Alex-from BS5839-1
If the system Category is such that automatic fire detection should be provided in any area that
contains a horizontal void of 800 mm or more in height, automatic fire detection should also be provided
in the void. Voids less than 800 mm in height need not be protected, unless either:
1) the void is such that extensive spread of fire or smoke, particularly between rooms and
compartments, can take place before detection; or
2) on the basis of a fire risk assessment, the fire risk in the void is such as to warrant protection of the
void.
NOTE 3 In Category P systems, it is common practice to protect all floor voids in electronic data processing installations,regardless of their depth, if the voids contain cables. Usually, this is on the basis of the probability of ignition and the amount of combustible material within the void, in conjunction with the consequences of a fire within the void in respect of business continuity. Guidance on protection of electronic data processing installations is given in BS 6266.
NOTE 4 If the fire risk within a void of 800 mm or more is considered to be low, consideration might be given to omission of fire detection from the void, subject to the agreement of the interested parties (see Clause 6), but this should be recorded as a variation on the relevant system certificate. This might arise, for example, if the probability of ignition and development of fire in the void were very low, or if the void were limited in extent so that spread of fire beyond the room of origin, via the void, were unlikely.
