Detector Heads

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These can be divided into four main types Heat detectors, Smoke detectors, Carbon Monoxide detectors and Multi sensors detectors.

Heat detectors

Heat sensitive point detectors

Point detectors can again be subdivided to a further two


  1. Fixed temperature which will operate when it is exposed to a pre-determined temperature. Normally fixed temperature detectors employ a fusible alloy element which must be replaced after the detector has operated. Different temperature rated elements are available to take account of varying ambient air temperatures.
  2. The second type operates on the rate of temperature rise. The rate of rise temperature detector may also include a fusible element for fixed temperature operation.

Both types are suitable for inclusion in open, closed or line monitored systems.

Linear detectors

These can take the form of a heat sensitive cable which will operate, at a predetermined temperature, as an open circuit device. Melting of the cable insulation provides a short-circuit between conductors. After operation the destroyed length of cable must be replaced. Linear detectors may be used in large areas such as warehouses. Alternative types of linear detector exist including the heat pneumatic operating on the

rate of rise principle.

Points to consider

  • Open, closed, fault monitored circuits
  • Temperature setting for fixed temperature fusible elements
  • Spare fusible elements
  • Surface or flush mountings
  • Temperature setting for fusible elements in the rate of rise detection., if included
  • Mounting height
  • Spacing to manufacturer’s recommendations
  • Rate of rise detectors located in positions where abnormal increase in temperature is likely, e.g. space heating equipment, industrial processes

Smoke Detectors

There are three basic types operating by ionization, light scattering and light obscuring.


These generally contain two chambers. One is used as a reference to compensate for changes in ambient temperature, humidity or pressure. The second contains a radioactive source, usually alpha particle, which ionizes the air passing through the chamber where a current flows between two electrodes. When any of the products of combustion enters the chamber the current flow decreases. This drop is used to initiate an alarm.

Light obscuring

In the obscuring type the smoke interferes with a light beam between a light source and photo cell, the variation in photo cell output being used to initiate an alarm. This type of  detection can be used to protect large areas with the source and photo cell positioned some distance apart.

Light scattering

The light scattering detector operates on the Tyndall effect, a photo cell and light source are separated from each other by a darkened chamber such that the light source does not fall on the photo cell. The passage of smoke into the chamber causes the light from the source to be scattered and fall on the photo cell, the cell output being used to initiate an alarm.

The light scattering and light obscuring detectors both,  detect visible smoke. The ionization detector and light scattering detector are normally each a single unit suitable for BESA conduit box mounting. In some models the smoke detector head is attached to the main body by a bayonet fixing for easy removal for maintenance or replacement. It should be noted that some detectors are suitable for two-wire circuits whereas others require three or four wire connections. Smoke detectors require a continuous power supply. Under quiescent conditions they draw a current of some 100 micro amps, and under alarm conditions, some 45 milliamps. This needs to be borne in mind when sizing the power supply. Smoke detectors generally operate on 24 d.c. Refer to British Standard Codes of Practice and manufacturers literature for information regarding the positioning of smoke detectors. Detectors are not suitable for positioning in kitchens, near fireplaces or areas with excessive exhaust fumes, or within 2m of air supply ducts or diffusers.

Carbon Monoxide detector

CO fire detectors are electronic detectors used to indicate the outbreak of fire by sensing the level of carbon monoxide in the air. Carbon monoxide, usually known by its chemical formula CO, is a poisonous gas produced by combustion. They are not the same as CO detectors used for home safety which are used to protect residents against carbon  monoxide produced by incomplete combustion in appliances such as gas fires or boilers.

CO fire detectors use the same type of sensor but are more sensitive and respond more quickly.CO detectors have an electrochemical cell, which senses carbon monoxide, but not smoke or any other combustion products. The cells do not require much power, so the detectors can be made electrically compatible with ordinary smoke and heat detectors. As fire detectors they are effective but only for certain types of fire. Deep-seated, smouldering fires produce carbon monoxide, which can be detected some distance from the seat of the fire. For this type of fire a CO fire detector will probably operate before a smoke detector. Smoke detectors, however, will almost always give a better response to a fire that has produced a rising plume of smoke. CO fire detectors will give a poor response to flaming fires. Because CO fire detectors work on different principles from smoke detectors, their false alarm behaviour will be different. For example, they will not be affected by steam, dust, or by most cooking fumes. However, because of their high sensitivity, they may false alarm from harmless transient levels of CO produced by gas heaters starting up, or from vehicle exhaust fumes entering through a window. These events would not affect an optical smoke detector.

As always, the detector must be selected for the application, to achieve the best balance between fire detection capability and false alarms. There are some known disadvantages of CO fire detectors. One is that the electrochemical cells at the heart of the detectors have a limited life typically seven years and that they are not failsafe. The detector might be “dead” with out this fact being apparent. For this reason a means of checking the CO cell has been incorporated. Another clear disadvantage is the poor response to many types of fire, especially life threatening flaming fires.

Multi sensor detector

This detector combines inputs from optical and heat sensors and processes them using a sophisticated algorithm. When polled by the control panel it returns an analogue count which is determined by combined responses from both optical and heat sensors. They are designed to be sensitive to a wide range of fires and may be used in place of an ionisation detector in many instances.

Operating principles

Signals from the optical smoke chamber and temperature sensor are independent, and represent the smoke level and air temperature respectively in the vicinity of the detector; the detector’s micro controller processes both signals. The temperature signal processing extracts only rate of rise information for combination with the smoke signal. The detector will not respond to slow increases in temperature but a large sudden change can cause an alarm without presence of smoke, if sustained for 20 seconds. The processing algorithms in the multi-sensor incorporate drift compensation.

Points to consider

  • Open or closed circuit
  • Fault monitored circuit
  • System voltage
  • Surface or flush mounting
  • Detector operated indicator
  • Two or three-wire system
  • Quiescent current demand
  • Smoke detector location
  • Spare detector heads

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