Heat detection

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Heat detection

Most heat detectors are of the ‘point’ type. Point detectors respond to the particular characteristic phenomenon that they are designed to detect, simply at a single point in space. Thus, a point-type heat detector relies principally on convection to transport hot gases from the fire to the detector. The relevant standard for point-type heat detectors

is BS EN 54-5.⁴⁵ In this standard, detectors are classified according to their temperature of operation or, in the case of detectors intended for normal ambient temperatures, their sensitivity.

Fixed temperature heat detectors behave like thermostats, in that they are designed to respond at a specific temperature. In view of the thermal inertia of the device, in a growing fire the fire gases in the vicinity of the detector will significantly exceed this temperature. In a fixed temperature/rate of rise heat detector, the detector will respond if the temperature rises sufficiently rapidly, even though the notional fixed temperature of operation has not yet been reached.

It is possible to manufacture heat detectors that respond only when the rate of rise of temperature is sufficient, and there is then no ‘long stop’

temperature at which the detector will operate when the temperature is increasing at a very slow rate. While the latter detectors have been used on the continent, they have never been accepted in the United Kingdom, as they might fail to detect a slowly developing fire. These detectors would not now comply with BS EN 54-5, as this standard now contains a test to confirm the ability of a detector to respond correctly to a slow rate of rise of air temperature.

In a line heat detection system, rather than detecting the hot gases at a point in space, the detector is capable of sensing heat along a line in space. Therefore, most line heat detection systems comprise a heat sensitive cable. Either the cable insulation melts catastrophically at a specific temperature, or the impedance of the insulating material is temperature dependent. Line heat detection systems tend to be most suitable where the geometry of the protected area is long and narrow.

Classic examples are cable tunnels and the areas below escalators, particularly in, for example, underground railway stations. In the case of underground railway stations, the line heat detection is installed to satisfy legislative requirements and operates an extinguishing system.

Heat detectors are relatively insensitive devices, compared with the most obvious alternative of smoke detectors. BS 5839-1:1988 suggested that, as a simple rule of thumb, flames will reach about one-third of the distance from the floor to the ceiling before heat detectors will operate.

For this reason, they are often regarded as somewhat old-fashioned and suitable for use only in situations in which a smoke detector would be unsuitable (e.g. because of the potential for false alarms). This is not exactly the stance adopted by the code, which, instead, specifies merely four areas in which heat detectors should not be used. These are discussed below.

45 BS EN 54-5, Fire detection and fire alarm systems — Heat detectors — Point detectors.

Which type of detector?

The first situation comprises areas of a Category P system in which a small fire has the potential to cause unacceptable damage. The obvious example of this would be a room containing critical electronic equipment, on which, for example, real time data processing or a production process might depend. In the latter situation, general use of heat detectors would also be precluded by the recommendations of BS 6266.⁴⁶ In these situations, extensive damage would occur to the equipment before a heat detector operated. Similar considerations might apply in, for example, a stately home that houses fine art that could be destroyed before operation of a heat detector and subsequent fire fighting action.

The second situation comprises escape routes within a Category L system. Again, it is essential that, in such a system, there is early warning of fire or smoke within escape routes. Heat detectors would be unsuitable for this purpose. However, the code accepts that heat detectors may be used in all other areas of any Category L system. The obvious example would be within rooms opening onto escape routes (e.g. bedrooms within sleeping risks, such as hotels and hostels) in a Category L3 system.

Equally, the code does not preclude the use of heat detectors in any areas, other than escape routes, even in a Category L1 system.

More generally, the code recommends against the use of heat detection in areas in which the production of smoke could present a threat to occupants’ escape before it is likely to be detected by people or heat detection. Again, however, a note within the code implies that this situation is not intended to relate to typically sized bedrooms, but that the recommendation against the use of heat detection could apply in the case of dormitory accommodation or rooms intended for mobility-impaired disabled people, who require additional time to escape from a fire in their bedroom.

Finally, as something of a truism, the code points out that heat detectors should not be used in areas in which they would have a high potential for false alarms. In practice, if the ambient temperature is sufficient to cause false alarms from heat detectors, heat detectors with a higher temperature of operation are used. Therefore, generally, if it is found that heat detectors are causing false alarms as a result of their environment, it is usually because heat detectors responding to rate of rise of temperature have been used in an area in which there is rapid fluctuation in temperature; an example would be commercial kitchens, in which the temperature above ovens can rise rapidly when oven doors are opened.

46 BS 6266, Code of practice for fire protection for electronic equipment installations.