Specifications

DESCRIPTION OF MAJOR
FEATURES
Smoke alarms incorporating either an Ionisation or an Optical sensor are available in mains only, mains
with Alkaline battery back-up, or mains with rechargeable Lithium back-up formats. Heat Alarms are
also available in these ranges, but these only sense heat, not smoke. A comprehensive Fire Alarm
system will contain all 3 elements of detection.
Ionisation Smoke Alarms contain a small sealed ionising source which causes a small electrical current to
flow through the air in the smoke chamber. Smoke particles entering the chamber reduce the current,
this is sensed by the electronics and when a pre-set threshold level is reached the alarm will sound.
Optical Smoke Alarms send a pulsed beam of infra red light through the smoke chamber periodically. If
visible smoke is present, it scatters the light on to the photodiode light receiver and this is detected by
the electronics causing the alarm to sound. Ei Optical Alarms are individually calibrated to ensure correct
operation and to reduce the risk of nuisance alarm.
These are fixed temperature alarms which incorporate a thermistor sensor. The thermistor is a heat
sensitive resistor, when the ambient temperature reaches a pre-set point of 58˚C, the resistance is
lowered and the alarm will sound. They are designed for use in areas where due to high levels of dust or
fumes, a conventional Smoke Alarm is not suitable. They are particularly suited for use in a kitchen
(source of 40% of domestic fires), garages or boiler rooms. Although they are stand alone units they
should be interconnected with our Smoke Alarms to provide an adequate warning system in the event
of a fire. BS 5839: Pt.6 recommends the use of fixed temperature heat detectors in kitchens in
preference to ‘rate of rise’ types due to the reduced risk of nuisance alarm which can be caused by
opening hot oven doors etc.
Both types have a sufficiently wide range of response to different types of fire to be of general use.
However:
Ionisation Alarms tend to respond quicker to fast flaming fires producing small smoke particles (e.g
flaming wood, petrol). This makes them particularly sensitive to the “invisible” particles produced by
toasting, grilling or frying. They are also susceptible to nuisance alarm when redecoration takes place. In
view of this, they should be located well away from the sources of such fumes or particles.
Optical alarms tend to respond faster to slow smouldering fires producing large smoke particles (e.g
smouldering wood, overheating PVC wiring). They are somewhat more susceptible to contamination from
dust and fibres so they should not be installed in particularly dusty areas. Both types should be
removed, or fully covered when renovation work is being carried out.
Both of the smoke alarm types give good performance in a range of situations. The BS 5839: Pt.6
British Standard that deals with fire alarm systems in dwellings was amended in 2004 and as before
recommends that the choice of detector should take account of the type of fire that may be expected
and the need to avoid false alarms. BS 5839: Pt.6 now strongly recommends the use of optical
alarms in circulation spaces of a property, such as hallways and landings and areas in which a likely
1. What types of
Smoke Alarms
are available?
3. How do Ei Heat
Alarms work?
4. What are the
advantages and
disadvantages
of both types
of Smoke
Alarms?
5. Which Alarm
should I use
where?
2. How do Ei
Ionisation and
Optical Smoke
Alarms work?
2