Chapter 11
Call and computer systems, telephone and
public address systems
Introduction
In many buildings, it is necessary to have a system of calling staff who are on duty in an
office or staff room to go to rooms elsewhere in the building. This happens, for example,
in hotels, hospitals and retirement homes. In the case of hospitals, patients wish to call a
nurse who is in the ward office to their own beds, and in hotels guests may wish to call
staff to their rooms. In retirement homes, a resident may wish to summon either domestic
or nursing staff. All such systems can be arranged electrically and form part of the
electrical services in a building.
Hospital call systems
In hospitals, it is desirable for each patient to be able to call a nurse to his/her bed. Figure
11.1 is a wiring diagram of a simple circuit for achieving this. There is a call unit at each
bed which contains a push button, a relay and an illuminated reset lamp push. When the
patient pushes the button, the relay is energized and holds itself in until it is released by
the reset lamp push. While the relay is energized, a lamp is illuminated; it can be either
next to the patient’s bed or over the door of the room. At the same time, a buzzer and
light are operated in the ward office or wherever else the nurse is to be called from. The
buzzer can be silenced by a muting switch, but the light can only be cancelled by the
resetting push on the patient’s unit. Secondary or pilot lamps and buzzers can be placed
elsewhere so that a nurse’s attention can be attracted in more than one place or so that a
nurse can supervise the activity of his/her staff.
When the call is made, the duty nurse has no indication from where it is coming and
must, therefore, walk around all the places on the system until s/he sees the individual
lamp which has been illuminated. The system is therefore limited to small areas. An
extended version which does not have such a limitation is shown in Figure 11.2. A call
unit is provided at each bed and also in each toilet; the toilet unit differs from the bed unit
by being
Figure 11.1 Hospital call system
operated by external push buttons instead of by buttons within it. There is a button in
each WC compartment and in each bath compartment, all of them being cabled in parallel
with each other to the operating contacts of the single toilet unit. Each call unit
illuminates a lamp over the door of the room in which that unit is. Thus all the units in a
ward illuminate the same lamp over the ward door. Several rooms are grouped in a
section and when any unit in one section is energized a signal lamp for that section is
illuminated in the nurse’s office. There can also be a parallel signal lamp at a suitable
position in the corridor.
When the duty nurse receives a call, the indicator in his/her office shows which
section the call is coming from. When s/he gets to that section of corridor, s/he will see
which door has a light over it, and once inside the room she can see which call unit has
the reset lamp alight. Further central stations can be added in parallel with the main one
in case staff have to be called from more than one area or in case supervision of staff
activities from another office is necessary.
It will have been noticed that the reset button on the call unit is in the form of a lamp
which stays alight until it has been pushed to reset the unit. One
Design of electrical services for buildings 176
Figure 11.2 Hospital call system
reason for this is to show which unit in a ward of several beds has been operated, but the
presence of the light also helps to reassure a nervous or frightened patient that assistance
is on the way. It is simpler and cheaper to make the reset push contain the light than to
install a separate button and lamp.
The call units and central station can also incorporate a combined microphone and
loudspeaker so that the nurse can speak to the patient from her office. The diagram of
Figure 11.2 is repeated with this facility added in Figure 11.3.
The indicator units and signal lamps are all made for extra-low voltage so that there is
no possibility of electrical shock to the users if any faults occur. Another reason for using
extra-low voltage is that it would be unnecessarily expensive to have mains voltage
signal lamps. It is, therefore, usual to make the whole system an extra-low voltage one
and to size the cables accordingly. The design of the wiring thus becomes similar to that
for fire alarms discussed in Chapter 10, but it is not so essential to protect the wiring from
fire damage. Ordinary PVC cables may be used and there is not the same objection to the
use of a transformer from the main building supply. In fact, call-system equipment is
normally made for a 24V d.c. supply, and the manufacturers also make a power unit
consisting of a transformer and rectifier. A 250V a.c. is supplied to the primary of the
power unit and 24V d.c. is taken from the secondary or output side. The cables must
obviously be sized to keep the voltage drop to an absolute minimum, and in a large
building it may be necessary to have several independent systems in order that the cables
are sufficiently short.
Electronic circuits enable relays to be replaced by transistor switches and lamps to be
replaced by light emitting diodes. This reduces the power consumption drastically and,
because currents are reduced to the order of milliamps, voltage drop ceases to be a major
consideration. Bells and buzzers still require appreciable power but can be operated
through amplifiers and relays from sources close to them. The operating principles are the
same as for the power-operated systems described and the methods of connection are
Call abd computer systems, telephone 177
similar but, because of the small currents, telephone-type cables can be used. In some
cases it may be convenient to use multicore cable.
A typical indicator to receive the calls in the central office is shown in Figure 11.4. An
indicating lamp suitable for mounting over a room door is shown in Figure 11.5. It has a
cast-iron or steel box with a lampholder to take a low-wattage sign-type lamp or a LED
and an overlapping brass cover with a ruby glass dome. The box is built into the wall so
that the cover is flush with the face of the wall.
A single call button is shown in Figure 11.6. This is contained in a box and is suitable
for fixing in a wall within reach of a bath or toilet. An example of a complete call unit is
shown in Figure 11.7. It contains the push button, reset light and also controls for the
patient’s bedside radio outlet. The call
Figure 11.3 Call system with speech
facility
Figure 11.4 Alarm indicator (Courtesy
of Gent & Co. Ltd)
Design of electrical services for buildings 178
Figure 11.5 Indicator lamp (Courtesy
of Edison Telecom Ltd)
Figure 11.6 Call button (Courtesy of
Edison Telecom Ltd)
Call abd computer systems, telephone 179
Figure 11.7 Call unit (Courtesy of
Edison Telecom Ltd)
Figure 11.8 Bedhead trunking
(Courtesy of Cableflow Ltd)
unit also contains a switch for a bedside lamp and the microphone for a speech system.
They are on the end of a wander lead and are designed so that they can be held
comfortably in the patient’s hand. The wander lead ends in a multiway connector plug
which fits into a mating socket in the bedhead trunking shown in Figure 11.8. The
permanent wiring of the installation ends in this the trunking.
Design of electrical services for buildings 180
Door indicators
A call system which is being increasingly installed is the door indicator. This is an
illuminated sign, with or without a buzzer, fixed outside an office door and operated by a
push at the desk in the office. The sign can be ‘Enter’, ‘Engaged’, ‘Next Patient’ or any
other message such as ‘Keep Out’. An enter sign is shown in Figure 11.9, together with
its wiring diagram. A transformer is contained in the same box as the sign and the system
operates at extra low voltage. The table push energizes a relay, which is also contained in
the sign box, and this connects the lamp behind the sign to the low-voltage supply. The
equipment can be arranged so that the light stays on until reset by the table push.
Standard equipment for these applications is made by a number of manufacturers. The
particular one shown is suitable for surface mounting,
Figure 11.9 Door sign
but they are also made for flush fixing, and clearly for a new building this would be
preferable.
The sign could be relay controlled, such as outside an X-ray room at a hospital. When
the X-ray equipment is about to be operated the relay automatically activates a ‘Keep
Out’, or other warning sign.
Telephone systems
The design of telephone systems is beyond the scope of this book, but we must consider
the provision that has to be made for them within a building. In many cases all that is
needed is a route by which the public telephone service, which in the UK is British
Telecom, can bring a telephone cable to an instrument. British Telecom telephones are
operated by batteries at the telephone exchanges and need no source of power within the
buildings they serve. Telephone cable is quite small and if the position of the outlet for
the telephone receiver is known it is sufficient to install a 20mm conduit from outside the
building to the outlet, with the same number and spacing of draw-in points as are used for
any other conduit system. It is usual for the electrical installer to fix the conduit and leave
draw cable in it, which the telephone engineers subsequently use for pulling their cable in
after the building is finished and occupied.
Call abd computer systems, telephone 181
The most common procedure used in the UK is for British Telecom or other cable
network supplier, to supply plastic ducts which the builder puts in the ground from the
telephone main in the road to a point just within the building, and for the electrical
contractor to supply and install metal conduit from the end of the plastic duct to the final
telephone outlet position. A conduit box is provided where the plastic duct meets the
metal conduit, and the electrical contractor puts draw cable into both the duct and the
conduit.
If the telephone cable is to come in overhead, as is likely to happen in rural areas, then
British Telecom will do all the outside work, including the fixing of a terminal on the
wall of the building. The electrical installer then has only to provide conduit with draw
cable from the entry point, which in this case will be at high level, to the final telephone
outlet position.
Some buildings have an internal telephone system which may consist of extensions to
the public telephones or may be an entirely separate installation. Here again the essential
matter for the electrical services designer is to agree the outlet positions with the
customer and to arrange for them to be linked to each other by conduit or trunking.
Trunking can be a useful alternative to conduit when the system is a complex one
needing many cables with a large number of junctions. Telephone cables do not have a
protective sheathing and therefore need the mechanical protection of conduit or trunking.
They can, however, be run exposed on surfaces and this is often done, but in a new
building it makes it rather apparent that the designers forgot about the telephones until it
was too late.
An internal telephone installation which is independent of the public telephones must
receive power from somewhere. All telephones work on extra-low voltage and this is
provided either by a battery or by an electronic power pack. A battery needs to be kept
charged by a battery charger which in turn has to be supplied by mains power. A power
pack usually contains its own transformer but this must then be fed from the mains. In
whichever way the telephone works, mains power has to be provided somewhere, usually
at the central exchange of the system. The power required is very small and can be
supplied from a socket outlet or fused connection unit on the nearest convenient generalpurpose
power circuit.
Most British Telecom telephones take their power from batteries at the Telephone
Exchange, and do not need power from the consumer’s supply. Some of the British
Telecom private branch exchange systems do however include a power pack which
requires a supply from the subscriber’s premises. The electrical designer should therefore
discuss the system to be used with British Telecom and make sure that any necessary
power outlets are provided. They can again be ordinary socket outlets or fused connection
units taken from a convenient power circuit at a point adjacent to the telephone
equipment. The more modern of telephone sets are connected to the permanent wiring by
a jack plug, which inserts into a telephone outlet socket. This socket forms part of a lid
which screws onto a standard conduit box.
Design of electrical services for buildings 182
Public address systems
Public address and loudspeaker systems are somewhat similar to telephones. The details
of the equipment to be used can be settled only with manufacturers’ catalogues and by
discussion with the manufacturers. Once the equipment and its location have been
selected, provision must be made for running cables from the announcing station to the
loudspeakers. Because these cables are likely to be put in after all other building work is
finished a conduit system is the almost inevitable choice. Loudspeaker cables are like
telephone cables in that they are small and do not have an outer sheath; this also makes it
difficult to find any alternative to putting them inside conduit.
Closed-circuit television systems
There is no real technical difference between the pictures we see at home on television,
and those delivered by a CCTV camera and monitor. The real difference is in the cost of
the equipment. Therefore we cannot expect to have the same quality of picture from
equipment bought for the sums of money a client is prepared to spend, compared with
broadcasting company equipment.
Closed-circuit television is used in many circumstances, including surveillance of
vehicular, and people traffic, entering and leaving a premises. The CCTV standard is
phase alternate line (PAL), also used in normal TV transmissions in Europe with the
exception of France, who use PAL only for CCTV.
The typical types of cable are: (a) coaxial, which is unbalanced, meaning that the
signal is a voltage with reference to ground. The video signal is between 0.3 to 1.0V
above ground; (b) twisted pair balanced, meaning that the video signal has been
converted for transmission along a medium other than coaxial. The signal level is the
voltage difference between each conductor; (c) fibre optics, which are immune to outside
interference and signals without needing amplification.
External interference is picked up en route by all types of cable, with the exception of
fibre optics. Unless suitably screened, power and signal cables should be kept well apart.
The longer the length of cables, the greater the losses. Unlike fibre cables, copper cables
will have a voltage drop over the length resulting in a lower signal level at the receiving
end than that processed by the camera. Provision of cable routes for CCTV is similar to
that discussed in telephone systems.
Computer systems
Computer networks are used extensively in organisations to utilise the storage space of a
server. To connect the workstations to the server, a network is used. The network wiring
provides a transmission path between the workstations and the server. Copper cable can
meet most demands at a relatively low cost. The data is transmitted in the form of lowvoltage
electrical signals, which are unfortunately subject to interference. The cables
Call abd computer systems, telephone 183
must, therefore, be run in situations where interference will not be a problem. Fibre optic
cables can transmit data over longer distances than copper cable, and the fibre optic cable
is not subject to interference. With fibre optic cables the data is converted into light by a
transducer and sent through the fibre optic cable. A second transducer converts the light
signals back into low-voltage signals at the server. Fibre optic cables are used for higher
bandwidth applications. They allow more information to be transmitted faster but,
however, are more expensive to install. Wireless networks use radio signals to transmit
data. They tend to be lower bandwidth than wired networks. The building services
engineer must liase with specialist installation companies to determine the requirements
of the company with regard to cable runs required.
The electrical mains supply to computer systems requires special consideration.
Information technology equipment inherently gives relatively high leakage current to
earth. To ensure that the circuit protective conductors remain secure in circuits supplying
IT equipment, BS 7671 Section 607 Earthing Requirements for the Installation of
Equipment having High Protective Conductor Currents sets out the special measures
required. For distribution and final circuits where the total protective conductor current
exceeds 10mA, the protective conductor must comply with the regulations with respect to
protective conductors and be not less than 10mm2, or 4mm2 if mechanically protected, or
two protective conductors may be used which may be of different types, e.g. copper, and
steel conduit.
For final circuits only, where the total protective conductor current will exceed 10mA,
the protective conductor is run in the form of a ring, irrespective of whether the circuit is
a ring or radial one. Alternatively, a connection is made between the remote end of the
copper protective conductor and the steel enclosure, if used. If two or more identical
radial circuits are used, a connection can be made between the remote ends of the
protective conductors, on each circuit. This gives a parallel path to earth, for each circuit.
The building service engineers may consider using more final circuits in computer
suites, etc., than would normally be installed in other parts of the building. Using more
final circuits would reduce the leakage current on each final circuit. However, the leakage
current on each final circuit, is cumulative on the associated distribution circuits, and
therefore must be addressed.
Standards relevant to this chapter are:
BS EN 50134–7 Social alarm systems
BS 5839 Fire detection and alarm systems in buildings
BS 6259 Code of practice for sound systems
BS EN 50132–7 Alarm systems; CCTV surveillance systems for use in security applications
BS 8220 Guide for security of buildings against crime
BS 7671 Section 607
Design of electrical services for buildings 184
Chapter 12
Reduced-voltage systems
The use of extra-low-voltage wiring for fire alarms and call systems has been discussed
in the previous two chapters. Other applications occur in laboratories, where permanently
installed reduced-voltage outlets are required for various experiments. Permanent outlets
are easier for the staff than the use of accumulator batteries which have to be carried from
preparation room stores and set up on the laboratory benches for each experiment.
Reduced-voltage supplies are also needed for microscopes which have a built-in light for
illuminating the slide.
The requirements for any particular laboratory must, of course, be agreed by the
electrical designer with the staff who will be using the laboratory. In some cases, as for
example illuminating microscope slides, the supply is needed at a constant voltage. Such
a supply can be provided by a transformer. Usually, this is comparatively small and can
be mounted on a bracket fixed to the wall either of the laboratory or of an adjacent store.
For a given power, the current on a reduced-voltage service is higher than on a mainsvoltage
circuit and, therefore, the cable sizes soon become substantial. To prevent the use
of excessively large cables, it is convenient to keep down the number of outlets on one
circuit and to use a separate transformer for each secondary circuit.
It would be possible to take the secondary of the transformer to a reduced-voltage
distribution board and split there to several reduced-voltage circuits. The cable from the
transformer to the distribution board would, however, be very large to take the necessary
current, and it is better to use a separate transformer for each secondary circuit. The kVA
rating required is calculated from the secondary voltage and total output power needed.
As usual in this kind of design, it is advisable to allow ample spare capacity so that the
transformer rating should be somewhat above the calculated requirement.
The voltage on the primary of the transformer is known, being the ordinary mains
supply voltage in the building, and this determines the transformer ratio. The ratio
determines the primary current and thus provides all the information necessary to design
the mains circuit feeding the transformer. A fuse can, if desired, be provided on the
secondary of the transformer, but an overload on the secondary would draw an overload
on the primary so that the fuse in the supply to the transformer will also protect the
secondary. This will not, however, be the case if the primary has been oversized while the
secondary has not. The exact carrying capacities of the primary and secondary sides
should be carefully compared before the fusing arrangements are finally decided on.
In other cases, a laboratory needs a variable reduced-voltage output. This makes it
possible to set up experiments with a choice of voltage. Reduced-voltage laboratory units
are made which contain a transformer, a rectifier and variable tappings on the output
switches. The one illustrated in Figure 12.1 gives a choice of outputs ranging from 6V to
24V a.c. and from 6V to 24V d.c. It can, therefore, be set to give the output required for
whatever experiment is in hand.
The wiring arrangements are exactly the same as for a fixed output transformer. The
cables must, of course, be sized for the largest current which will be taken, which will
correspond to the lowest voltage. It is also at the lowest voltage that voltage drop along
the cable is most serious.
Figure 12.1 Reduced-voltage unit
A number of accessories is available for terminating the reduced-voltage wiring at the
benches, and some of these have been described in Chapter 1. For laboratory work, the
type described is most suitable because the cables used for setting up the bench
experiment can be readily connected to these terminals. Microscopes or other permanent
equipment requiring a reduced-voltage supply will usually have a trailing flexible lead
with a plug at the end of it. For these, a socket outlet is clearly more convenient than a
laboratory type of terminal, but it should not be possible to plug a reduced-voltage piece
of equipment into a mains voltage socket. Reduced-voltage wiring for such applications
Design of electrical services for buildings 186
should, therefore, terminate in 5A two, or three-pin socket outlets which should be clearly
labelled with the voltage. Two, or three-pin plugs can then be supplied and fixed to the
equipment, and it will not be possible for anyone to push these into ordinary socket
outlets.
Intrinsically safe circuits
Equipment for hazardous areas is discussed in Chapter 1. One technique in areas where
there is a risk of fire is the use of intrinsically safe circuits. The principle of these is that
the energy of any spark which occurs shall be limited so that it is not sufficient to ignite
the vapour. This is achieved by using reduced voltages and equipment which does not
take high currents at these voltages. The power in the circuit is thus low and there is not
enough energy available to initiate combustion.
Not all equipment can be designed on this basis, but it is often possible to have
intrinsically safe circuits within a hazardous area operating relays which control normal
equipment outside the danger zone. This may be cheaper than installing flameproof
equipment within the zone.
Standards relevant to this chapter are:
BS 1259 Intrinsically safe electrical apparatus and circuits
IEE Wiring Regulations particularly applicable to this chapter are:
Section 411
Regulation 553–3
Reduced-voltage systems 187
Call and computer systems, telephone and
public address systems
Introduction
In many buildings, it is necessary to have a system of calling staff who are on duty in an
office or staff room to go to rooms elsewhere in the building. This happens, for example,
in hotels, hospitals and retirement homes. In the case of hospitals, patients wish to call a
nurse who is in the ward office to their own beds, and in hotels guests may wish to call
staff to their rooms. In retirement homes, a resident may wish to summon either domestic
or nursing staff. All such systems can be arranged electrically and form part of the
electrical services in a building.
Hospital call systems
In hospitals, it is desirable for each patient to be able to call a nurse to his/her bed. Figure
11.1 is a wiring diagram of a simple circuit for achieving this. There is a call unit at each
bed which contains a push button, a relay and an illuminated reset lamp push. When the
patient pushes the button, the relay is energized and holds itself in until it is released by
the reset lamp push. While the relay is energized, a lamp is illuminated; it can be either
next to the patient’s bed or over the door of the room. At the same time, a buzzer and
light are operated in the ward office or wherever else the nurse is to be called from. The
buzzer can be silenced by a muting switch, but the light can only be cancelled by the
resetting push on the patient’s unit. Secondary or pilot lamps and buzzers can be placed
elsewhere so that a nurse’s attention can be attracted in more than one place or so that a
nurse can supervise the activity of his/her staff.
When the call is made, the duty nurse has no indication from where it is coming and
must, therefore, walk around all the places on the system until s/he sees the individual
lamp which has been illuminated. The system is therefore limited to small areas. An
extended version which does not have such a limitation is shown in Figure 11.2. A call
unit is provided at each bed and also in each toilet; the toilet unit differs from the bed unit
by being
Figure 11.1 Hospital call system
operated by external push buttons instead of by buttons within it. There is a button in
each WC compartment and in each bath compartment, all of them being cabled in parallel
with each other to the operating contacts of the single toilet unit. Each call unit
illuminates a lamp over the door of the room in which that unit is. Thus all the units in a
ward illuminate the same lamp over the ward door. Several rooms are grouped in a
section and when any unit in one section is energized a signal lamp for that section is
illuminated in the nurse’s office. There can also be a parallel signal lamp at a suitable
position in the corridor.
When the duty nurse receives a call, the indicator in his/her office shows which
section the call is coming from. When s/he gets to that section of corridor, s/he will see
which door has a light over it, and once inside the room she can see which call unit has
the reset lamp alight. Further central stations can be added in parallel with the main one
in case staff have to be called from more than one area or in case supervision of staff
activities from another office is necessary.
It will have been noticed that the reset button on the call unit is in the form of a lamp
which stays alight until it has been pushed to reset the unit. One
Design of electrical services for buildings 176
Figure 11.2 Hospital call system
reason for this is to show which unit in a ward of several beds has been operated, but the
presence of the light also helps to reassure a nervous or frightened patient that assistance
is on the way. It is simpler and cheaper to make the reset push contain the light than to
install a separate button and lamp.
The call units and central station can also incorporate a combined microphone and
loudspeaker so that the nurse can speak to the patient from her office. The diagram of
Figure 11.2 is repeated with this facility added in Figure 11.3.
The indicator units and signal lamps are all made for extra-low voltage so that there is
no possibility of electrical shock to the users if any faults occur. Another reason for using
extra-low voltage is that it would be unnecessarily expensive to have mains voltage
signal lamps. It is, therefore, usual to make the whole system an extra-low voltage one
and to size the cables accordingly. The design of the wiring thus becomes similar to that
for fire alarms discussed in Chapter 10, but it is not so essential to protect the wiring from
fire damage. Ordinary PVC cables may be used and there is not the same objection to the
use of a transformer from the main building supply. In fact, call-system equipment is
normally made for a 24V d.c. supply, and the manufacturers also make a power unit
consisting of a transformer and rectifier. A 250V a.c. is supplied to the primary of the
power unit and 24V d.c. is taken from the secondary or output side. The cables must
obviously be sized to keep the voltage drop to an absolute minimum, and in a large
building it may be necessary to have several independent systems in order that the cables
are sufficiently short.
Electronic circuits enable relays to be replaced by transistor switches and lamps to be
replaced by light emitting diodes. This reduces the power consumption drastically and,
because currents are reduced to the order of milliamps, voltage drop ceases to be a major
consideration. Bells and buzzers still require appreciable power but can be operated
through amplifiers and relays from sources close to them. The operating principles are the
same as for the power-operated systems described and the methods of connection are
Call abd computer systems, telephone 177
similar but, because of the small currents, telephone-type cables can be used. In some
cases it may be convenient to use multicore cable.
A typical indicator to receive the calls in the central office is shown in Figure 11.4. An
indicating lamp suitable for mounting over a room door is shown in Figure 11.5. It has a
cast-iron or steel box with a lampholder to take a low-wattage sign-type lamp or a LED
and an overlapping brass cover with a ruby glass dome. The box is built into the wall so
that the cover is flush with the face of the wall.
A single call button is shown in Figure 11.6. This is contained in a box and is suitable
for fixing in a wall within reach of a bath or toilet. An example of a complete call unit is
shown in Figure 11.7. It contains the push button, reset light and also controls for the
patient’s bedside radio outlet. The call
Figure 11.3 Call system with speech
facility
Figure 11.4 Alarm indicator (Courtesy
of Gent & Co. Ltd)
Design of electrical services for buildings 178
Figure 11.5 Indicator lamp (Courtesy
of Edison Telecom Ltd)
Figure 11.6 Call button (Courtesy of
Edison Telecom Ltd)
Call abd computer systems, telephone 179
Figure 11.7 Call unit (Courtesy of
Edison Telecom Ltd)
Figure 11.8 Bedhead trunking
(Courtesy of Cableflow Ltd)
unit also contains a switch for a bedside lamp and the microphone for a speech system.
They are on the end of a wander lead and are designed so that they can be held
comfortably in the patient’s hand. The wander lead ends in a multiway connector plug
which fits into a mating socket in the bedhead trunking shown in Figure 11.8. The
permanent wiring of the installation ends in this the trunking.
Design of electrical services for buildings 180
Door indicators
A call system which is being increasingly installed is the door indicator. This is an
illuminated sign, with or without a buzzer, fixed outside an office door and operated by a
push at the desk in the office. The sign can be ‘Enter’, ‘Engaged’, ‘Next Patient’ or any
other message such as ‘Keep Out’. An enter sign is shown in Figure 11.9, together with
its wiring diagram. A transformer is contained in the same box as the sign and the system
operates at extra low voltage. The table push energizes a relay, which is also contained in
the sign box, and this connects the lamp behind the sign to the low-voltage supply. The
equipment can be arranged so that the light stays on until reset by the table push.
Standard equipment for these applications is made by a number of manufacturers. The
particular one shown is suitable for surface mounting,
Figure 11.9 Door sign
but they are also made for flush fixing, and clearly for a new building this would be
preferable.
The sign could be relay controlled, such as outside an X-ray room at a hospital. When
the X-ray equipment is about to be operated the relay automatically activates a ‘Keep
Out’, or other warning sign.
Telephone systems
The design of telephone systems is beyond the scope of this book, but we must consider
the provision that has to be made for them within a building. In many cases all that is
needed is a route by which the public telephone service, which in the UK is British
Telecom, can bring a telephone cable to an instrument. British Telecom telephones are
operated by batteries at the telephone exchanges and need no source of power within the
buildings they serve. Telephone cable is quite small and if the position of the outlet for
the telephone receiver is known it is sufficient to install a 20mm conduit from outside the
building to the outlet, with the same number and spacing of draw-in points as are used for
any other conduit system. It is usual for the electrical installer to fix the conduit and leave
draw cable in it, which the telephone engineers subsequently use for pulling their cable in
after the building is finished and occupied.
Call abd computer systems, telephone 181
The most common procedure used in the UK is for British Telecom or other cable
network supplier, to supply plastic ducts which the builder puts in the ground from the
telephone main in the road to a point just within the building, and for the electrical
contractor to supply and install metal conduit from the end of the plastic duct to the final
telephone outlet position. A conduit box is provided where the plastic duct meets the
metal conduit, and the electrical contractor puts draw cable into both the duct and the
conduit.
If the telephone cable is to come in overhead, as is likely to happen in rural areas, then
British Telecom will do all the outside work, including the fixing of a terminal on the
wall of the building. The electrical installer then has only to provide conduit with draw
cable from the entry point, which in this case will be at high level, to the final telephone
outlet position.
Some buildings have an internal telephone system which may consist of extensions to
the public telephones or may be an entirely separate installation. Here again the essential
matter for the electrical services designer is to agree the outlet positions with the
customer and to arrange for them to be linked to each other by conduit or trunking.
Trunking can be a useful alternative to conduit when the system is a complex one
needing many cables with a large number of junctions. Telephone cables do not have a
protective sheathing and therefore need the mechanical protection of conduit or trunking.
They can, however, be run exposed on surfaces and this is often done, but in a new
building it makes it rather apparent that the designers forgot about the telephones until it
was too late.
An internal telephone installation which is independent of the public telephones must
receive power from somewhere. All telephones work on extra-low voltage and this is
provided either by a battery or by an electronic power pack. A battery needs to be kept
charged by a battery charger which in turn has to be supplied by mains power. A power
pack usually contains its own transformer but this must then be fed from the mains. In
whichever way the telephone works, mains power has to be provided somewhere, usually
at the central exchange of the system. The power required is very small and can be
supplied from a socket outlet or fused connection unit on the nearest convenient generalpurpose
power circuit.
Most British Telecom telephones take their power from batteries at the Telephone
Exchange, and do not need power from the consumer’s supply. Some of the British
Telecom private branch exchange systems do however include a power pack which
requires a supply from the subscriber’s premises. The electrical designer should therefore
discuss the system to be used with British Telecom and make sure that any necessary
power outlets are provided. They can again be ordinary socket outlets or fused connection
units taken from a convenient power circuit at a point adjacent to the telephone
equipment. The more modern of telephone sets are connected to the permanent wiring by
a jack plug, which inserts into a telephone outlet socket. This socket forms part of a lid
which screws onto a standard conduit box.
Design of electrical services for buildings 182
Public address systems
Public address and loudspeaker systems are somewhat similar to telephones. The details
of the equipment to be used can be settled only with manufacturers’ catalogues and by
discussion with the manufacturers. Once the equipment and its location have been
selected, provision must be made for running cables from the announcing station to the
loudspeakers. Because these cables are likely to be put in after all other building work is
finished a conduit system is the almost inevitable choice. Loudspeaker cables are like
telephone cables in that they are small and do not have an outer sheath; this also makes it
difficult to find any alternative to putting them inside conduit.
Closed-circuit television systems
There is no real technical difference between the pictures we see at home on television,
and those delivered by a CCTV camera and monitor. The real difference is in the cost of
the equipment. Therefore we cannot expect to have the same quality of picture from
equipment bought for the sums of money a client is prepared to spend, compared with
broadcasting company equipment.
Closed-circuit television is used in many circumstances, including surveillance of
vehicular, and people traffic, entering and leaving a premises. The CCTV standard is
phase alternate line (PAL), also used in normal TV transmissions in Europe with the
exception of France, who use PAL only for CCTV.
The typical types of cable are: (a) coaxial, which is unbalanced, meaning that the
signal is a voltage with reference to ground. The video signal is between 0.3 to 1.0V
above ground; (b) twisted pair balanced, meaning that the video signal has been
converted for transmission along a medium other than coaxial. The signal level is the
voltage difference between each conductor; (c) fibre optics, which are immune to outside
interference and signals without needing amplification.
External interference is picked up en route by all types of cable, with the exception of
fibre optics. Unless suitably screened, power and signal cables should be kept well apart.
The longer the length of cables, the greater the losses. Unlike fibre cables, copper cables
will have a voltage drop over the length resulting in a lower signal level at the receiving
end than that processed by the camera. Provision of cable routes for CCTV is similar to
that discussed in telephone systems.
Computer systems
Computer networks are used extensively in organisations to utilise the storage space of a
server. To connect the workstations to the server, a network is used. The network wiring
provides a transmission path between the workstations and the server. Copper cable can
meet most demands at a relatively low cost. The data is transmitted in the form of lowvoltage
electrical signals, which are unfortunately subject to interference. The cables
Call abd computer systems, telephone 183
must, therefore, be run in situations where interference will not be a problem. Fibre optic
cables can transmit data over longer distances than copper cable, and the fibre optic cable
is not subject to interference. With fibre optic cables the data is converted into light by a
transducer and sent through the fibre optic cable. A second transducer converts the light
signals back into low-voltage signals at the server. Fibre optic cables are used for higher
bandwidth applications. They allow more information to be transmitted faster but,
however, are more expensive to install. Wireless networks use radio signals to transmit
data. They tend to be lower bandwidth than wired networks. The building services
engineer must liase with specialist installation companies to determine the requirements
of the company with regard to cable runs required.
The electrical mains supply to computer systems requires special consideration.
Information technology equipment inherently gives relatively high leakage current to
earth. To ensure that the circuit protective conductors remain secure in circuits supplying
IT equipment, BS 7671 Section 607 Earthing Requirements for the Installation of
Equipment having High Protective Conductor Currents sets out the special measures
required. For distribution and final circuits where the total protective conductor current
exceeds 10mA, the protective conductor must comply with the regulations with respect to
protective conductors and be not less than 10mm2, or 4mm2 if mechanically protected, or
two protective conductors may be used which may be of different types, e.g. copper, and
steel conduit.
For final circuits only, where the total protective conductor current will exceed 10mA,
the protective conductor is run in the form of a ring, irrespective of whether the circuit is
a ring or radial one. Alternatively, a connection is made between the remote end of the
copper protective conductor and the steel enclosure, if used. If two or more identical
radial circuits are used, a connection can be made between the remote ends of the
protective conductors, on each circuit. This gives a parallel path to earth, for each circuit.
The building service engineers may consider using more final circuits in computer
suites, etc., than would normally be installed in other parts of the building. Using more
final circuits would reduce the leakage current on each final circuit. However, the leakage
current on each final circuit, is cumulative on the associated distribution circuits, and
therefore must be addressed.
Standards relevant to this chapter are:
BS EN 50134–7 Social alarm systems
BS 5839 Fire detection and alarm systems in buildings
BS 6259 Code of practice for sound systems
BS EN 50132–7 Alarm systems; CCTV surveillance systems for use in security applications
BS 8220 Guide for security of buildings against crime
BS 7671 Section 607
Design of electrical services for buildings 184
Chapter 12
Reduced-voltage systems
The use of extra-low-voltage wiring for fire alarms and call systems has been discussed
in the previous two chapters. Other applications occur in laboratories, where permanently
installed reduced-voltage outlets are required for various experiments. Permanent outlets
are easier for the staff than the use of accumulator batteries which have to be carried from
preparation room stores and set up on the laboratory benches for each experiment.
Reduced-voltage supplies are also needed for microscopes which have a built-in light for
illuminating the slide.
The requirements for any particular laboratory must, of course, be agreed by the
electrical designer with the staff who will be using the laboratory. In some cases, as for
example illuminating microscope slides, the supply is needed at a constant voltage. Such
a supply can be provided by a transformer. Usually, this is comparatively small and can
be mounted on a bracket fixed to the wall either of the laboratory or of an adjacent store.
For a given power, the current on a reduced-voltage service is higher than on a mainsvoltage
circuit and, therefore, the cable sizes soon become substantial. To prevent the use
of excessively large cables, it is convenient to keep down the number of outlets on one
circuit and to use a separate transformer for each secondary circuit.
It would be possible to take the secondary of the transformer to a reduced-voltage
distribution board and split there to several reduced-voltage circuits. The cable from the
transformer to the distribution board would, however, be very large to take the necessary
current, and it is better to use a separate transformer for each secondary circuit. The kVA
rating required is calculated from the secondary voltage and total output power needed.
As usual in this kind of design, it is advisable to allow ample spare capacity so that the
transformer rating should be somewhat above the calculated requirement.
The voltage on the primary of the transformer is known, being the ordinary mains
supply voltage in the building, and this determines the transformer ratio. The ratio
determines the primary current and thus provides all the information necessary to design
the mains circuit feeding the transformer. A fuse can, if desired, be provided on the
secondary of the transformer, but an overload on the secondary would draw an overload
on the primary so that the fuse in the supply to the transformer will also protect the
secondary. This will not, however, be the case if the primary has been oversized while the
secondary has not. The exact carrying capacities of the primary and secondary sides
should be carefully compared before the fusing arrangements are finally decided on.
In other cases, a laboratory needs a variable reduced-voltage output. This makes it
possible to set up experiments with a choice of voltage. Reduced-voltage laboratory units
are made which contain a transformer, a rectifier and variable tappings on the output
switches. The one illustrated in Figure 12.1 gives a choice of outputs ranging from 6V to
24V a.c. and from 6V to 24V d.c. It can, therefore, be set to give the output required for
whatever experiment is in hand.
The wiring arrangements are exactly the same as for a fixed output transformer. The
cables must, of course, be sized for the largest current which will be taken, which will
correspond to the lowest voltage. It is also at the lowest voltage that voltage drop along
the cable is most serious.
Figure 12.1 Reduced-voltage unit
A number of accessories is available for terminating the reduced-voltage wiring at the
benches, and some of these have been described in Chapter 1. For laboratory work, the
type described is most suitable because the cables used for setting up the bench
experiment can be readily connected to these terminals. Microscopes or other permanent
equipment requiring a reduced-voltage supply will usually have a trailing flexible lead
with a plug at the end of it. For these, a socket outlet is clearly more convenient than a
laboratory type of terminal, but it should not be possible to plug a reduced-voltage piece
of equipment into a mains voltage socket. Reduced-voltage wiring for such applications
Design of electrical services for buildings 186
should, therefore, terminate in 5A two, or three-pin socket outlets which should be clearly
labelled with the voltage. Two, or three-pin plugs can then be supplied and fixed to the
equipment, and it will not be possible for anyone to push these into ordinary socket
outlets.
Intrinsically safe circuits
Equipment for hazardous areas is discussed in Chapter 1. One technique in areas where
there is a risk of fire is the use of intrinsically safe circuits. The principle of these is that
the energy of any spark which occurs shall be limited so that it is not sufficient to ignite
the vapour. This is achieved by using reduced voltages and equipment which does not
take high currents at these voltages. The power in the circuit is thus low and there is not
enough energy available to initiate combustion.
Not all equipment can be designed on this basis, but it is often possible to have
intrinsically safe circuits within a hazardous area operating relays which control normal
equipment outside the danger zone. This may be cheaper than installing flameproof
equipment within the zone.
Standards relevant to this chapter are:
BS 1259 Intrinsically safe electrical apparatus and circuits
IEE Wiring Regulations particularly applicable to this chapter are:
Section 411
Regulation 553–3
Reduced-voltage systems 187
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