Fire alarm monitoring of external circuits

12.Fire alarm Monitoring, integrity and reliability of circuits external to control equipment
12.1 Commentary
External circuits external to the fire alarm control equipment, need to be arranged such that the probability of faults that could prevent the fire system from giving a fire warning is minimized. Such faults will, however, occur occasionally. Work on the fire alarm system, for
the purpose of modification, repair or routine fire alarm maintenance, may also cause an impairment of the fire alarm system to operate as designed in the event of fire.

Measures are incorporated within this code of practice to limit the probability of impairments, whether as a result of faults or work on the fire system, and to limit the duration of any impairment. The wiring of all critical signal paths is protected against mechanical damage and damage by fire. Critical signal paths are monitored, so that faults will be identified quickly. Compliance with the recommendations for fire alarm maintenance arrangements ensures that any such fault is quickly repaired. The probability that a fault will exist at the
time of a fire is, therefore, considered to be extremely low. In the event of fire, fire damage to wiring during the period required for evacuation is unlikely to occur in view of the recommendations of this code of practice for the fire resistance of cables.
As a result of the above considerations, duplication of, or redundancy in, external circuits is not normally necessary. However, in the case of circuits serving fire alarm devices, special measures need to be taken to ensure that, during, or following, evacuation, damage to any wiring, with resultant loss of the evacuation signal, does not imply to occupants of the building that it is safe to remain in, or re-occupy, the building.
This can be achieved by ensuring that, in the event of a short circuit developing on any fire alarm device circuit during a fire (e.g. at a cable termination), a minimum of one audible fire alarm device continues to
sound as a reminder that the system has not been silenced or reset. Normally, this sounder is located close to the control and indicating equipment, as this equipment will be sited in a suitable area (e.g. a reception area, security control room or main entrance foyer) at which the reminder needs to be given.