Fire alarms and detectors

The simplest form of fire alarms are manually operated devices such as gongs or bells. These are suitable only for the smallest premises where one of the devices can be heard throughout the whole building. 

Audible messages can also be part of the alarm system particularly where members of the public are present. These will alert the occupants to the fact of a fire and can be used to direct them to the nearest fire exits by the safest routes. Tests have shown that the public react more quickly to an audible message than they do to a bell or other sound. The standards required for voice alarms are contained in BS 5839 Part 8 and EN 60849.  

Automatic fire alarms have the advantage of being able to raise the alarm in the event of a fire in an unoccupied or unmann area. They operate by detecting particular changes in the environment. Different types of detectors have been developed to react to the different stages in the development of a fire. The different characteristics that may be used in detecting a fire include  

1 variations in the strength of transmitted beams or rays between transmitter and receiver, whether visible light, infra-red or radioactive, caused by the rising hot products of combustion  

2 the visual interference (obscuration) of a light beam caused by smoke from burning materials  

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