| 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.
- 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.
- When large particle smoke
is detected, the light beam will be scattered onto the light receptor.
- This will then send an
electrical signal to the IC (Integrated Circuit).
- If two consecutive signals
are received by the IC, the alarm will sound.
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.
- 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.
- When smoke enters the sensor
chamber, the balance of the current is disturbed.
- This is detected by the
electronics in the alarm circuitry and a signal is sent to the
Integrated Circuit (IC).
- This causes the alarm sounder
to operate.
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 alarms 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.
- A thermistor (a heat sensitive
resistor) is sited in the sensor chamber of the alarm.
- When the temperature rises
the resistance of the thermistor reduces.
- 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.
- The alarm sounder then
operates.
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