Fire Alarm design, protection of life

(17) In some buildings, a fire risk assessment might determine that an unacceptable risk to occupants can be reduced by the installation of a fire alarm or a wireless fire alarm, with fire detectors in only selected rooms or areas in the building. A Category L5  fire system might then be suitable, A Category L5 fire system may form part of a fire engineering
solution, in which automatic fire detection is just one component of the solution.
if after conducting a fire risk assessment, there is a need for a reliable means of warning of the presence of smoke in escape routes, a Category L4 designed fire alarm system, in which smoke detectors are installed in escape routes, such as corridors and stairways, may be appropriate. For example, this can arise from the presence of lone workers in a large, generally unoccupied area of a building in which the fire hazard level is assessed to be relatively high.

In buildings that are listed or it is difficult to install wiring, a wireless fire alarm should be considered. In buildings in which people sleep within rooms accessed by corridors, other than short lengths of corridor, research has shown that passage of hot fire gases through a door crack can produce smoke sufficiently dense
and cool for a corridor to become smoke-logged before adequate warning can be given by detectors in the corridor. In this case, a Category L3 fire alarm system, in which detectors are installed in rooms that open onto the escape routes, as well as within the escape routes themselves, is likely to be appropriate. Nowadays, this decision is taking after a fire risk assessment has been carried out to identify the possible risk from fire. The purpose of the
detectors in these rooms is to give an early enough warning to occupants, other than, possibly, the person in the room of fire origin, such that they have adequate time to escape before their escape route is impassable as a result of smoke. Accordingly, within the rooms, the use of one or more of the following types of detectors
is satisfactory: heat, smoke, or wireless fire alarm detection.