This article is a continuation of the series of advices given by Rafiki, starting with the fire risk assessment and Fire Detection & Alarm Systems BS5839, and continuing with the system zoning, manual call points, automatic detectors, smoke detectors, heat detectors, detection in apex roofs, fire alarm sounders, and cabling.
- A ‘Manual Call Point’ is a device which enables personnel to raise an alarm in the event of a fire incident by pressing a frangible element to activate the alarm system.
- The Manual Call Points should be installed at a height of 1.2m above floor level at easily accessible, conspicious positions, on exit routes, at the entry floor landings of staircases and at all exits to the open air.
- The Manual Call Points should be spaced so that one may always be found within a maximum distance of 45m apart or 25m for disabled person.
When deciding on the type of detector to be used in any area it is important to remember that the detector has to discriminate between a genuine fire and the normal conditions existing therin, like the smoking and staff rooms, steam from ensuite bathrooms, kitchen fumes, vehicle and forklift truck fumes in warehouses, etc. Generally, all types of detectors should be sited on the ceiling at the highest point of the area to be covered. Detectors mounted at greater heights have a reduced efficiency and in these cases further advice should be sought.
In open spaces under flat horizontal ceilings, every point should lie within 7.5m of a smoke detector.
Smoke detection should be generally avoided in the following areas to avoid unwanted alarms. They should be protected by means of other detectors such as heat detectors.
- Contamination in dusty areas may cause unwanted alarms and reduce the life of the detector.
- Damp or humid conditions such as showers, bathrooms and external areas should be avoided as the water vapor may cause unwanted alarms and reduce the life of the detector.
- Detectors should never be used at low temperature where ice or condensation can affect detector sensitivity.
- Kitchens, garages, welding shops and boiler houses should generally be avoided.
- In open spaces under flat horizontal ceilings, every point should lie within 5.3m of a heat detector.
- Heat detectors are designed to either detect a rapid rise in temperature or to operate at a fixed temperature. Although they provide a slower response time than smoke detectors they do provide a method of protection for areas where smoke detectors cannot be used.
- Heat detectors should not be used for the protection of life or where extensive property loss may be expected.
Types of heat detectors
- ‘Rate of Rise’ Heat Detectors
‘Rate of Rise’ heat detectors respond to both rapid increases of temperature and to fixed top temperatue.
- ‘Fixed Temperature’ Heat Detectors
Fixed temperature heat detectors are available with different temperature settings, and are normally installed in kitchens, boiler rooms, etc.
Detection In Apex Roofs
If the ceiling is pitched or sloping, smoke will tend to rise towards the highest point (apex) of the roof, therefore detection should be placed in the apex. As the slope tends to reduce the delay before smoke or heat reaches the detectors, it is permissible to use a greater spacing between the detectors mounted there.
The spacing of the smoke detectors in the apex only, may be increased by 1% for every degree of slope of the ceiling up to a maximum of 25%.
- Fire Alarm Sounders should be installed throughout the building with an even distribution, to generally provide a minimum sound level of 65dB(A) or 5dB(A) above any background noise which is likely to persist for more than 30 seconds.
- Where the alarm may have to arouse sleeping persons e.g. Hotel bedrooms, nursing homes, etc, a minimum sound level of 75dB(A) is required, at the bed head with all the doors shut.
- All fire alarm sounders in a building should produce the same sound, distinct from any other audible warning devices in the building.
- Where fire alarm sounders are required in extremely noisy areas (e.g. machine shops), it may be necessary to install additional ‘Visual Indication Beacons’.
The operation of a ‘Fire Alarm and Detection System’ depends on the cabling and connections between the components. It is essential that connection between Manual Call Points and Detectors function correctly when they are operated. The cables within the system are required to function correctly for significant periods after being attacked by fire. These include the power supply cables to the control panel, the detection circuits and the fire alarm sounder circuits. Thus the cables chosen must be correctly rated to withstand these conditions.
This is the second part of the Rafiki advices for Fire Alarms(the first part is here), some of the guidelines provided by Rafiki(intelligent fire technology) – check out the Rafiki series of industrial/commercial fire alarm systems (like the Rafiki Twinflex), found at The Conventional and Analogue Fire Alarms. For more details about the fire alarms, visit the Fire-Detect website(see a list of articles about the fire alarms here and here).