Last edited 29 Apr 2019

Fire detection and alarm system FDAS

Most fire detection and alarm systems operate on the same basic principles. If a fire is detected, then an alarm is triggered. This warns building managers and occupants that there may be a fire and that evacuation may be necessary. Some systems include remote signalling equipment which can alert the fire brigade or a remote monitoring centre.

Fire can be detected by; heat detectors, flame detectors, smoke detectors, carbon monoxide detectors and multi sensor detectors, or an alarm can be triggered at manual call points. Alarms may consist of bells, sirens, horns, lights or a combination these. Two power supplies are required, generally a mains supply and batteries providing 24 hours back up.

It is important that a thorough assessment of need is undertaken before a fire detection and alarm system is designed or purchased.

In the UK, fire alarm systems are categorised as:

L, (L1 to L5): automatic systems intended for the protection of life.
M: manual systems, fitted with sounders and call points.
P, (P1 and P2): automatic systems intended for the protection of property.

Fire detection and alarm systems can be divided into a number of general types:

Conventional systems.
Addressable systems.
Analogue addressable systems.
Wireless systems.
Self-contained units.

Conventional systems generally consist of a series of detectors and call points wired to a control panel which drives the detectors and a minimum of two sounder circuits, includes LED indicators and allows de-activation and resetting. Typically, separate circuits will be provided for each fire 'zone' (usually a floor of a building or a fire compartment). This separation into zones means that the approximate location of the fire is known and so the appropriate response can be instigated. It also allows for easier diagnosis of faults.

Addressable systems are similar to conventional systems, but the central control panel can identify exactly which detector or call point triggered the alarm (rather than just a zone), or whether communication has been lost with a detector. In this system the circuit is wired in a loop, with a number of detectors or call points on each loop. The loop can be powered from both ends, so that it continues to function even if there is a break in the loop (separate loops may still be provided for each zone).

The control panel can be programmed to show specific information, or trigger specific responses for different detectors within the system. Addressable systems are generally used for larger or more complex installations because of the benefits of more accurate detection, and so fault finding, and the reduced wiring requirement.

Analogue addressable systems, or intelligent systems can include an analytical capability in each detector which can assess local parameters to determine whether there is a fire, a fault or a maintenance requirement. This can be useful in preventing the occurrence of false alarms. A pre-alarm warning may be indicated if a detector is approaching a trigger condition.

Wireless fire alarm systems connect detectors and call-points to the control panel using wireless signals.

Self-contained fire alarm units are generally only suitable for small installations. They consist of a single unit, including break glass contact, sounder, power supply, battery and charger.

It is important that fire alarm and detection systems are regularly inspected, serviced and tested, and fire drills may be held to ensure occupants are familiar with fire and evacuation procedures.

NB: The 2014 BRE briefing paper, The causes of false fire alarms in buildings, found that; 'False alarms generated from remotely monitored fire detection and fire alarm systems cost businesses and Fire and Rescue Service (FRS) authorities an estimated £1bn per year in the UK. In the period 2011-2012 a total of 584,500 fire and false alarms were reported in Britain, 53.4% of these were not fires and therefore considered false alarms. This is a considerable drain on FRS authorities as well as causing business disruptions leading to a loss of productivity and reducing the confidence of the general public.'

The paper proposed that education could contribute significantly to reducing false alarms and that the increased use of multi-sensor detectors is a cost effective way of averting false alarms from common causes such as cooking fumes and steam.