System Sensor BS5839

Note: This document is based on the recommendatio ns of BS5839 Part 1: 2002. I t is intended only as a guide to the ap plication of ﬁre detection systems.

Reference must be made to relevant national and local stan dards.

Application Guide

heat sensors, which can give a response to f ast ﬂaming

ﬁres similar to that of ionisation detecto rs. Other sensor

combinations are also available.

Figure 2.3.2. Photo-Thermal Detector Response

2.3.4. CO Detectors

A recent addition to BS5839 is CO d etectors. These generally

use an electro-chemical sensor to de tect carbon monoxide

given off by incomplete combustion. T hey provide reliable

detection of incipient ﬁres whilst givin g good assurance

against nuisance alarms. However the chemical c ells used

in these detectors have a limited life span, an d they cannot

detect fast burning ﬁres due to the low CO l evels produced.

2.3.5. He at Detectors

Heat detectors are normally used in env ironments where a

smoke detector might generate false alar ms, for example

kitchens or shower rooms.

Rate of Rise heat detectors will alarm if t he temperature

rises very quickly, or if the temperature rea ches a set

threshold. This type of detector wo uld be the ﬁrst choice in an

environment where a smoke detector could no t be used.

In some environments, such as boiler roo ms, fast rates of

rise of temperature can be expected nor mally, meaning that

there would be a risk of false alarms when using a ra te-of-rise

device. In this case a ﬁxed temperature det ector would be

suitable. As their name implies, ﬁxed temper ature detectors

give an alarm once the temperature has reac hed a preset

threshold, most commonly 58°C or 78°C for E N54-5 Class AS

or BS respectively.

2.3.6. Op tical Beam Detectors

Optical beam detectors work on the pr inciple of projecting a

beam of light across a room, which is atten uated 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), re quiring separate wiring both to the

emitter and receiver, and reﬂective in which t he emitter and

receiver are mounted in the same box, and the be am is shone

onto a reﬂective material at the far side o f the room (dual

path).

Since an optical beam detector senses smo ke across the

entire smoke plume, it tends to be less affec ted by smoke

dilution as the ceiling height increases than p oint type smoke

detectors. In addition, a single beam de tector can protect a

large area; hence they are partic ularly suitable for protect ing

large high rooms such as sports aren as, warehouses and

shopping malls.

2.3. SELECTION OF AUTO MATIC FIRE DETECTORS

Smoke detectors are the most sensiti ve automatic means of

detecting a ﬁre and should be used wherever co nditions allow.

2.3.1. Ionisation smoke detectors

Ionisation smoke detectors use a weak ra dioactive source to

ionise the air between two electrode s, creating positive and

negative ions and so allowing a small current to ﬂow ac ross

the chamber. Smoke particles attract th ese ionised particles,

and allow positive and negative ions to recombi ne, thus

reducing the number of ions and hence the curren t ﬂow.

Environmental regulations concern ing the radioactive source

used in ion detectors means that they are n ow becoming

obsolete, and most major manufacture rs are no longer

including ionisation detectors in new ra nges.

2.3.2. Pho toelectric smoke detector s

Photoelectric or optical smoke detec tors work by generating

pulses of infra red light and measuring any dif fracted light.

If smoke is present in the sensing chamber, the light is

diffracted by the smoke particle s onto a photodiode, which

senses the presence of the smoke (see ﬁgur e 2.3.1). They

are now largely replacing ionisation detec tors as a general

purpose detector.

Figure 2.3.1. - Operation of Optical Chamber

Photoelectric smoke detectors ar e tested across the complete

range of EN54 ﬁres, however they are most sen sitive to smoke

containing large particles from ar ound 0.4 to 10 microns,

such as that given off by smouldering ﬁres. A p hotoelectric

detector would therefore be a good choi ce in an environment

where a slow burning ﬁre could be expected, such a s a room

containing modern fabrics and furni shings.

2.3.3. Mul ti-criteria Detector s

Multi-criteria detectors com prise two or more sensors within

the same housing, integrated by the detec tor electronics or

software to give a rapid response t o a broader range of ﬁres

and greater immunity to nuisance alarms . The most common

type at present is a combination of opti cal and rate of rise

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.

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.

Time

ChamberValue

Alarm

Threshold

Optical

Alarm

Chamber

Response

Heat

Response

Multi-Criteria

Alarm