Ionisation Alarms
where should they be used?
Ionisation type sensors are particularly sensitive to the almost invisible
smoke produced by fast flaming fires.This makes them more liable to
false alarm due to cooking fumes if sited in a hallway close to a
kitchen. Ionisation alarms are less vulnerable to false alarms caused
by dense tobacco smoke,excessive dust and insect ingress. The
BS 5839: Pt.6: 2004 Standard recommends that ionisation alarms
should not be used in hallways and landings,where there is a risk of
false alarms caused by cooking fumes.
The Importance of Type & Interconnection of4
Heat Alarms
where should they be used?
Heat alarms are less likely to cause false alarm problems as they
are not responsive to any type of smoke or fumes,only heat.
Because of the potential for a slower response than smoke alarms,
they should only be used in a fire alarm system that also includes
smoke alarms,and all of the alar ms must be interconnected.The
BS 5839: Pt.6: 2004 recommends that heat alarms should be used in
kitchens.It goes on to suggest that they may also have a role to
play in the main living room but they should not be installed in
circulation spaces or areas where fast response to fire is required.
Optical Alarms
where should they be used?
Optical sensors are more responsive to smouldering fires producing
large particle smoke typical of fires involving furniture and bedding.
They are more immune to invisible smoke produced by 'burning the
toast' and similar cooking fumes.This makes them ideal for siting in
hallways close to kitchens where false alarms from ionisation alarms
may be a particular problem.The BS 5839: Pt.6: 2004 Standard
recommends the use of optical alarms in circulation spaces of a
dwelling,such as hallways and landings. Optical alarms are prone to
false alarm if exposed to steam and should not be located too close
to poorly ventilated bathrooms or shower rooms.
Note: Either type of sensor is generally suitable.The choice of sensor type should, if possible,take into account the type of fire
that might be expected and the need to avoid false alarms.
1. A light beam is pulsed in the sensor chamber every 10 seconds to ‘look’for
smoke.Any smoke present has to be visible to the naked eye so that the
receptor can ‘see’it.If no smoke is detected, the alarm will remain in a
standby state.
2. When large particle smoke is detected,the light beam will be scattered onto
the light receptor.
3. This will then send an electrical signal to the IC (Integrated Circuit).
4. If two consecutive signals are received by the IC,the alarm will sound.
1. Inside the sensor chamber is a minute (safe) radioactive element that ionises
the air within.This causes a small current to flow in the chamber and this will
remain constant for the life of the alarm unless smoke particles enter.
2. When smoke enters the sensor chamber,the balance of the current
is disturbed.
3. This is detected by the electronics in the alarm circuitry and a signal is sent to
the Integrated Circuit (IC).
4. This causes the alarm sounder to operate.
1. A thermistor (a heat sensitive resistor) is sited in the sensor chamber of
the alarm.
2. When the temperature rises the resistance of the thermistor reduces.
3. The IC continuously monitors the resistance of the thermistor.When this
indicates the temperature is over 54 ºC the IC sends a signal to the
sounder circuit.
4. The alarm sounder then operates.