Alarm Sensor Types - How to choose which Sensor to Use

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 sitting 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.

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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.

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.

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.

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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.

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.

Wiring Diagram for Mains Powered Smoke and Heat Alarms

Use standard BS 6004 PVC cable typically 1.5mm2 6243Y three core and earth.
For 'R' models a fourth interconnect wire is required. Please ask for wiring diagrams.

Important - Interconnection is vital to ensure the alarm is heard throughout the property by all occupants.

• Incorrectly wiring the Live and Neutral wires will result in irreparable circuit damage and the unit will have to be replaced.
• The alarms must not be connected when the house wiring insulation is being tested with high voltages i.e. do not use an insulation tester on the alarms, as this will cause irreparable damage.