System Sensor BS5839 manual Selection of Automatic Fire Detectors

2.3. SELECTION OF AUTOMATIC FIRE DETECTORS

Smoke detectors are the most sensitive automatic means of detecting a fire and should be used wherever conditions allow.

2.3.1. Ionisation smoke detectors

Ionisation smoke detectors use a weak radioactive source to ionise the air between two electrodes, creating positive and negative ions and so allowing a small current to flow across the chamber. Smoke particles attract these ionised particles, and allow positive and negative ions to recombine, thus reducing the number of ions and hence the current flow.

Environmental regulations concerning the radioactive source used in ion detectors means that they are now becoming obsolete, and most major manufacturers are no longer including ionisation detectors in new ranges.

2.3.2.Photoelectric smoke detectors

Photoelectric or optical smoke detectors work by generating pulses of infra red light and measuring any diffracted light. If smoke is present in the sensing chamber, the light is diffracted by the smoke particles onto a photodiode, which senses the presence of the smoke (see figure 2.3.1). They are now largely replacing ionisation detectors as a general purpose detector.

Without Smoke: Chamber is designed so that light from the IR-LED does not reach the receiver

Smoke Present : Light from the IR-LED is reflected off the smoke particles onto the receiver, triggering an alarm signal.

heat sensors, which can give a response to fast flaming		Application Guide
fires similar to that of ionisation detectors. Other sensor
combinations are also available.
Multi-Criteria	Optical
Alarm
	Alarm
ChamberValue	Chamber
	Response
	Alarm
	Threshold
	Heat
	Response
	Time

Figure 2.3.2. Photo-Thermal Detector Response

2.3.4.CO Detectors

A recent addition to BS5839 is CO detectors. These generally use an electro-chemical sensor to detect carbon monoxide given off by incomplete combustion. They provide reliable detection of incipient fires whilst giving good assurance against nuisance alarms. However the chemical cells used in these detectors have a limited life span, and they cannot detect fast burning fires due to the low CO levels produced.

2.3.5. Heat Detectors

Heat detectors are normally used in environments where a smoke detector might generate false alarms, for example kitchens or shower rooms.

Rate of Rise heat detectors will alarm if the temperature rises very quickly, or if the temperature reaches a set threshold. This type of detector would be the first choice in an environment where a smoke detector could not be used.

In some environments, such as boiler rooms, fast rates of rise of temperature can be expected normally, meaning that there would be a risk of false alarms when using a rate-of-rise device. In this case a fixed temperature detector would be suitable. As their name implies, fixed temperature detectors give an alarm once the temperature has reached a preset threshold, most commonly 58°C or 78°C for EN54-5 Class AS or BS respectively.

2.3.6.Optical Beam Detectors

Figure 2.3.1. - Operation of Optical Chamber

Photoelectric smoke detectors are tested across the complete range of EN54 fires, however they are most sensitive to smoke containing large particles from around 0.4 to 10 microns, such as that given off by smouldering fires. A photoelectric detector would therefore be a good choice in an environment where a slow burning fire could be expected, such as a room containing modern fabrics and furnishings.

2.3.3.Multi-criteria Detectors

Multi-criteria detectors comprise two or more sensors within the same housing, integrated by the detector electronics or software to give a rapid response to a broader range of fires and greater immunity to nuisance alarms. The most common type at present is a combination of optical and rate of rise

Optical beam detectors work on the principle of projecting a beam of light across a room, which is attenuated when smoke is present thus allowing an alarm to be given (Figure 2.3.3). There are two forms of beam detector: emitter and receiver separate (single path), requiring separate wiring both to the emitter and receiver, and reflective in which the emitter and receiver are mounted in the same box, and the beam is shone onto a reflective material at the far side of the room (dual path).

Since an optical beam detector senses smoke across the entire smoke plume, it tends to be less affected by smoke dilution as the ceiling height increases than point type smoke detectors. In addition, a single beam detector can protect a large area; hence they are particularly suitable for protecting large high rooms such as sports arenas, warehouses and shopping malls.

Note: This document is based on the recommendations of BS5839 Part 1: 2002. It is intended only as a guide to the application of fire detection systems.

Reference must be made to relevant national and local standards.

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