribution boards are all single phase.
An exception must, of course, be made for any sub-mains and distribution boards serving
three-phase machinery. Single-phase outlets near each other should be on the same phase,
and distribution boards close to each other should also be on the same phase.
The IEE Regulations, which are almost invariably made to apply to installations in this
country, stipulate that where fixed live parts between which there is more than 250V are
inside enclosures which although separate from each other are within reach of each other,
a notice must be displayed giving warning of the maximum voltage between the live
parts. Within reach is usually taken to mean 6ft or 2m. It is clearly not pleasant to have a
lot of notices in a building saying ‘Danger—400 volts’ and one therefore plans the
distribution within a building so that circuits on different phases are kept more than 2m
away from each other.
A very convenient way of doing this is to divide the building into three zones each of
which is served by one phase. These zones must be of such sizes that each takes
approximately the same load, in order that the total load is spread as nearly as possible
equally over all three phases. The zones are not necessarily of the same area; for example
a three-storey school may have classrooms on two floors and laboratories on the third and
the laboratories may account for more than a third of the total load. In such a case, a
convenient division could be half the laboratories plus a third of the first floor on the red
(brown) phase, the other half of the laboratories plus a further third of the first floor on
the yellow (black) phase, and the remaining third of the first floor plus the whole of the
ground floor on the blue (grey) phase, but obviously the division must depend on the
building and no general rules can be laid down. The designer must have enough personal
judgement to settle each case on its merits. An advantage of having mixed phases is that
if one phase goes down, there is lighting supplied on other phases.
It is helpful to label the three zones on a plan of the building. This will avoid
confusion and draw attention to anomalies. It has the added and very important advantage
Design of electrical services for buildings 94


of making the designer’s intentions clear to the workmen on site and so reducing the risk
of mistakes.
When a building is zoned it is worth checking all two-way light switches. Suppose, for
example, that ground and first floors are on different phases, and that the light on the
stairs is cabled on one of the first-floor circuits. If it has two-way switching, the
downstairs switch may be next to a corridor light switch which is on a ground-floor
circuit and, therefore, on a different phase. This should be avoided and is best prevented
by careful labelling of circuits and phases on the drawings during design.
Even where three-phase equipment has to be connected, the rules for separation of
phases should still be applied to all single-phase outlets near the three-phase equipment.
Operators and maintenance workers will be expecting a high voltage on the three-phase
equipment, but will not be expecting a high voltage to appear between a pair of singlephase
outlets near it.
Another point which has to be considered when the distribution through a building is
being planned is the position of other services. If a fault develops on an electric cable its
sheath or protective metal casing, an exposed conductive part, could become live. The
various methods of giving protection against the consequences of such faults are
described in Chapter 9, but the dangers arising from cable faults should also be guarded
against when the cable routes are selected. If a live conductor breaks and touches the
outer sheath or conduit the exposed parts become live. It must automatically disconnect
quickly.
For this reason good practice requires electrical wiring to be separated from other
services. There should be at least 150mm between them. The earth metalwork of the
other services should be bonded to the main earthing terminal of the electrical supply.
This ensures that whatever happens the other service is at the same potential as the main
earthing terminal.
The tariff which the electricity company will apply should be discussed with them
when the installation is being planned. There are many tariffs available, and they differ
from company to company. The consumer may find that electricity can be obtained more
cheaply on a tariff which employs different rates for current used for power and for light.
If this is so, the power and the light must be metered separately, and the designer must
keep power and light circuits separate. Clearly a decision on this must be made before the
design is drawn up, and this can only be achieved after negotiations with the electricity
company; these should, therefore, be the first part of a designer’s job.
If separate metering is decided on, it will be necessary to use split busbars at the
intake. These are simply two lengths of busbars separated by a short distance from each
other, but contained within the same casing. Externally, the appearance is the same as
that of a single metered system, but internally the connections to the light and power
systems are kept separate. The electricity company, of course, supplies two meters.
Cubicle-type main panels can be similarly arranged to keep the two services separate.
Each distribution board must be either for power only or for light only and must be fed
by a sub-main from the appropriate set of busbars. In a large building with many outlets
and, therefore, a considerable number of final circuits, it is normally quite easy to have
separate boards for light and power. For example, a technical college may need perhaps
two lighting circuits and two power circuits in each laboratory, and a group of four
laboratories may be so arranged that they can be conveniently served from one place. A


pair of twelve-way distribution boards in this place will provide the additional circuits
needed for corridors and leave reasonable spare capacity. As two boards would be needed
in any case and there are almost equal numbers of lighting and power circuits to be
accommodated, it is quite convenient to have one board for power only and one board for
lighting only. In small buildings, on the other hand, separation of light and power may
make it necessary to use more distribution boards than would otherwise be required. Thus
a sports hall may have four lighting circuits and only two socket outlets which can go on
one power circuit. A six or eight-way board would conveniently serve them all, leaving
some spare capacity for future extensions. If the power is metered separately, a separate
board will be needed for the single-power circuit and it will probably have to be a threeway
board because this is the smallest size commercially available. A second sub-main
will also be required and consequently an additional switch fuse at the intake. The result
is a substantial increase in the cost of the installation, and the design engineer should
keep this in mind when assessing the relative merits of different tariffs.
We have so far been talking in terms of distribution within a single large building. The
same principles apply to distribution to dwellings but are modified by the fact that each
occupier is a separate customer of the supply authority and must have a separately
metered supply. In a suburban development of detached and semi-detached houses, the
supply authority will bring a single-phase service cable into each house. The number of
circuits in a house is small enough to be accommodated on a single distribution board
Figure 6.8 Domestic service
so that there is no need for sub-main distribution. The scheme thus becomes as shown
diagrammatically in Figure 6.8. The incoming cable goes to a service cut out which is
supplied, fixed and sealed by the electricity company. As a domestic supply does not
normally exceed 100A the whole can go through the meter and there is no occasion for
Design of electrical services for buildings 96


the use of transformers. From the meter the cable goes directly to the distribution board.
This must be supplied and fixed by the installer, who also provides the short tail to
connect it to the meter. The tail is left loose at the meter for the electricity company to
make the connection into the meter. In the case of a serious fault the service cut out will
disconnect the consumer from the service cable and thus prevent the fault affecting the
rest of the board’s distribution network. It is obviously convenient to have all these pieces
of equipment close together.
There are distribution boards made specially for domestic use, and these are known as
consumer control units (CCU). They are very similar to ordinary fuseboards but are
single-phase only and incorporate a double-pole isolating switch on the incoming side, a
residual current device may also be used as the main isolator. Manufacturers also supply
CCUs with split busbars, one busbar supplied through an isolator, the other busbar
supplied through a residual current device. Whichever method is employed, this makes it
possible to cut off the supply from fuses or circuit breakers before changing or
withdrawing them. On a larger building, it is not so necessary to have an isolator on each
board because the sub-main and board can be cut off together at the intake end. CCUs are
available with 60A or 100A isolators and are made with up to 12 fuseways/CBs. A fairly
typical arrangement of circuits for a semi-detached house is given in Table 6.1a and for a
flat in a block with central heating in Table 6.1b. If a house requires a bigger supply than
100A, it cannot be cabled through a domestic CCU and it must be
Circuits Serving Rating amps
1 Upstairs lights 6
2 Downstairs lights 6
3 Garage and outside lights 6
4 Upstairs ring main 32
5 Downstairs ring main 32
6 Immersion heater 15
7 Shower unit 45
8 Cooker 45
8 Spare –
(a)
Circuits Serving Rating amps
1 Lights 6
2 Ring main 32
3 Cooker 32
4 Bathroom ventilation fan 10
5 Clothes dryer 10
6 Motorized valve on heating 10
Distribution 97


7 Spare –
8 Spare –
(b)
Table 6.1 Domestic circuits
treated as a small commercial building with an intake put together from standard
commercial fuse and distribution gear.
In a block of flats the arrangement within each flat is the same as that within a house,
as just described, but some means must be found to take the supply authority’s cable
through the block to each flat. In a small block, it may be practicable to bring into the
block as many cables as there are flats and run them within the block to the individual
flats. In this case, they will probably be PVC/SWA/PVC cables, but in a block of any size
this would be a cumbersome solution, and the distribution on the board’s side of the
meter is similar to that which is used on the consumer’s side of the meter in commercial
buildings. The supply authority brings in one service cable to a service head which
contains a fuse. From this, it takes a main cable feeding a series of distribution boards,
and from each way on a distribution board it takes a sub-main into one flat. The
protection device on the distribution board protects the flat supply cable and, therefore a
service cut-out is not needed in the flat itself.
Usually there are only a small number of flats on each floor and they can be served
individually from a single distribution board. Successive floors in a block of flats are
either identical or very similar, so that the various distribution boards are vertically above
each other, and the main to them rises vertically. A convenient and very common way of
carrying out the vertical distribution is by means of bare rising mains of the type
illustrated in Figure 2.4 (Chapter 2). A distribution scheme of this type is shown
diagrammatically in Figure 6.9, and a typical floor plan is shown in Figure 6.10. It will be
seen from Figure 6.9 that the rising main carries all three phases, but that each
distribution board is connected to only one phase. In this way, the flats are allocated
between the phases so as to make the total load as nearly balanced as possible. It has to
be assumed that all flats impose the same load.
In this example, the electricity company service cable enters the building through an
earthenware duct through the foundations, and terminates in a service head in a cubical
recessed in a wall at ground-floor level. From this, the bare rising mains, enclosed in a
sheet steel case, rises through the building. The architect has chosen to have the enclosing
case semi-recessed in the corridor wall, but it could equally well be fully recessed,
completely hidden inside a builder’s work duct or completely exposed. On each alternate
floor a six-way distribution board is attached to the rising main bars. The system used
enables the board to form part of the enclosing case. From each distribution board three
25mm conduits rise to the floor slab above and three drop to the slab below, and within
the slabs these conduits continue to a position just inside each flat. They terminate where
the meter will be fixed and the CCU is fixed next to the meter. A three-core 25mm2 PVC
insulated cable is pulled through this conduit from the distribution board to each meter
position. Alternatively PVC/SWA/PVC cables may be used instead of cables in conduit.
Design of electrical services for buildings 98

It will be noticed from the first-floor plan that there are actually two identical
distribution systems within the building, each serving half the flats on each floor. This
was done to avoid long and difficult horizontal runs at each level and also to keep the
load on each rising main to below 400A per phase. The whole of this distribution system
is the property of the electricity company, who remain responsible for its maintenance. It
may be installed during construction of the block by the company’s own staff or by
contractors employed by the developer, in which case the contractors must work to the
company’s specification. Which method is to be employed must be agreed beforehand by
the developer and the company; the plan of the installation must also be agreed between
them when the building and its electrical system is being designed.
People do not stay at home all day and it is often difficult for meter readers to enter to
read the meter. It is an advantage if the meter can be read from outside the dwelling, and
on many new developments the supply is arranged to make this possible. One method is
to have the meter in a purpose-made cupboard in the outside wall of the dwelling. The
CCU can be in an adjacent cupboard on the inside of the dwelling. Alternatively the CCU
can be in any other convenient position inside the dwelling and instead of a short tail
from meter to CCU there is a fairly long piece of cable. As this is likely to be at least a
25mm2 cable which cannot be bent too sharply the route it is to take should be planned
beforehand to make sure the cable can be drawn in. In the case of a block of flats, instead
of having the meter outside each flat, all the meters can be grouped together in some
convenient central point. Figure 6.11 is a modification of Figure 6.10 with external
meters. The six meters served by each distribution board are housed in a meter cupboard
next to the distribution board, each meter being connected by a short tail to the
corresponding fuse in the distribution board. The conduit to each flat runs from the meter
cupboard instead of from the distribution board, and goes straight to the CCU in the flat.
The second way of reading meters from outside is to use repeaters or slave indicators.
A repeater, sometimes known as a slave indicator, is an indicating dial, identical to that in
the meter itself, driven remotely from the meter. If this method were adopted in the
example already discussed, the meters would be within the flats as shown in Figure 6.10,
Next to the distribution board would be a cupboard containing six repeaters, which would
take the place of the meter cupboard shown in Figure 6.11. 25mm2 cable would be taken
through the conduit from the distribution board to each meter. An additional
Distribution 99


1.5mm2 cable would run in the same conduit back from the meter into the distribution
board and through the side of the board into the repeater cupboard. Here it would be
connected to the repeater belonging to the flat in question, and the repeater would
reproduce the reading on the meter.
Standards relevant to this chapter are:
BS EN 60947–2 Specification for low voltage switchgear and controlgear
BS EN 60947–3 Specification for low-voltage switchgear and controlgear
BS 5486 Low voltage switchgear and controlgear
BS 6121 Mechanical cable glands for elastomer and plastics insulated cables
BS EN 50262 Metric cable glands for electrical installations
BS 6480 Impregnated paper insulated cables for voltages up to 33000V
IEE Wiring Regulations particularly applicable to this chapter are:
Section 537