Fire Design – fire safety design, stage 4 – Siting automatic detectors

In this article, which is a continuation of the Fire Safety Design stages 1, 2, and 3, you can read about: the spacing of detectors in normal applications: for pitched roof, near obstructions or unusual ceilings; siting optical beam detectors, and other considerations.

Spacing of detectors in normal applications

Under flat horizontal ceilings – smoke and heat detectors:

Spacing the smoke detectors(categories P + L) on the centre line in escape routes and corridors
Width of area Radius of detection To wall Between
1.00 7.50 7.50 15.00
2.00 7.50 7.50 15.00
3.00 7.50 7.35 14.70
4.00 7.50 7.23 14.46
5.00 7.50 7.07 14.14
6.00 7.50 6.87 13.75
7.00 7.50 6.63 13.27
7.50 7.50 6.50 12.99
8.00 7.50 6.34 12.69
9.00 7.50 6.00 12.00
10.00 7.50 5.59 11.18
10.60 7.50 5.30 10.60
Spacing heat detectors(Category P) on the centre line in corridors(not escape routes)
Width of area Radius of detection To wall Between
1.00 5.30 5.30 10.60
2.00 5.30 5.30 10.60
3.00 5.30 5.08 10.17
4.00 5.30 4.91 9.82
5.00 5.30 4.67 9.35
6.00 5.30 4.37 8.74
7.00 5.30 3.98 7.96
7.50 5.30 3.75 7.50

Design stage 4.1: Detector spacing for pitched roofs

Generally mount the first row of detectors in apex except where woof rise is less than 150mm for heat or 600mm for smoke, in which case consider as flat ceiling. Coverage increases by 1% for every degree of angle of pitch to a maximum of 25%. If slope differs either side, so will coverage of detectors in apex.

Design stage 4.2: Siting detectors near obstructions or unusual ceilings

Detectors should be sited no nearer than twice the depth of the obstruction.

  • If greater than 250mm, do not site the detector nearer than 500mm.
  • If the obstruction is greater than 10% of floor to ceiling height, treat it as a wall.
  • If the gap between partition or top of racking is less than 300mm, treat as wall.
  • Remember to keep a clear space of greater than 500mm from any detector.
  • Low beams or rafters – refer to table 2 of BS 5839-1:2002, detectors may not be required between each one depending on their construction.
  • For ceiling of floor voids – mount detector in top 10% or 125mm, whichever is the greater.
  • More info – consult section 22.3 of BS 5839-1:2002

Design stage 4.3: siting optical beam detectors

Like “Point Detectors” the furthest distance between the wall and line of detector should be more than 7.5m.

  • If used under a pitched ceiling, you may increase coverage by 1% for every degree of angle of pitch up to 25%. If pitch varies either side, so will the coverage.
  • If used below 600mm of ceiling height in order to provide supplementary detection of a rising smoke plume, for ex. the width of the area protected on each side of the beam should be regarded as 12.5% of the height of the beam, and the nearest likely seat of fire.

Notes:

  • Transmitters and receivers should be mounted on a solid construction, not something that will move with temperature or wind;
  • Avoid mounting beams where high probability of people or equipment may obstruct the beam;
  • Do not hire beam less than 500mm from along side any wall/obstruction;
  • Additional beam detectors can be sited at lower levels if smoke is unlikely to reach the apex due to a natural heat barrier.

Design stage 4.4: further considerations

  • For a type P1 system, every part of the building should be suitably protected. For this purpose each enclosed space should be considered separately.
  • Areas covered by a Type P2 system should be separated from unprotected areas by a fire resisting construction.
  • Voids which are not more than 800mm deep need to be protected unless fire can spread through them from one to the other or it is identified in a Fire Risk Assessment, as a potential High Risk;
  • Where rooms are divided by partitions or storage racks reaching to within 300mm of the ceiling each section should be protected separately.
  • Shafts for elevators, lifts, hoists, escalators and enclosed chutes through floors and stairways should be protected by detectors at the top of the shaft and within 1.5m of openings of each floor.
  • Enclosed staircases should be protected by detectors on each main landing within the staircase.
  • Lantern lights should be protected by a detector unless they are less than 800mm in height above the ceiling and are not used as ventilation.
  • Extra detectors may be needed to cope with structural features within a room.
  • Consideration must be given to possible adverse air flows when air conditioning and ventilation systems are in use.
  • If a detector is concealed, it may be desirable to provide a remote visible indication of its operation.
  • Care should be taken when siting to ensure that adverse conditions such as high levels of shock or vibration are not encountered.

To be continued with the next steps in Fire Safety and Fire Design.

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