Fire detection and fire alarm systems for buildings - code of practice

April 2003 Fire & Safety

Electrical contractors are now required to certify that their work meets the designed specification and the requirements of BS5839-1. ‘Variation’ from the recommendations will require documentation of the rationale and principles used to ensure adequate protection. Transition to the new standard will be complete by July 2003.

The fire industry is traditionally very conservative; its working practices have evolved from the experience gained over many years. In October 2002, a new, dramatically revised edition of BS 5839-1 was released. Why were changes needed and how does this affect the requirements for a fire alarm installation?

Why change?

The last major review of BS 5839-1 was in 1988, making this code of practice 14 years old. Although there have been minor amendments, it was widely accepted that it was out of touch with current custom and practice and did not take account of recent advances in technology.

A more modern approach, likely to be endorsed by forthcoming changes in the UK Fire Safety Legislation, is based on risk assessment and fire-engineered solutions. A fire-engineered solution is where science and engineering are applied to achieve a specific fire safety objective without necessarily following the prescribed recommendations. To allow a fire safety engineer to fully understand and make judgments about what is required needed a more detailed explanation of the intent of a safety measure. Another problem was that the guidelines did not reflect the practicalities of how a typical fire alarm contract worked; they were neither complementary with certification schemes nor suitable for the auditing of a job.

BS 5839-1 has a new structure

BS 5839-1 has a completely new structure following the practice specification format (BS 5588). A useful feature, intended to help understand the requirements, is the provision of informative commentary, separate from the more succinct and auditable recommendations. Greater use of diagrams makes the document easier to read and the structure of the document reflects the progression of a typical fire alarm contract: Design considerations, Limitation of false alarms, Installations, Commissioning and handover, Maintenance and user responsibilities.

Several annexes augment the recommendations with more detailed guidance:

* An annex indicating the system category typically used in various types of premises. This is to assist in choosing the appropriate category of fire detection and alarm system.

* An informative annex providing data on the typical sound levels found in buildings to assist in calculating the requirements for sounders.

* A normative annex that gives the requirements for control and transmission equipment for tactile alarm devices for people with impaired hearing.

* Another normative annex detailing the required calculations for standby battery capacity.

* A flow chart that explains the design process to mitigate false alarms.

* A model system log to record events in managing the system.

* A set of model certificates covering the normal stages of a fire alarm contract: design, installation, commissioning, acceptance, verification, servicing and modification allow separate companies to be responsible for each stage. If separate companies are used, the verification contract acknowledges that a responsible person must verify that the integrated services have achieved the overall objective of satisfying the requirements of BS 5839-1.

What has changed?

Most of the changes address restructuring and clarification of requirements with many of the basic requirements such as the spacing for fire detectors remaining unchanged. However, several advances in fire technology such as multisensors, carbon monoxide sensors and video detection have been included.

New terms and definitions

There are new definitions of the roles for people involved in the process of providing a fire detection and alarm system: Responsible Person, Designer, Installer and User. Each of these roles now has a clear definition of responsibility and the required exchange of information.

A new term 'variation' has replaced 'deviation'; this is a less emotive description of justifiable differences from prescriptive requirements. Variations allow a formal way to identify, document, explain and authorise non-prescriptive solutions. There is recognition of the arbitrary nature of some of the given limits, for example zone size, maximum travel distance, minimum sound pressure levels, etc.

'Two-state' replaces 'conventional' in describing the main types of fire detection systems. There is discussion of the benefits of addressable systems such as reducing false alarms by using alarm verification, coincidence detection filters and other complex signal processing.

System category

The word 'category' is used instead of 'type' when describing a fire alarm system to avoid confusion with the type of detector used.

Fundamental to the design of a system is establishing the level of protection that is required, a process often based on a fire risk assessment by a competent person.

The previous descriptions of system 'Type' have been transferred to an equivalent 'Category' but two new definitions are now provided:

Category L4 is for systems installed within the circulation areas of escape routes, typically corridors and stairways. The objective is to enhance the safety of occupants by warning of smoke in escape routes.

Category L5 is for systems designed to satisfy a specific fire safety objective that does not fall within Category L1, L2, L3 or L4.

Requirements for sounders

In general, there is more flexibility in the requirements for sounders. The general recommendation for sound level remains at 65 dB. However, because a variation of 3 dB is not normally noticeable to the human ear, limited areas with sound levels as low as 60 dB are accepted. Another practical consideration is to ignore a 500 mm border of rooms since people are not likely to occupy this space. Sound levels for stairways and offices up to 60 m2 have been reduced to 60 dB.

Alarm zones - phased evacuation

Another important new term is 'alarm zone'. This term is generally used when describing systems in large buildings that may require phased evacuation or wish to minimise the disruption of a false alarm. An alarm zone typically covers more than one detection zone. For example, each floor of a multistorey building may be an alarm zone containing several detection zones. This will allow separate alert and evacuate signals to be given in different parts of the building. There are some basic rules for alarm zones:

* Alarm and detection zone boundaries should coincide and alarm zone boundaries must be fire resisting.

* All the zones in a building must have the same alert signal and the same evacuation signal, (alert and evacuation signals will be different).

* If signals from adjacent alarm zones overlap, no confusion should ensue.

It has become common practice to use a fire alarm to evacuate a building for events other than fire, such as a bomb threat. For this, a manual evacuation by alarm zone is required.

Alarm zoning and the associated evacuation strategies are typically complex and would require approval by the relevant enforcing authority.

Manual call points

Manual call points, (MCP) are required to operate an evacuation signal within three seconds. This has long been a requirement of BS5839-1 but is somewhat controversial because it is more onerous than and conflicts with the 10 second requirement of EN 54-2. However, after consideration the BS working group decided that this was such an important part of the UK fire alarm strategy that a three second response would be retained. Typically, this is not a difficult specification for modern fire alarm panels to achieve. A note in BS 5839-1 acknowledges the EN 54-2 requirement and suggests that 10 seconds may be an acceptable variation if agreed with the relevant fire authority.

This edition of BS 5839-1 increases the minimum walk distance to an MCP:

* 45 m actual, or 30 m straight line travel distance.

* 25 m actual, or 16 m straight line travel distance for high risk areas.

It is now allowed to mount MCP at 1,4 m + 200 mm. This will allow architects to aesthetically align features.

For multistorey buildings with phased evacuation, MCPs should be mounted inside accommodation rather than on stairways. This is to prevent people escaping from a fire operating an MCP several floors below that on which the fire is actually located and causing an inappropriate evacuation signal.

Cable requirements

BS 5839-1 has now defined two levels of cable fire resistance with guidance on the application of each type.

Standard cables must meet EN 50200: PH 30 and survive 15 minutes shock and fire at 830°C followed by a further 15 minutes of shock and water spray.

Enhanced cables must meet EN 50200: PH 120 and survive 60 minutes shock and fire at 930°C followed by a further 60 minutes of shock and water. Enhanced cables are recommended if the cable routing is non-diverse and for application in any one of the following buildings without sprinklers:

* Buildings with four or more phases of evacuation.

* Greater than 30 metres high.

* Where parts of the building may remain occupied (eg, hospitals).

System monitoring

False alarms can cause costly disruption of a business and waste valuable fire fighting resources. Modern fire alarm systems, (particularly analog systems) include features that will reduce the incidence of false alarms. To help record and monitor such events, false alarms have been categorised as follows:

* Unwanted alarms.

* Equipment false alarms.

* Malicious false alarms.

* False alarms with good intent.

BS 5839-1 also suggests acceptable rates of alarm. A typical office should incur less than one false alarm per 100 detectors per year. An industrial site should incur less than one false alarm per 75 detectors per year. Any site with more than 40 false alarms per year requires investigation.

Other requirements

To achieve high reliability and to reduce false alarms, the components of a fire detection and alarm system should have third party certification to the relevant standard. In addition BS 5839-1 defines several other requirements for a system:

All critical signal paths must be monitored.

Large systems must be configured such that a system fault will not cause the loss of more than 512 detectors.

Recommendations are given for wireless systems and for hearing impaired systems.

Conclusion

Introduced in October 2002, BS 5839-1 provides a clear explanation of the requirements and guidelines for fire detection and alarm systems. Those involved in the design, installation and maintenance of fire detection and alarm systems must fully understand BS 5839-1. The BFPSA and other reputable bodies offer suitable courses.

For more information contact Honeywell SA, 011 695 8000.





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