Design & Zoning — Fire Alarm Systems (BS 5839-1) Practice

Sample questions

1. 01What is the maximum number of detectors that should be connected to a single zone?

   • ANo specific limit
   • B20
   • C32
   • D50

   Why: BS 5839-1 places no limit on the number of detectors in a zone. A zone is bounded by floor area (2 000 m², clause 12.3a) and by search distance (60 m, clause 12.3b) — both concerned with how quickly someone responding can locate the fire, not with device counts. Figures like 32 per zone or 99 per loop are panel and loop-card constraints set by the manufacturer, not recommendations of the standard.

2. 02Where should the sensitive element of a ceiling-mounted smoke detector sit relative to the ceiling?

   • ABetween 25 mm and 600 mm below the ceiling
   • BFlush with the ceiling surface
   • CAt least 600 mm below the ceiling
   • DBetween 25 mm and 150 mm below the ceiling

   Why: Clause 21.2.4a): the sensitive element of a smoke detector should be between 25 mm and 600 mm below the ceiling; 21.2.4b) gives 25 mm to 150 mm for a heat detector. 600 mm is the maximum depth, not a minimum. The 25 mm lower bound keeps the element out of the stagnant boundary layer held against the ceiling itself, and the upper bound keeps it inside the layer of hot smoke spreading beneath it.

3. 03In open-plan offices, what factor determines detector spacing?

   • AThe number of staff employed in the open-plan area
   • BBoth ceiling height and furniture layout
   • CThe colour of the floor covering that is used
   • DThe brand of detector the contractor has fitted

   Why: Detector spacing depends on both ceiling height and furniture layout, as obstructions and ceiling height affect smoke flow patterns.