System Sensor BS5839 manual Intelligent Fire Alarm Systems, PRE-ALARM Facility, Fire System Zones

Intelligent Fire Alarm Systems

1.6. DRIFT COMPENSATION AND MAINTENANCE ALARM

The sensitivity of a smoke detector tends to change as it becomes contaminated with dirt or dust (see figure 1.6.1). As contamination builds up, it usually becomes more sensitive, leading to the risk of a false alarm, but in some cases can become less sensitive, so delaying the alarm if a fire is detected. To counter this, if a detector drifts outside its specification, a maintenance signal may be sent to the panel warning that the detector needs cleaning.

To further increase the maintenance interval, many systems incorporate a “drift compensation” function, included in either the detector or the control panel algorithms. These functions use algorithms that monitor the sensitivity of a detector, and modify its response to compensate for a build up of dust in the chamber over time. Once the detector reaches the “drift limit” when the dirt build up can no longer be compensated for, a fault can be signalled. Some systems also incorporate a warning to signal that a detector is approaching its compensation limit and requires cleaning.

Threshold increased to compensate for increased chamber clean air value.

Value	Smoke required to
Chamber	reach alarm
	threshold reduces -
	Detector sensitivity
	increases

Time

Clean Air	Uncompensated	Uncompensated	Compensated
Value	Alarm Threshold	Chamber Value	Threshold

Figure 1.6.1 Chamber Contamination and Drift

Compensation

1.7. PRE-ALARM FACILITY

use control modules to operate additional electrical equipment such as air conditioning units and door releases to prevent the spread of smoke and fire.

The alarm signals can either be a zone of conventional sounders and strobes activated via control modules on the loop or directly from the control panel, or addressable loop powered devices connected on the same loop as the detectors and activated by direct command from the panel. Loop powered sounders tend to have lower wiring costs, however the number permissible on the loop may be restricted by current limitations.

On larger sites, it may be desirable to use zoned alarms. 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 detectors into ‘zones’ for faster location of a fire, with all the detectors in a particular zone being connected on one circuit. Although intelligent systems allow the precise device that initiated an alarm to be identified, zones are still used in order to make programming the system and interpreting the location of a fire easier. The control panel will have individual fire indicators for each zone on the system, and the control panel response to an alarm is often programmed according to the zone of the device in alarm rather than its individual address.

Whilst the division of a loop into zones is achieved within the panel software, BS5839 part 1 recommends that a single wiring fault in one zone should not affect the operation

of the system in other zones of the building. To meet this recommendation, a short circuit isolator should be placed on each boundary between zones (figure 1.9.1). In this instance, a short circuit in one zone would cause the isolators on either side of the zone to open, thereby disabling that zone. Any devices in neighbouring zones would be protected by the short circuit isolators and remain operational.

One advantage of intelligent type systems is that since the data sent by a detector to the panel varies with the local environment, it can be used to detect when the device is approaching an alarm condition. This “Pre-Alarm” can be signalled at the panel and can therefore be investigated to check if there is a real fire, or if it is caused by other signals, for example steam or dust from building work. This can avoid the inconvenience and expense of evacuating a building or calling out the fire brigade unnecessarily because of a nuisance alarm. The Pre-Alarm Threshold is typically set at 80% of the alarm threshold.

1.8. FIRE ALARMS

When a fire is detected, the control panel indicates an alarm by activating the fire indicator for the relevant zone on the control panel, sending a command to the relevant detector to illuminate its LED and activate alarm signals to start evacuation. Most intelligent fire system control panels include alphanumeric displays enabling them to show information

FIRE ALARM SYSTEM OK 28 January 2003 12:15 pm

SYSTEM OK

FIRE ALARM

FAULT

SYSTEM RESET

INTELLIGENT FIRE ALARM CONTROL PANEL

ISOLATOR

Zone 1

Zone 2

ISOLATOR

Zone 3

Zone 4

ISOLATOR

on the source of the alarm. This may simply be a zone and detector address, or could be more descriptive for example “Smoke Detector, Bedroom 234”. The control panel may also

Figure 1.9.1 Intelligent System Fire Zones

6

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.