Design considerations

Whereas the previous section should be read by all interested parties who need to use the code, the clauses within Section 2 are directed towards ‘the designer’. Indeed, with an element of tautology, the designer is defined in the code as the person or organization taking responsibility for the work outlined in Section 2 of the code.

8 Relationship between system category and areas protected

In effect, this clause takes forward the concept of system categories and, other than in the case of a Category M system, relates these to the rooms or areas that should be protected by automatic fire detection.

Consideration is also given to the type of automatic fire detection that should be installed (e.g. heat, smoke, combustion gas, or multi-sensor detectors).

9 Actuation of other fire protection systems or safety facilities This clause is very short, since the actuation of other fire protection systems or safety facilities is outside the scope of the code. The prime purpose of the clause is to refer the reader to other codes and to ensure that the interconnection of the fire alarm system to another system does not impair the reliability of the fire alarm system to give warning in the event of fire. Thus, it would be a misconception to assume that reference should be made to BS 5839-1 for information on, for example, the siting of smoke detectors that are provided purely for the specific purpose of, say, causing the automatic closure of fire resisting doors or shutters.

(This is addressed in BS 7273-4.¹⁰)

10 Systems in explosive gas or dust atmospheres

This clause is the shortest in the code and simply refers the reader to the relevant European standards on these matters.

11 System components

This clause is primarily a list of product standards, to which the various components of the system (e.g. manual call points, various types of fire detector, control and indicating equipment, power supply equipment, cables, etc.) should comply. In the majority of cases, these are the relevant parts of BS EN 54.¹¹

12 Monitoring, integrity and reliability of circuits external to control equipment

This is the first ‘meaty’ clause of the code. The recommendations of the clause are subdivided into those concerned with fault monitoring and those concerned with system integrity. The purpose of the clause is to ensure that the probability of faults that could prevent the system from giving a fire warning is minimized and that, when such faults occur, or work on the system occurs, the extent and duration of the impairment of the system is limited.

10 BS 7273-4:2007, Code of practice for the operation of fire protection measures – Actuation of release mechanisms for doors.

11 BS EN 54, Fire detection and fire alarm systems.

Contents of the code

Within this clause are, arguably, the recommendations of the code that have been most misunderstood within previous versions of the code, namely those relating to the principle whereby, in the event of fire damage to a sounder circuit, a limited audible warning can still be given at a single point in the building. In order to amplify and clarify this recommendation, the code contains two relevant diagrams.

13 Detection zones

In previous versions of the code ‘detection zones’ were merely referred to as ‘zones’. In the current code, the adjective ‘detection’ is intended to distinguish these zones from ‘alarm zones’. While some of the recommendations of this clause are applicable to all detection zones (i.e. those containing either manual call points or automatic detectors or both), separate recommendations are given for detection zones that contain only manual call points, detection zones that contain non-addressable automatic fire detectors and those containing non-addressable automatic fire detectors.

One parameter that may limit the size of a detection zone is the

‘search distance’ (see Chapter 12 of this guide). This concept, which was first introduced in the 1988 code, has been commonly misunderstood.

Accordingly, a diagram within this clause assists in the understanding of search distance.

14 Alarm zones

Alarm zones were a new concept introduced into the 2002 code. By definition, an alarm zone is a geographical subdivision of the protected premises, in which the fire alarm warning can be given separately, and independently, of a fire alarm warning in any other alarm zone. The code provides some simple recommendations concerning the subdivision of the premises into alarm zones. The need for this will, of course, not occur in premises in which single phase evacuation is used (i.e. all areas of the building are evacuated simultaneously).

15 Communication with the fire and rescue service

This clause highlights the importance of rapid summoning of the fire and rescue service on every occasion that the fire alarm system operates, unless, in order to avoid false alarms being passed to the fire and rescue

service, filtering arrangements are appropriate (see Chapter 25). In previous versions of BS 5839-1, means for automatic transmission of fire alarm signals to the fire and rescue service were discussed and, if such facilities for automatic transmission were provided, recommendations were given.

However, as well as providing recommendations to ensure the early summoning of the fire and rescue service and the reliability of automatic transmission facilities, the 2002 code, for the first time, introduced recommendations regarding the circumstances in which automatic transmission facilities positively should be provided. These are discussed in Chapter 13 of this guide.

16 Audible alarm signals

The prime purpose of this clause is to set minimum sound pressure levels for the audible fire alarm signal. Guidance is given on a minimum sound pressure level for all buildings, but with an increased sound pressure level for bedrooms in premises in which people sleep. A new feature, introduced in 2002, is a small relaxation in minimum sound pressure level for stairways, small cellular rooms and very limited areas. Separate recommendations are given for hospitals and residential care premises.

In the former case, reference is made to HTM 05-03 Part B,¹² whereas, in the latter case, minimum sound pressure level in bedrooms depends on whether or not the alarm is intended to arouse the occupants or whether reliance is placed on staff for this purpose.

Guidance is also given in this clause on the practical measurement of sound pressure level. This guidance is quite important, as, according to whether or not it is adopted, a system may ‘pass’ or ‘fail’ compliance with the code.

17 Visual alarm signals

This clause is analogous to the previous clause, but relates to practical considerations in the siting and performance of visual alarm signals.

Generally, of course, visual alarm signals are only used to supplement audible alarm signals, primarily in areas of high ambient noise level,

12 HTM 05-03 Part B, Alarm and detection systems. Published by the Department of Health in England and Wales. (In Scotland, the relevant code of practice is SHTM 82 published by NHS Scotland.)

Contents of the code

where ear defenders may be used. However, the guidance given in this clause would also be applicable to situations in which visual alarm signals are used to warn deaf people of fire or where the disturbance of an audible warning is undesirable (e.g. television and radio studios, cinemas, theatres and hospital operating theatres). The Loss Prevention Certification Board (part of BRE Certification), in conjunction with the Fire Industry Association, have prepared a new code of practice on visual alarm devices, to supplement the guidance in BS 5839-1. This new guidance has been taken into account in the 2013 version, and part of it is reproduced as Annex F in the 2013 edition of BS 5839-1.

18 Fire alarm warnings for people with impaired hearing

Given the need to make all buildings accessible and usable by disabled people, this is a very important clause of the code, which was first introduced in the 2002 version. The majority of the recommendations in this clause are concerned with systems that use vibrating pagers to warn deaf and hard of hearing people of fire. The purpose of the clause is to ensure that such an arrangement has, as far as practicable, a similar degree of reliability and integrity to the audible fire warning facilities provided for people with normal hearing. It is important to note that, in the case of vibrating pager systems, the recommendations of this clause are supplemented by the recommendations given in Annex C of the code.

19 Staged fire alarms

This clause deals with fire alarm systems in which the initial warning of fire is given only in a restricted area, or is even restricted to key staff, but can be extended in further stages so that, ultimately, all occupants of the premises are given an evacuation signal. Thus, the clause deals with both systems that can give what is normally described as a two-stage alarm and those that have a facility for a ‘staff alarm’. In the latter case, the warning of fire is restricted to key staff, either so that they can prepare for an evacuation or can investigate to determine whether the incident is a false alarm. Staff alarm arrangements are becoming increasingly common to avoid unnecessary summoning of the fire and rescue service to false alarms.

20 Manual call points

This clause recommends the type of call points that should be used in all premises, and it provides recommendations for the location and siting of manual call points. Recommendations are also given for the maximum delay that should occur between operation of a manual call point and the sounding of the fire alarm signal within the detection zone in which the manual call point is located.

This clause also introduces the suggestion that, in public car parks, where there is a risk of malicious operation of call points, an emergency voice communication system (e.g. an intercom system) might be used as a means of raising the alarm, rather than manual call points. Such systems should comply with BS 5839-9.¹³

21 Types of fire detector and their selection

This clause ‘sets out the stall’ of automatic fire detectors that may be used in a system (i.e. heat detectors, smoke detectors, combustion gas detectors, flame detectors and multi-sensor fire detectors). The principle of operation of each type of detector is briefly examined and there is considerable discussion regarding the factors to consider in selection of detector type. This is, possibly, the most textbook-type clause of the code, and, therefore, the commentary is considerably longer than the recommendations. However, recommendations are given regarding the situations in which the various types of detector should, or should not, be used.

22 Spacing and siting of automatic fire detectors

This very lengthy clause provides all the detail required by the designer regarding the spacing of automatic fire detectors and their siting, particularly siting in relation to physical features of the building, such as structural beams, partitions, etc. Special guidance is given for what has become a common perplexity, namely the siting and arrangement of detectors in ‘honeycomb’ ceilings and between structural joists and beams. Maximum ceiling heights at which the various types of fire detector should be used are also given.

13 BS 5839-9, Fire detection and fire alarm systems for buildings – Code of practice for the design, installation, commissioning and maintenance of emergency voice communication systems.

Contents of the code 23 Control and indicating equipment

This clause provides recommendations for the indicating facilities that should be provided, the siting of control and indicating equipment and the environment in which the equipment should be sited.

24 Networked systems

This clause considers a number of special recommendations that apply when a number of fire alarm panels are ‘networked’ together. The rec-ommendations that apply depend on whether the resulting networked system is, in effect, a single fire alarm system with outstations or a number of separate fire alarm systems reporting back to a single point.

25 Power supplies

It is a common misconception that the 230 V mains power supply for the system is not actually part of the fire alarm system, but is simply a local mains supply within the building. Clause 25 of the code makes it very clear that this is incorrect, and it provides a considerable number of recommendations that apply specifically to the mains power supply.

Equally, the clause provides guidance on standby power supplies (i.e. batteries), and gives recommendations for the duration for which the standby supplies should be capable of operating the system. The actual method by which the capacity should be calculated from the recommended duration is given in a normative annex, which forms part of the code, thereby ensuring that all designers adopt the same approach in the determination of the appropriate battery size.

26 Cables, wiring and other interconnections

The prime purpose of the recommendations in this clause is to ensure that the cable type and installation methodology is suitable to maintain the integrity of the fire alarm system for a sufficient duration in the event of fire. In order to achieve this, various recommendations in respect of installation practice are contained within this clause.

However, the overwhelmingly most important matter addressed by this clause is that of the fire resistance of the cable itself. Two different performance levels for fire resisting cables are described, thereby defining two ‘grades’ of fire resisting cable (‘standard’ and ‘enhanced’).

Having defined these grades of fire resistance, the recommendations of this clause include a description of the situations in which only the enhanced grade of fire resisting cable should be used.

27 Radio-linked systems

This clause provides recommendations for systems in which manual call points, detectors and sounders may be linked to control equipment by radio, rather than by wiring. A recommendation is also made in respect of compliance with BS EN 54-25, the standard with which radio-linked systems should comply.

28 Electromagnetic compatibility

This clause provides some basic recommendations designed to ensure that the fire alarm system is not unduly prone to malperformance or failure as a result of electromagnetic interference.

29 Electrical safety

This clause provides recommendations in respect of matters such as earthing and avoidance of electric shock to people from the fire alarm system.