System Sensor BS5839

6Note: 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.

1.6. DRIFT COMPENSATION AND MAINT ENANCE ALARM

The sensitivity of a smoke detector t ends to change as it

becomes contaminated with dirt or du st (see ﬁgure 1.6.1). As

contamination builds up, it usually become s more sensitive,

leading to the risk of a false alarm, but in some ca ses can

become less sensitive, so delaying the alarm if a ﬁr e is

detected. To counter this, if a detector drif ts outside its

speciﬁcation, a maintenance signal may be se nt to the panel

warning that the detector needs cleaning.

To further increase the maintenance inter val, many systems

incorporate a “drift compensatio n” function, included in either

the detector or the control panel algo rithms. These functions

use algorithms that monitor the sensiti vity of a detector, and

modify its response to compensate f or a build up of dust in

the chamber over time. Once the detector reac hes the “drift

limit” when the dirt build up can no longer be comp ensated

for, a fault can be signalled. Some systems also incor porate

a warning to signal that a detector is approac hing its

compensation limit and requires cleaning.

Figure 1.6.1 Chamber Contamination and Drift

Compensation

1.7. PRE-ALARM FACILITY

One advantage of intelligent type sys tems is that since the

data sent by a detector to the panel varies wi th the local

environment, it can be used to detect wh en the device is

approaching an alarm condition. This “Pr e-Alarm” can be

signalled at the panel and can therefore be inve stigated to

check if there is a real ﬁre, or if it is caused by oth er signals,

for example steam or dust from building work . This can

avoid the inconvenience and expense of evacuating a buil ding

or calling out the ﬁre brigade unnecessarily b ecause of a

nuisance alarm. The Pre-Alarm Thr eshold is typically set at

80% of the alarm threshold.

1.8. FIRE ALARMS

When a ﬁre is detected, the control pan el indicates an alarm

by activating the ﬁre indicator for the rele vant zone on the

control panel, sending a command to the relevan t detector

to illuminate its LED and activate alarm sig nals to start

evacuation. Most intelligent ﬁre system c ontrol panels include

alphanumeric displays enabling them to show info rmation

on the source of the alarm. This may simply be a zone an d

detector address, or could be more desc riptive for example

“Smoke Detector, Bedroom 234”. The control panel may a lso

use control modules to operate additio nal electrical equipment

such as air conditioning units and door rele ases to prevent the

spread of smoke and ﬁre.

The alarm signals can either be a zone of convent ional

sounders and strobes activated vi a control modules on the

loop or directly from the control pan el, or addressable loop

powered devices connected on the same loop a s the detectors

and activated by direct command from the p anel. Loop

powered sounders tend to have lower wiring cost s, however

the number permissible on the loop may be restr icted by

current limitations.

On larger sites, it may be desirable to use zoned a larms. This

allows a phased evacuation to be carried out , with areas

at most immediate risk being evacuated ﬁrst , then less

endangered areas later.

1.9. FIRE SYSTEM ZONES

Conventional ﬁre alarm systems group de tectors into

‘zones’ for faster location of a ﬁre, with a ll the detectors in

a particular zone being connected on one cir cuit. Although

intelligent systems allow the precise device t hat initiated an

alarm to be identiﬁed, zones are still used i n order to make

programming the system and interpret ing the location of a ﬁre

easier. The control panel will have individual ﬁre indi cators

for each zone on the system, and the control p anel response

to an alarm is often programmed accor ding to the zone of the

device in alarm rather than its individua l address.

Whilst the division of a loop into zones is achieve d within the

panel software, BS5839 p art 1 recommends that a single

wiring fault in one zone should not affect t he operation

of the system in other zones of the building. To meet this

recommendation, a short circuit isola tor should be placed on

each boundary between zones (ﬁg ure 1.9.1). In this instance,

a short circuit in one zone would cause the isol ators on either

side of the zone to open, thereby disabling that zo ne. Any

devices in neighbouring zones would be protec ted by the short

circuit isolators and remain operati onal.

Figure 1.9.1 Intelligent System Fire Zones

Intelligent Fire Alarm Systems

ChamberValue

Time

Clean Air

Value Uncompensated

AlarmThreshold

Uncompensated

ChamberValue Compensated

Threshold

Smoke required to

reach alarm

threshold reduces -

Detector sensitivity

increases

Threshold

increased to

compensate for

increased chamber

clean air value.

ISOLATOR

Zone 1

Zone 2

Zone 3

Zone 4

INTELLIGENT

FIRE ALARM

CONTROL

PANEL

FIREALARMSYSTEMOK

28January2003

12:15pm

SYSTEM OK

SYSTEM RESET

FIRE ALARM

FAULT