Chapter 10
Fire alarms
Introduction
A fire alarm circuit, as its name implies, sounds an alarm in the event of a fire. There can
be one or several alarms throughout a building, and there can be several alarm points
which activate the warning. The alarm points can be operated manually or automatically;
in the latter case they may be sensitive to heat, smoke or ionization. There are clearly
many combinations possible, and we shall try in this chapter to give some systematic
account of the way they are built up. The external circuitry is similar whether the control
panel consists of electronic components or electromechanical relays.
Circuits
The simplest scheme is shown in Figure 10.1. Several alarm points are connected in
parallel, and whenever one of them is actuated the circuit is completed and the alarm
sounds. This is described as an open circuit, and it will be seen that it is not fail-safe,
because if there is a failure of supply, the fire alarm cannot work. Another characteristic
of this circuit is that every
Figure 10.1 Fire alarm open circuit
Figure 10.2 Fire alarm closed circuit
alarm point must be capable of carrying the full current taken by all the bells or hooters
working together. An end-of-line resistor placed at the remote end of the break-glass
points or detectors allows a monitoring current to flow so that any breaks in the wiring
can be detected by the cessation of a current flow.
A slightly more elaborate scheme is shown in Figure 10.2. The alarm points are
connected in series with each other and with a relay coil. The relay is normally closed
when de-energized, and opens when the coil is energized. Thus when an alarm point is
activated the relay coil is de-energized, the relay closes and the alarm sounds. The system
fails safe to the extent that if the coil circuit fails the main circuit operates the alarm. It is
not of course safe against total failure of the supply because in that event there is no
supply available to work the bells. The alarm points do not have to carry the operating
current of the bells or hooters. This arrangement is called a closed circuit in contrast to
the open circuit of Figure 10.1. We can notice that in an open circuit the alarm points are
cabled in parallel and are normally open, while in a closed circuit they are cabled in
series and are normally closed.
When an alarm has been given it is often desirable to silence the audible alarm before
the operating point which actuated the alarm is replaced or reset. An alarm stop/reset unit
is made commercially which diverts the current from the general alarm to a supervisory
buzzer or indicator but restores the current to its normal condition when the alarm
initiating point has been reset. An open circuit including this unit is shown in Figure 10.3.
In a large building it may be desirable to have an indicator at some central position to
show which warning point in the building has caused the alarm to sound. Figure 10.4
shows a closed scheme in which each pair of points is connected to a separate signal on
an indicator board. The board can have either flags or luminous signs. The circuit can
easily be adapted so that each
Fire alarms 165
Figure 10.3 Fire alarm with relay unit
Figure 10.4 Fire alarm with indicator
individual point has its own signal or so that a larger number of points is grouped
together to one signal. All the points so grouped are cabled in series and are connected to
their own operating relay in the relay box. The alarm contacts are closed when any one of
the relays is energized. The bells can be silenced when required, but neither the
supervisory buzzer nor the indicator can be reset until the alarm initiating point has been
restored to its normal position.
Other refinements can be made for more complicated schemes in large buildings. The
exact circuit arrangement must depend largely on the features of the equipment used, and
in practice a satisfactory scheme can only be designed round a chosen manufacturer’s
equipment and with the aid of data from the relevant catalogue.
Design of electrical services for buildings 166
Electronic systems
Solid state electronics, printed circuit boards and microprocessors have made it possible
to include more facilities within fire alarm systems.
The simplest development is for the inputs from the fire detectors to be fed into a
microprocessor which is programmed for various functions. These would include the
operation of audible alarms and visual indicators and could also include output signals to
remote repeater panels and relays which in turn could activate fire-fighting services. It
may for example be useful to operate automatic door closers or door releasers, and stop,
start or even reverse some of the ventilation fans. Both detectors and audible alarms can
be arranged in zones. The programme can be such that a signal from one zone activates
the alarms in that zone and in some but not all of the other zones.
A further possibility is circuits which monitor the state of every detecting sensor. In
some systems it is possible to distinguish between normal, fire alarm and fault conditions
and illuminate indicator lights on the control panel accordingly. It is possible to detect
deterioration of heat, smoke or ionization detectors and indicate that maintenance or
replacement is necessary. The monitoring is usually done by checking each part of the
system in turn in a sequence which is repeated every two or three seconds. These are
termed ‘addressable systems’.
For large systems, the monitoring process can be displayed on a visual display unit
and a permanent record provided by a printer. The addition of a keyboard with the
appropriate circuitry enables an operator or supervisor to interrupt the automatic
monitoring sequence and test parts of the system at will. The central panel of such a
system is shown in Figure 10.5.
These systems can also be combined with burglar or intruder alarms. In factories it
may be useful to have detection and warning of particularly important plant failures, and
even further extensions are possible. One manufacturer offers a system which combines
fire detection, sprinkler supervision, security, plant monitoring and control and airconditioning
monitoring and control, as well as incorporating voice communication.
For sites covering many acres with several buildings it becomes necessary to have an
independent alarm system in each building with the output information of each repeated
in a central security or management office. This can be done economically by
telecommunication multiplexing techniques which allow a large number of panels of
many types to be connected together with only three cables and be able to send signals
between each other in both directions.
With this type of equipment it is essential that all the components are compatible with
each other. In the language of electronics, the designer must
Fire alarms 167
Figure 10.5 Fire detection central
station (Courtesy of Gent & Co. Ltd)
ensure that the right interfaces are provided. In practice it is necessary for the building
services designer to work with a selected system manufacturer to ensure that all
components are compatible with each other.
Wiring
The wiring of a fire alarm installation follows exactly the same principles as any other
wiring, but greater consideration has to be given to the protection of the cables and to
their ability to withstand fire damage. Cables used in fire alarm systems fall into two
general categories. In group 1, cables are not required to operate after the fire has been
detected; in group 2, cables are required to operate after the fire has been detected. It is
obviously necessary for a fire alarm to go on working for quite some time after a fire has
started. The wiring of group 2 must, therefore, be entirely separate from any other wiring.
In conduit or trunking systems, it should be segregated from all other services and run in
conduit or trunking of its own. It must be able to withstand high temperature, which in
practice means that it is MICC to BS 6207 part 1, to BS 6387 categories AWX, SWX, A
or S. Other types of cable may be used if they are embedded in 12mm of plaster or
equivalent, protecting them from significant fire risk for half-an-hour. In any case, BS
5839 specifies a number of requirements on the cables used in fire-alarm circuits. For
electronic systems the cable may need to be a coaxial or screened type, and the
manufacturer’s requirements must be checked to ascertain this. The supply to the fire
alarm must also be separate from any other supply, and this at the very least means that it
must be fed from its own circuit breaker or switch at the main service entry into the
building. Some authorities go further and think that the fire-alarm system should be at
Design of electrical services for buildings 168
extra low voltage, and to satisfy this requirement fire alarm equipment is made for 24V
a.c. and 12, 24 or 48V d.c. operation as well as for mains voltage operation. BS 5839
requires that a risk formal assessment forms the basis of design.
A system working on 24V a.c. has to be fed from a transformer. The primary of the
transformer is fed from its own circuit breaker or switch at the main service entry. D.c.
systems are fed from batteries of the accumulator type, which are kept charged by a
charger unit connected to the mains. As the battery will operate the system for a
considerable period before losing all its charge, this method provides a fire alarm which
is independent not only of the mains within the building, but also of all electrical services
into the building. For this reason, some factory inspectors and fire-pevention officers
insist that a battery system be used. However, when the system voltage is low the
currents required to operate the equipment are higher and the voltage drops which can be
tolerated are much smaller. We can see this by reflecting that a motor wound for 240V
will work without noticeable diminution of speed or performance if the voltage at its
terminals drops by 6V to 234V, which is a reduction of 21.5 per cent, whereas a 24V bell
may not sound at all if a potential of 18V is applied to it. In this case a drop of 6V in the
line is a reduction of 25 per cent. At the same time for a given sound output a 24V bell
needs ten times the current that a 240V bell does.
Thus voltage drop becomes a very serious factor in the design of any extra-low
voltage system. The equipment to be used must be carefully checked to see what voltage
drop it can accept and the cables sized to keep the drops very low. This usually results in
large cables having to be used. There is a great deal to be said for obviating these
difficulties as far as possible by not using systems at less than 48V. Electronic systems
operate with much smaller currents so that for most of the system, voltage drop is not a
critical consideration. But the bells or horns still require appreciable power and voltage
drop should not be overlooked, particularly in discussion with electronics specialists who
are not normally concerned with it.
In the author’s opinion, there is very little to be said for a 24V transformer system. It
has all the voltage drop problems of a d.c. system without the independence of the
incoming service that batteries give. In other words it appears to have the disadvantages
of both mains and battery systems without the advantage of either.
The current carrying capacity of each component has to be taken into account in the
design and layout of the installation. It may happen, for example, that the total current of
all the alarm signals sounding together exceeds the current which can be taken by the
contacts of the alarm initiating points. The closed circuit of the type shown in Figure 10.2
overcomes this problem because the current to the alarm bells does not go through the
initiating points. This is a considerable advantage of the closed system, and can be a
deciding factor in choosing it in preference to the open system. Even with closed systems,
however, the total current of the alarm signals may exceed the capacity of the contacts in
the indicator panel. If this happens, a further relay must be interposed between the
indicator and the alarm signals.
Fire alarms 169
Fire-alarm points
A typical manually operated fire-alarm point is shown in Figure 10.6. It is contained in a
robust red plastic case with a glass cover. The material is chosen for its fire-resisting
properties. The case has knock-outs for conduit entries at top and bottom but the material
can be sufficiently easily cut for the site electrician to make an entry in the back if
needed. Alternative terminals are provided for circuits in which the contacts have to close
when the glass is smashed (as in Figure 10.1) and for circuits in which the contacts have
to open when the glass is smashed (as in Figure 10.2). In the former case, there is a test
switch which can be reached when the whole front is opened with an Allen Key. In the
latter case, the test push is omitted because the circuit is in any case of the fail-safe type.
The alarm point illustrated is suitable for surface mounting. Similar ones are available
for flush fixing and in weatherproof versions. The current carrying capacity of the
contacts should always be checked with the maker’s catalogue.
Automatic detectors can respond to heat, smoke or ionization, and in the first case they
can respond either at a fixed temperature or to a given rate of rise in temperature. A
thermally operated detector for use with power-type systems is shown in Figure 10.7. It
consists of a bi-metal strip which deflects
Figure 10.6 Fire alarm point (Courtesy
of Gent & Co. Ltd)
Design of electrical services for buildings 170
Figure 10.7 Heat detector (Courtesy of
Gent & Co. Ltd)
when the temperature rises, and thereby tilts a tube half-full of mercury. When the tube
tilts, the mercury flows into the other half of the tube where it completes the circuit
between two contacts previously separated by air. Alternatively, the arrangement within
the tube can be such that the mercury breaks the circuit when the tube is tilted. The casing
of the detector illustrated is of stainless steel, and it would be suitable for use in places
such as boiler houses. For other areas similar detectors are available with the working
parts within aesthetically more pleasing enclosures. They are usually set to operate at
65°C.
Smoke detectors can be of the optical type. In this a small lamp shines a light across to
a photoelectric cell. When smoke enters the detector the light beam is interrupted and the
output of the photoelectric cell changes, which initiates the alarm. In modern units the
light source can be an LED which requires little space and consumes negligible power.
When first introduced smoke detectors often caused nuisance operation of the alarm by
reacting to small quantities of smoke which had not been caused by a fire; on some
occasions they sounded the alarm as a result of cigarette smoke in an office. Modern ones
have adjustable sensitivity so that they can be set to avoid nuisance operation.
Detectors of the ionization type are also referred to as smoke detectors, but they will
respond to ionization, which is normally caused by a fire, even if there is no visible
smoke. An ionization detector contains a chamber which houses some low-strength
radioactive material and a pair of electrodes. The radioactive material makes the air in the
chamber conductive so that a small current flows between the electrodes. The size of the
current varies with the nature of the gas in the chamber and as soon as any combustion
products are added to the air there is a sudden change in the current flowing. The
Fire alarms 171
Figure 10.8 Automatic fire detectors
(Courtesy of Gent & Co. Ltd)
detector also has a second chamber which is permanently sealed so that the current
through it never changes. As long as the currents through the two chambers are equal
there is no output; as soon as they become unbalanced there is a net output which is used
to operate a transistor switch in the detection circuit.
Various types of detector which are suitable for use with a monitoring electronic
system are illustrated in Figure 10.8. This manufacturer provides a common base which
will accept any of the detector heads, so that at any location the means of detection can be
very easily changed.
Bells
Any bell could, of course, be used to sound an alarm. However, the voltage at which it
operates must be that of the system and the current consumption must be taken into
account in the design of the system. Most manufacturers of fire-alarm equipment make
bells intended for use with their systems, and it is clearly advantageous to design a
system making use of only one maker’s equipment. Typical bells have power
consumptions of between 1 and 6W.
Design of electrical services for buildings 172
Sirens
Electrically operated sirens can be used as an alternative to bells. They are much louder
and can be heard at a distance of half to three quarters of a mile. This makes them very
suitable for factories in which there may be a lot of background noise. A typical rating is
60W and we see that a siren takes a much bigger current than a bell. As a consequence,
there is a bigger voltage drop in the circuit feeding it. In extra-low voltage circuits, the
drop can be sufficiently serious to prevent the sirens from sounding at all, and it becomes
especially important to check the layout for voltage drop.
Horns
A horn is an alternative to both bell and siren, and because of its penetrating, raucous
note it is particularly suitable where a distinctive sound is needed. Its volume is easily
adjustable and its power consumption is intermediate between that of a bell and that of a
siren.
The manufacturers of fire-alarm equipment also provide standard indicators, relays
and reset units for use with the various circuits which we have described in this chapter.
Typical indicators for use with power systems are illustrated in Figure 10.9. The
appropriate indicator light is illuminated, or
Figure 10.9 Fire alarm indicators
(Courtesy of Gent & Co. Ltd)
a mechanical flag dropped, by a relay which may be either normally energized or
normally de-energized. In the first case, the relay holds the signal off and the signal
comes on when the activation of the fire alarm point interrupts the circuit. In the second
case, the action of the alarm point energizes the relay and brings the signal on. In either
case, the indicator must be such that the relay cannot be reset until the alarm point has
Fire alarms 173
been returned to its normal position. Indicators for use with electronic systems have
already been described.
Self-contained battery and charger units are also available for low-voltage d.c.
systems. Alternatively, separate batteries and constant potential chargers can be used. In
either case, the designer should check that the ampere-hour capacity of the battery is
sufficient for the load it will have to supply. This is of particular importance in those
systems which have a current permanently flowing through the alarm initiating points.
Standards relevant to this chapter are:
BS 2740 Simple smoke alarms and alarm metering devices
BS EN
54–1
Fire detection and fire-alarm systems; introduction
BS EN
54–2
Fire detection and fire-alarm systems; control and indicating equipment
BS EN
54–4
Fire detection and fire-alarm systems; power supply equipment
BS 5446–
1
Fire detection and fire-alarm devices for dwellings; spec. for smoke/heat alarms
BS 5839 Fire detection and alarm systems in buildings CoP for system design, installation,
commissioning and maintenance
BS 6387 Specification for performance requirements for cables required to maintain circuit
integrity under fire conditions
BS 6004 Electric cables; single-core unsheathed heat-resisting cables for voltages up to and
including 450/750V, for internal wiring
BS 6007 Electric cables; single-core unsheathed heat-resisting cables for voltages up to and
including 450/750V, for internal wiring
BS 6346 Specification for 600/1000V and 1900/3300V armoured electric cables having PVC
insulation
BS 5467 Specification for 600/1000V and 1900/3300V armoured electric cables having
thermosetting insulation
BS 2316 Specification for radio-frequency cables; general requirements and tests; British
Government Services requirements
IEE Wiring Regulations particularly applicable to this chapter are:
Part 4
Section 531
Section 533
Section 537
Chapter 54
Appendix 3
Design of electrical services for buildings 174
Fire alarms
Introduction
A fire alarm circuit, as its name implies, sounds an alarm in the event of a fire. There can
be one or several alarms throughout a building, and there can be several alarm points
which activate the warning. The alarm points can be operated manually or automatically;
in the latter case they may be sensitive to heat, smoke or ionization. There are clearly
many combinations possible, and we shall try in this chapter to give some systematic
account of the way they are built up. The external circuitry is similar whether the control
panel consists of electronic components or electromechanical relays.
Circuits
The simplest scheme is shown in Figure 10.1. Several alarm points are connected in
parallel, and whenever one of them is actuated the circuit is completed and the alarm
sounds. This is described as an open circuit, and it will be seen that it is not fail-safe,
because if there is a failure of supply, the fire alarm cannot work. Another characteristic
of this circuit is that every
Figure 10.1 Fire alarm open circuit
Figure 10.2 Fire alarm closed circuit
alarm point must be capable of carrying the full current taken by all the bells or hooters
working together. An end-of-line resistor placed at the remote end of the break-glass
points or detectors allows a monitoring current to flow so that any breaks in the wiring
can be detected by the cessation of a current flow.
A slightly more elaborate scheme is shown in Figure 10.2. The alarm points are
connected in series with each other and with a relay coil. The relay is normally closed
when de-energized, and opens when the coil is energized. Thus when an alarm point is
activated the relay coil is de-energized, the relay closes and the alarm sounds. The system
fails safe to the extent that if the coil circuit fails the main circuit operates the alarm. It is
not of course safe against total failure of the supply because in that event there is no
supply available to work the bells. The alarm points do not have to carry the operating
current of the bells or hooters. This arrangement is called a closed circuit in contrast to
the open circuit of Figure 10.1. We can notice that in an open circuit the alarm points are
cabled in parallel and are normally open, while in a closed circuit they are cabled in
series and are normally closed.
When an alarm has been given it is often desirable to silence the audible alarm before
the operating point which actuated the alarm is replaced or reset. An alarm stop/reset unit
is made commercially which diverts the current from the general alarm to a supervisory
buzzer or indicator but restores the current to its normal condition when the alarm
initiating point has been reset. An open circuit including this unit is shown in Figure 10.3.
In a large building it may be desirable to have an indicator at some central position to
show which warning point in the building has caused the alarm to sound. Figure 10.4
shows a closed scheme in which each pair of points is connected to a separate signal on
an indicator board. The board can have either flags or luminous signs. The circuit can
easily be adapted so that each
Fire alarms 165
Figure 10.3 Fire alarm with relay unit
Figure 10.4 Fire alarm with indicator
individual point has its own signal or so that a larger number of points is grouped
together to one signal. All the points so grouped are cabled in series and are connected to
their own operating relay in the relay box. The alarm contacts are closed when any one of
the relays is energized. The bells can be silenced when required, but neither the
supervisory buzzer nor the indicator can be reset until the alarm initiating point has been
restored to its normal position.
Other refinements can be made for more complicated schemes in large buildings. The
exact circuit arrangement must depend largely on the features of the equipment used, and
in practice a satisfactory scheme can only be designed round a chosen manufacturer’s
equipment and with the aid of data from the relevant catalogue.
Design of electrical services for buildings 166
Electronic systems
Solid state electronics, printed circuit boards and microprocessors have made it possible
to include more facilities within fire alarm systems.
The simplest development is for the inputs from the fire detectors to be fed into a
microprocessor which is programmed for various functions. These would include the
operation of audible alarms and visual indicators and could also include output signals to
remote repeater panels and relays which in turn could activate fire-fighting services. It
may for example be useful to operate automatic door closers or door releasers, and stop,
start or even reverse some of the ventilation fans. Both detectors and audible alarms can
be arranged in zones. The programme can be such that a signal from one zone activates
the alarms in that zone and in some but not all of the other zones.
A further possibility is circuits which monitor the state of every detecting sensor. In
some systems it is possible to distinguish between normal, fire alarm and fault conditions
and illuminate indicator lights on the control panel accordingly. It is possible to detect
deterioration of heat, smoke or ionization detectors and indicate that maintenance or
replacement is necessary. The monitoring is usually done by checking each part of the
system in turn in a sequence which is repeated every two or three seconds. These are
termed ‘addressable systems’.
For large systems, the monitoring process can be displayed on a visual display unit
and a permanent record provided by a printer. The addition of a keyboard with the
appropriate circuitry enables an operator or supervisor to interrupt the automatic
monitoring sequence and test parts of the system at will. The central panel of such a
system is shown in Figure 10.5.
These systems can also be combined with burglar or intruder alarms. In factories it
may be useful to have detection and warning of particularly important plant failures, and
even further extensions are possible. One manufacturer offers a system which combines
fire detection, sprinkler supervision, security, plant monitoring and control and airconditioning
monitoring and control, as well as incorporating voice communication.
For sites covering many acres with several buildings it becomes necessary to have an
independent alarm system in each building with the output information of each repeated
in a central security or management office. This can be done economically by
telecommunication multiplexing techniques which allow a large number of panels of
many types to be connected together with only three cables and be able to send signals
between each other in both directions.
With this type of equipment it is essential that all the components are compatible with
each other. In the language of electronics, the designer must
Fire alarms 167
Figure 10.5 Fire detection central
station (Courtesy of Gent & Co. Ltd)
ensure that the right interfaces are provided. In practice it is necessary for the building
services designer to work with a selected system manufacturer to ensure that all
components are compatible with each other.
Wiring
The wiring of a fire alarm installation follows exactly the same principles as any other
wiring, but greater consideration has to be given to the protection of the cables and to
their ability to withstand fire damage. Cables used in fire alarm systems fall into two
general categories. In group 1, cables are not required to operate after the fire has been
detected; in group 2, cables are required to operate after the fire has been detected. It is
obviously necessary for a fire alarm to go on working for quite some time after a fire has
started. The wiring of group 2 must, therefore, be entirely separate from any other wiring.
In conduit or trunking systems, it should be segregated from all other services and run in
conduit or trunking of its own. It must be able to withstand high temperature, which in
practice means that it is MICC to BS 6207 part 1, to BS 6387 categories AWX, SWX, A
or S. Other types of cable may be used if they are embedded in 12mm of plaster or
equivalent, protecting them from significant fire risk for half-an-hour. In any case, BS
5839 specifies a number of requirements on the cables used in fire-alarm circuits. For
electronic systems the cable may need to be a coaxial or screened type, and the
manufacturer’s requirements must be checked to ascertain this. The supply to the fire
alarm must also be separate from any other supply, and this at the very least means that it
must be fed from its own circuit breaker or switch at the main service entry into the
building. Some authorities go further and think that the fire-alarm system should be at
Design of electrical services for buildings 168
extra low voltage, and to satisfy this requirement fire alarm equipment is made for 24V
a.c. and 12, 24 or 48V d.c. operation as well as for mains voltage operation. BS 5839
requires that a risk formal assessment forms the basis of design.
A system working on 24V a.c. has to be fed from a transformer. The primary of the
transformer is fed from its own circuit breaker or switch at the main service entry. D.c.
systems are fed from batteries of the accumulator type, which are kept charged by a
charger unit connected to the mains. As the battery will operate the system for a
considerable period before losing all its charge, this method provides a fire alarm which
is independent not only of the mains within the building, but also of all electrical services
into the building. For this reason, some factory inspectors and fire-pevention officers
insist that a battery system be used. However, when the system voltage is low the
currents required to operate the equipment are higher and the voltage drops which can be
tolerated are much smaller. We can see this by reflecting that a motor wound for 240V
will work without noticeable diminution of speed or performance if the voltage at its
terminals drops by 6V to 234V, which is a reduction of 21.5 per cent, whereas a 24V bell
may not sound at all if a potential of 18V is applied to it. In this case a drop of 6V in the
line is a reduction of 25 per cent. At the same time for a given sound output a 24V bell
needs ten times the current that a 240V bell does.
Thus voltage drop becomes a very serious factor in the design of any extra-low
voltage system. The equipment to be used must be carefully checked to see what voltage
drop it can accept and the cables sized to keep the drops very low. This usually results in
large cables having to be used. There is a great deal to be said for obviating these
difficulties as far as possible by not using systems at less than 48V. Electronic systems
operate with much smaller currents so that for most of the system, voltage drop is not a
critical consideration. But the bells or horns still require appreciable power and voltage
drop should not be overlooked, particularly in discussion with electronics specialists who
are not normally concerned with it.
In the author’s opinion, there is very little to be said for a 24V transformer system. It
has all the voltage drop problems of a d.c. system without the independence of the
incoming service that batteries give. In other words it appears to have the disadvantages
of both mains and battery systems without the advantage of either.
The current carrying capacity of each component has to be taken into account in the
design and layout of the installation. It may happen, for example, that the total current of
all the alarm signals sounding together exceeds the current which can be taken by the
contacts of the alarm initiating points. The closed circuit of the type shown in Figure 10.2
overcomes this problem because the current to the alarm bells does not go through the
initiating points. This is a considerable advantage of the closed system, and can be a
deciding factor in choosing it in preference to the open system. Even with closed systems,
however, the total current of the alarm signals may exceed the capacity of the contacts in
the indicator panel. If this happens, a further relay must be interposed between the
indicator and the alarm signals.
Fire alarms 169
Fire-alarm points
A typical manually operated fire-alarm point is shown in Figure 10.6. It is contained in a
robust red plastic case with a glass cover. The material is chosen for its fire-resisting
properties. The case has knock-outs for conduit entries at top and bottom but the material
can be sufficiently easily cut for the site electrician to make an entry in the back if
needed. Alternative terminals are provided for circuits in which the contacts have to close
when the glass is smashed (as in Figure 10.1) and for circuits in which the contacts have
to open when the glass is smashed (as in Figure 10.2). In the former case, there is a test
switch which can be reached when the whole front is opened with an Allen Key. In the
latter case, the test push is omitted because the circuit is in any case of the fail-safe type.
The alarm point illustrated is suitable for surface mounting. Similar ones are available
for flush fixing and in weatherproof versions. The current carrying capacity of the
contacts should always be checked with the maker’s catalogue.
Automatic detectors can respond to heat, smoke or ionization, and in the first case they
can respond either at a fixed temperature or to a given rate of rise in temperature. A
thermally operated detector for use with power-type systems is shown in Figure 10.7. It
consists of a bi-metal strip which deflects
Figure 10.6 Fire alarm point (Courtesy
of Gent & Co. Ltd)
Design of electrical services for buildings 170
Figure 10.7 Heat detector (Courtesy of
Gent & Co. Ltd)
when the temperature rises, and thereby tilts a tube half-full of mercury. When the tube
tilts, the mercury flows into the other half of the tube where it completes the circuit
between two contacts previously separated by air. Alternatively, the arrangement within
the tube can be such that the mercury breaks the circuit when the tube is tilted. The casing
of the detector illustrated is of stainless steel, and it would be suitable for use in places
such as boiler houses. For other areas similar detectors are available with the working
parts within aesthetically more pleasing enclosures. They are usually set to operate at
65°C.
Smoke detectors can be of the optical type. In this a small lamp shines a light across to
a photoelectric cell. When smoke enters the detector the light beam is interrupted and the
output of the photoelectric cell changes, which initiates the alarm. In modern units the
light source can be an LED which requires little space and consumes negligible power.
When first introduced smoke detectors often caused nuisance operation of the alarm by
reacting to small quantities of smoke which had not been caused by a fire; on some
occasions they sounded the alarm as a result of cigarette smoke in an office. Modern ones
have adjustable sensitivity so that they can be set to avoid nuisance operation.
Detectors of the ionization type are also referred to as smoke detectors, but they will
respond to ionization, which is normally caused by a fire, even if there is no visible
smoke. An ionization detector contains a chamber which houses some low-strength
radioactive material and a pair of electrodes. The radioactive material makes the air in the
chamber conductive so that a small current flows between the electrodes. The size of the
current varies with the nature of the gas in the chamber and as soon as any combustion
products are added to the air there is a sudden change in the current flowing. The
Fire alarms 171
Figure 10.8 Automatic fire detectors
(Courtesy of Gent & Co. Ltd)
detector also has a second chamber which is permanently sealed so that the current
through it never changes. As long as the currents through the two chambers are equal
there is no output; as soon as they become unbalanced there is a net output which is used
to operate a transistor switch in the detection circuit.
Various types of detector which are suitable for use with a monitoring electronic
system are illustrated in Figure 10.8. This manufacturer provides a common base which
will accept any of the detector heads, so that at any location the means of detection can be
very easily changed.
Bells
Any bell could, of course, be used to sound an alarm. However, the voltage at which it
operates must be that of the system and the current consumption must be taken into
account in the design of the system. Most manufacturers of fire-alarm equipment make
bells intended for use with their systems, and it is clearly advantageous to design a
system making use of only one maker’s equipment. Typical bells have power
consumptions of between 1 and 6W.
Design of electrical services for buildings 172
Sirens
Electrically operated sirens can be used as an alternative to bells. They are much louder
and can be heard at a distance of half to three quarters of a mile. This makes them very
suitable for factories in which there may be a lot of background noise. A typical rating is
60W and we see that a siren takes a much bigger current than a bell. As a consequence,
there is a bigger voltage drop in the circuit feeding it. In extra-low voltage circuits, the
drop can be sufficiently serious to prevent the sirens from sounding at all, and it becomes
especially important to check the layout for voltage drop.
Horns
A horn is an alternative to both bell and siren, and because of its penetrating, raucous
note it is particularly suitable where a distinctive sound is needed. Its volume is easily
adjustable and its power consumption is intermediate between that of a bell and that of a
siren.
The manufacturers of fire-alarm equipment also provide standard indicators, relays
and reset units for use with the various circuits which we have described in this chapter.
Typical indicators for use with power systems are illustrated in Figure 10.9. The
appropriate indicator light is illuminated, or
Figure 10.9 Fire alarm indicators
(Courtesy of Gent & Co. Ltd)
a mechanical flag dropped, by a relay which may be either normally energized or
normally de-energized. In the first case, the relay holds the signal off and the signal
comes on when the activation of the fire alarm point interrupts the circuit. In the second
case, the action of the alarm point energizes the relay and brings the signal on. In either
case, the indicator must be such that the relay cannot be reset until the alarm point has
Fire alarms 173
been returned to its normal position. Indicators for use with electronic systems have
already been described.
Self-contained battery and charger units are also available for low-voltage d.c.
systems. Alternatively, separate batteries and constant potential chargers can be used. In
either case, the designer should check that the ampere-hour capacity of the battery is
sufficient for the load it will have to supply. This is of particular importance in those
systems which have a current permanently flowing through the alarm initiating points.
Standards relevant to this chapter are:
BS 2740 Simple smoke alarms and alarm metering devices
BS EN
54–1
Fire detection and fire-alarm systems; introduction
BS EN
54–2
Fire detection and fire-alarm systems; control and indicating equipment
BS EN
54–4
Fire detection and fire-alarm systems; power supply equipment
BS 5446–
1
Fire detection and fire-alarm devices for dwellings; spec. for smoke/heat alarms
BS 5839 Fire detection and alarm systems in buildings CoP for system design, installation,
commissioning and maintenance
BS 6387 Specification for performance requirements for cables required to maintain circuit
integrity under fire conditions
BS 6004 Electric cables; single-core unsheathed heat-resisting cables for voltages up to and
including 450/750V, for internal wiring
BS 6007 Electric cables; single-core unsheathed heat-resisting cables for voltages up to and
including 450/750V, for internal wiring
BS 6346 Specification for 600/1000V and 1900/3300V armoured electric cables having PVC
insulation
BS 5467 Specification for 600/1000V and 1900/3300V armoured electric cables having
thermosetting insulation
BS 2316 Specification for radio-frequency cables; general requirements and tests; British
Government Services requirements
IEE Wiring Regulations particularly applicable to this chapter are:
Part 4
Section 531
Section 533
Section 537
Chapter 54
Appendix 3
Design of electrical services for buildings 174
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