In the context of the code, the circuits regarding which advice on design is given are purely those external to control equipment; the code is not concerned with the internal circuitry of control equipment or devices, as this is a matter for the relevant product standards. The relevant clause of the code is clause 12, which is entitled ‘Monitoring, integrity and reliability of circuits external to control equipment’.
Before examining the ‘meat’ of this clause, which is quite considerable in that there are 20 specific recommendations, we should examine the title of the clause quite closely. The terms ‘monitoring’, ‘integrity’ and
‘reliability’ have been chosen quite carefully. Monitoring of critical signal paths ensures that, when a fault occurs, a suitable warning is given. The purpose of this warning is, of course, to ensure that the fault is rectified as quickly as possible. This reduces the system ‘down time’, during which the fire protection afforded by the system may be impaired or have greater exposure to impairment.
Monitoring does not, however, affect the likelihood of impairment.
Nevertheless, if we regard ‘reliability’ as the probability that the system will perform correctly on demand (i.e. when a fire occurs), monitoring obviously does increase reliability, since the shorter the period of down time the lesser the probability that a fire will occur during the down time. Reliability, in the narrower sense of immunity to faults in the operation of devices, is a matter for product standards, rather than the code; it is for this reason that the code recommends the use of products conforming to the relevant product standards (see Chapter 10).
The integrity of the installation may be considered to comprise three aspects. First, the installation should have a certain immunity to damage; it may be considered that recommendations to further this aim percolate throughout the code (e.g. in the clause on cables, recommendations for protection of cables against mechanical damage).
Secondly, integrity must surely include the ability of the system to
operate, for at least a limited period, during the course of a fire; again, recommendations to this effect extend beyond merely those contained in clause 12 (e.g. recommendations for fire resistance of cables, contained within clause 26).
However, given that, notwithstanding all the measures described so far, faults may occur (either in the non-fire state or the fire state), integrity also involves a limitation of the effect of such faults, so that, for example, the entire system is not compromised. As we shall see below, clause 12 of the code contains a number of recommendations that have this specific aim.
The code recognizes the distinction between fault monitoring and system integrity. Accordingly, subclauses of clause 12 deal with these two design aspects quite independently.
With regard to the recommendations on fault monitoring, some of these overlap with the product requirements of BS EN 54-222 and BS EN 54-4.23 Thus, the code recommends that a fault indication be given at the control and indicating equipment within 100 seconds of various specified faults occurring (or specified longer periods in the case of specified faults associated with power supplies). In each case, the specified faults are merely those for which BS EN 54-2 would require that a fault warning be given within the same specified periods.
It may be argued that this overlap with BS EN 54-2 is unnecessary.
However, users of the code (including installation designers, installers and those charged with auditing installations) may be much less familiar with BS EN 54-2 than with BS 5839-1. Inclusion of these recommend-ations, many of which are easily verifiable in the field, within the code provides an opportunity for ‘double checking’ that the products used will satisfy the requirements of BS EN 54-2 when assembled into an actual installation; it is just possible that, in some installations, this might not be the case, although such a situation would probably result from a failure of the installer to comply with the recommendations of the equipment manufacturer. Moreover, the language used within the code is arguably clearer than that of BS EN 54-2 and BS EN 54-4. This can obviate any ambiguity.
A particular point to note within these recommendations is that concerning monitoring of wiring between main and repeat control and/or indicating equipment. Consider, for example, a repeater mimic,
22 BS EN 54-2, Fire detection and fire alarm systems — Control and indicating equipment.
23 BS EN 54-4, Fire detection and fire alarm systems — Power supply equipment.
Design of fire alarm circuits
perhaps located at a secondary entrance to a building and provided for the information of the fire and rescue service.
It is not unknown, particularly in the case of conventional systems, for this mimic to comprise, in effect, a box of LEDs connected to the main control panel by unmonitored wiring. In order to consider how this should be viewed under the code, we need to return to an assertion within an earlier chapter that the purpose of a fire alarm system is to support a fire safety strategy. If the mimic is provided in furtherance of that strategy, it clearly forms part of the BS 5839-1 system. In such a case, the wiring between the mimic and the main control panel must be monitored if the installation is to comply with the code.
This is somewhat implicit in the requirements of BS EN 54-2, which requires that a fault warning be given in the event of short circuit or interruption in transmission paths between parts of the control and indicating equipment contained in more than one mechanical cabinet, if the fault is capable of affecting ‘a mandatory function’. This requirement of BS EN 54-2 is translated in the code into a recommendation that a fault warning be given in the event of a short circuit or open circuit in the wiring between separate control and/or indicating equipment that is provided in order to satisfy the recommendations of the code. However, to avoid any ambiguity, a further recommendation in the code is more specific; it is recommended that a fault warning be given in the event of a short circuit or open circuit in the wiring between main and any repeat control and/or indicating equipment (such as a mimic diagram) that is provided in order to satisfy the recommendations of the code.
A further requirement of BS EN 54-2 that has become a recommend-ation in clause 12 of the code concerns the connection between the control and indicating equipment and any facility for transmission of alarm signals to an alarm receiving centre (i.e. for automatic transmission of fire alarm signals to the fire and rescue service). The code recommends that a short circuit or open circuit in the wiring between control equipment and any separate enclosure of equipment used for transmission of alarm signals to an alarm receiving centre should result in a fault indication at the fire alarm control and indicating equipment.
This recommendation raises a number of issues. First, it would not be acceptable for the interconnection to ‘fail safe’, such that a fire alarm signal resulted from either a short circuit or an open circuit fault; a fault warning must be given for compliance with the code. Secondly, the fault warning should be given at the fire alarm control and indicating equipment; compliance could not be achieved by either the provision of an indication at the transmitting equipment or an indication at the alarm receiving centre.
It can be very difficult, in practice, to comply in full with this recommendation. It has, traditionally, been common practice to use the common fire relay within the control and indicating equipment as the ‘trigger’ for the transmitting equipment used to transmit signals to an alarm receiving centre. Without additional sophistication in the monitoring of the interconnection (involving facilities not normally provided within the transmitting equipment), this arrangement clearly cannot comply with the code. The use of, for example, a switched, unmonitored 24 V output from the fire alarm panel to the transmission equipment would clearly be an equal compliance. The non-compliance is often exacerbated by use of transmission equipment that is also used to transmit intruder alarm signals and so is located within, or adjacent to, intruder alarm control equipment; this may be sited some considerable distance from the fire alarm control equipment, resulting in a significant length of unmonitored cable.
Obviously, one solution is to incorporate the transmission equipment within the fire alarm control equipment (or in sufficient mechanical contact with it for the two to be regarded as one mechanical enclosure).
The need for monitoring does not then arise, as the recommendation only applies to situations in which the two enclosures are separate.
Another possibility is to use a monitored output from the fire alarm control and indicating equipment (e.g. a monitored 24 V auxiliary circuit) as the trigger circuit for the transmission equipment. Note that a sounder circuit cannot be used for this purpose, as, in clause 15, the code recommends that automatic transmission of alarm signals should not be prevented by the act of silencing fire alarm sounders.
A further recommendation of clause 12 goes beyond the European standard. The code recommends that, if a standby power supply comprises a number of batteries connected in parallel, a fault indication should be given in the event of disconnection of any one battery. Parallel connection of batteries tends to occur only in larger installations, or those in which a standby duration of greater than 24 hours is necessary (see also Chapter 21). Under these circumstances, installers do sometimes parallel batteries in order to increase the available capacity. Some control equipment is specifically designed to cater for this situation, and each of the parallel batteries are separately charged and monitored. However, possibly more commonly, no such special arrangements exist, and the effect is that it is possible to disconnect totally one of the parallel batteries without the system detecting this and giving a fault warning. As this clearly affects system integrity, in that the standby duration is halved, the code recommends that a fault warning be given.
Design of fire alarm circuits
Two further ‘Achilles heels’ in monitoring are highlighted by the code.
These are the interconnections that occur between the fire alarm sys-tem and either a voice alarm syssys-tem or another fire protection syssys-tem (e.g. a gaseous extinguishing system). In the former case, as the voice alarm system forms a crucial part of the fire warning system within the premises, it is appropriate for the code to deal with the monitoring of the interconnections between the fire detection and fire alarm system and the voice alarm system. Interconnections between a fire alarm system and any transmission equipment used to transmit signals to vibrating pagers that give fire warning to deaf people are also covered.
Thus, the code recommends that, if the fire alarm system and the voice alarm system (or fire warning system for deaf people) are separate, any short circuit or disconnection of the communicating link between the two should be indicated at the fire detection and alarm system control and indicating equipment within 100 seconds. In effect, therefore, the code treats the interconnecting wiring between the two systems in the same way as it treats the wiring to a normal fire alarm sounder. Equally, in the case of voice alarm systems, the code refers the reader to BS 5839-8,24 which merely supports, and slightly amplifies, this recommendation.
It should be noted that, with regard to this recommendation, the code refers to a separate voice alarm system. This would seem to imply that, for example, if the fire detection and alarm system control equipment and the voice alarm system rack were virtually integrated (e.g. were installed side-by-side and in contact with each other), so that the interconnecting wiring effectively becomes internal wiring within a single mechanical enclosure, the recommendation might not apply.
The same situation would arise in the case of paging transmission equipment provided to warn deaf people in the event of fire.
With regard to the interconnecting wiring between the fire detection and fire alarm system and any other fire protection system or safety facility, as discussed in Chapter 9 the interface is not strictly within the scope of the code. However, clause 12 recommends that, for recommend-ations regarding monitoring of the interconnections, reference should be made to BS 7273 or other applicable codes of practice.
As discussed in Chapter 14, the code gives considerable advice on fire alarm warnings for deaf people. Most of the recommendations of the code in this respect are concerned with systems that use tactile devices, including vibrating pagers. The code recommends that ‘circuits’ serving
24 BS 5839-8, Fire detection and alarm systems for buildings — Code of practice for the design, installation, commissioning and maintenance of voice alarm systems.
these devices should be monitored. This includes wiring of circuits to vibrating pads, although, in practice, these are not always monitored up to the pad itself.
The recommendations in respect of system integrity are somewhat more subtle, and certainly more complex, than those relating to monitor-ing. Indeed, within these recommendations is one that, since it was introduced into the code in 1980, has become the most misunderstood aspect of engineering design within the entire code. Accordingly, it is this recommendation that we will consider first.
At the time of drafting BS 5839-1:1980, concern arose that, if fire damaged a fire alarm sounder circuit, causing the circuit protection to trip, fire alarm sounders would stop operating throughout the building, unless there were more than one circuit. In practice, it was considered that the potential for this to occur is not generally significant enough to have a major impact on the safety of those occupying a building at the time of a fire. After all, it has long been recommended within the code that fire alarm sounder circuits be protected against fire, normally by the use of a fire resisting cable. Moreover, the code has long recommended that joints in cables (which are potential points of weakness in the fire resistance of the overall cable system) be avoided.
Accordingly, the probability that fire disables a sounder circuit before it is detected and occupants have evacuated the building, is very low;
there is certainly no evidence that, in practice, systems designed in accordance with the code have failed to protect occupants adequately as a result of fire damage to a sounder circuit.
However, what of those who have evacuated the building and are awaiting instructions as to whether it is safe to re-enter the building?
The technical committee were concerned that, if all fire alarm sounders stopped operating, these people might reasonably assume that the fire alarm signal had been a false alarm (given that most fire alarm signals are, indeed, false alarms), or that the incident was merely a fire drill.
Again, it could be argued that this is an unlikely event, given that the fire and rescue service should have been summoned and that staff training should be such that occupants do not re-enter a building after evacuation, until they are instructed by someone in responsible charge to do so (after agreement by the fire and rescue service).
Even so, given that even fire resisting cable systems may fail after a prolonged period of time (often at the terminations within devices), it was considered appropriate to address this scenario within the recommend-ations of the 1980 version of the code. The issue was addressed, in the 1980 version of the code, by a recommendation that, in the event of the scenario described, the audible alarm signal should continue to be
Design of fire alarm circuits
given at, at least, a single point in the building. The intention of this recommendation was that, for example, a single fire alarm sounder, located somewhere in the vicinity of the control equipment, would continue to operate. As a result of the recommendation, it became necessary for fire alarm control panels to incorporate at least two sounder circuits; previously, this would not have been necessary.
The control equipment is normally located close to the entrance to the building in a relatively sterile area. This has two benefits. First, the short length of cable between the control equipment and the single bell envisaged in the code is unlikely to be affected by fire. Secondly, the location at which the signal would be given would be appropriate to warn those who might otherwise re-enter the building.
Unfortunately, this recommendation was widely misunderstood. It was interpreted by many as a recommendation that there be ‘dual’
circuits throughout the building, so that, if any one circuit failed, the fire alarm signal would be audible throughout all areas of the building.
While this may, or may not, be a laudable design principle, it was never the intent of the recommendations in the code to achieve this level of integrity.
Moreover, it is not uncommon for those who strive to achieve this level of integrity to ‘interleave’ two fire alarm sounder circuits in each area, so that adjacent sounders are each on the alternate circuit. While such an arrangement would not actually contravene the recommendations of the code, it could be argued that this arrangement less satisfactorily addresses the scenario that the code attempts to address than the single sounder on the second circuit; if fire is severe enough to cause failure of one circuit, it is very likely that it could also damage a second circuit within the same area.
The above considerations only address, of course, a failure of a sounder circuit as a result of fire damage. What of mechanical damage to the circuit or disconnection of the circuit during maintenance?
With regard to mechanical damage, the code already recommends that, where cables are exposed to mechanical damage, there should be suitable mechanical protection. Also, this is where monitoring becomes of importance. An indication of cable fault will be given, and the code recommends that, where maintenance is carried out by a third party, such as a fire alarm maintenance organization, there should be an agreement for emergency call-out to deal with any fault or damage that occurs to the system. Thus, mechanical damage to cables should be a rare event of short duration.
Fire, itself, is also, happily, a rare event. The probability of fire occurring during the short period between occurrence of a cable fault
and repair of the fault is therefore extremely low, to the extent that it has not been considered necessary to further address this scenario within the code. Similarly, durations for which circuits are disabled during maintenance should be short and, in the event of fire, it is likely to be possible to reinstate the circuit.
In view of the above considerations, the recommendation of the code is that, in the event of a single open circuit or short circuit fault on any circuit that serves fire alarm sounders, at least one single fire alarm sounder, normally located in the vicinity of the control and indicating equipment, should still sound correctly if a fire alarm condition occurs anywhere within the building. This fire alarm sounder should have an identical sound to the general fire alarm sounders in the building
In view of the above considerations, the recommendation of the code is that, in the event of a single open circuit or short circuit fault on any circuit that serves fire alarm sounders, at least one single fire alarm sounder, normally located in the vicinity of the control and indicating equipment, should still sound correctly if a fire alarm condition occurs anywhere within the building. This fire alarm sounder should have an identical sound to the general fire alarm sounders in the building