Choice of detector and detector positioning

Question 1

It is essential when choosing a fire detection system
that the designer selects a system best suited to
the needs of the building. The final choice will
depend on:

Answer 1

(i) The speed of response required to satisfy the
fire safety objective, which might be life
safety or property protection or both.
(ii) The need to minimise false alarms from the
system.
(iii) The nature of the fire hazard.
(iv) The form of evacuation strategy to be
employed in a building, e.g. single or staged

Question 2

General points re heat detectors

Answer 2

1. less sensitive to fires in their early
   stages
2. suitable in areas where smoke or fumes could create false alarms.
3. not suitable for protecting areas where a warning of smoke is crucial, e.g. escape routes.

Question 3

General points re ionisation smoke detectors

Answer 3

1. good at small particles found in clean burning
   fires
2. less sensitive to larger
   particles such as those generated by a smoldering
   fire.

Question 4

general points re optical smoke detectors

Answer 4

1. better at detecting large particles
2. less sensitive to clean burning fires producing little visible smoke
3. light scatter principle are more sensitive than those using smoke obscuration.

Question 5

general points re beam detectors

Answer 5

1. Beam detectors are both economical and efficient in large undivided areas, such as cathedrals or large concert halls
2. they must be mounted onto a rigid structure that is unlikely to distort or move by wind or load etc

Question 6

general points re infra red and ultra violet flame detectors

Answer 6

1, Infra-red and ultra-violet detection is suitable
where flaming fires can be expected.
2. inability of ultra-violet detectors to penetrate
smoke should be noted

Question 7

Aspirating systems are useful where

Answer 7

n Highly sensitive equipment, such as computers,
are housed in enclosed cabinets.
n Aesthetics preclude the use of point detectors
but where holes for small diameter capillary
tubes are acceptable.
n Areas where the environment is unsuitable
for point detectors, e.g. cold rooms and
where sampling tubes can be connected to
monitors outside of the cold area.

Question 8

general points re gas detectors

Answer 8

1. Carbon monoxide detectors may be immune to
   influences such as tobacco, dust or steam that
   could result in false alarms if conventional smoke
   detectors are used
2. They react faster than if heat
   detectors were used as an alternative to smoke in
   those circumstances.

Question 9

In areas with flat ceilings, the horizontal distance
between any point and a detector should not
exceed

Answer 9

n 7.5m for smoke detectors

n 5.3m for heat detectors