There are four main types of fire or smoke alarm that are in common use in homes: ionisation, optical, heat and combination alarms. Each type has its particular advantages and disadvantages in detecting the two most common types of fire: flaming fires, which produce light smoke particles; and smouldering fires, which produce heavier smoke particles. 

 

Ionisation Smoke Alarms 

 

Ionisation detectors are inexpensive and consist of simple circuitry in which a small current is continuously passed through an ionised air gap between two charged plates. If smoke is present, it disrupts the flow of current, which triggers the alarm. Ionisation alarms are most sensitive to the light smoke particles produced by flaming fires, such as those caused by fast burning material like paper or wood. They are slightly less sensitive to the heavier smoke particles produced by smouldering fires. 

 

Optical Smoke Alarms 

 

Optical smoke detectors work by making use of the photo-electric effect. A light-producing emitter and sensor are placed together in a chamber but aligned so that the light misses the sensor. When smoke enters the chamber, it deflects the light onto the sensor, which sets off the alarm. Optical alarms are efficient at detecting the heavier smoke particles of smouldering fires, such as from burning blankets, cushions or mattresses, but are slightly less responsive to the lighter smoke particles of flaming fires. 

 

Heat Alarms 

 

Heat alarms, as their name suggests, are sensitive to heat rather than smoke. When the surrounding temperature exceeds a safe level, the alarm is triggered. Being insensitive to smoke is both an advantage and disadvantage. It cuts down on false alarms caused by normal cooking and is the only type that is of practical use in a kitchen. A disadvantage is that smoke from a fire in another part of the house may be present but not yet hot enough to trigger the alarm. 

 

Combination Alarms 

 

Where it’s impossible to predict what type of fire is most likely to occur, combination alarms can provide the best overall safeguard. These are simply a combination of two different types of detector. Working together ensures that the disadvantage of each type is compensated by the advantage of the other. Depending on the particular model, fire alarms of all types can be powered by long-life batteries, by mains electricity, or by both. Some alarm systems can also be linked so that if one alarm is triggered, it triggers others in all parts of the building, ensuring that the alarm will be heard by everyone.