System-sensor A05-1003-002 Manuel d'utilisateur télécharger pdf (Page 15)

APPLICATIONS GUIDE: SYSTEM SMOKE DETECTORS

What to Do About

Unwanted Alarms

No detection system is imper-

vious to unwanted alarms.

Statistically, as the system size and the total number of

detectors increases, the total number of nuisance alarms

per year tends to increase. Historical experience in a given

installation or data on similar sized buildings with similar

utilization patterns can provide a basis for a rough indica-

tion of how many nuisance alarms are probable during a

12 month span; however, no two installations are identical.

In small- to moderate-sized detection systems protecting

relatively combustion-free environments, like office build-

ings, more than one or two unwanted alarms per year

would be unusual. In more adverse environments, such as

laboratory or manufacturing facilities where combustion

processes are present, more frequent alarms can be antici-

pated. In very adverse environments, one alarm per month

might not be considered excessive.

After the first few months, which serve as a shakedown

period, it should be possible to arrive at some reasonable

expectation for probable unwanted alarms from the sys-

tem. After that, any unexpected change in frequency or dis-

tribution indicates a problem that should be investigated.

The best way to monitor alarm frequency and distribution

is to maintain an alarm log.

Reasons for Unwanted Alarms

Unwanted alarms can result from a wide variety of causes,

including:

• Improper locations are environments where they will

not operate properly because of temperature extremes;

excessive dust, dirt, or humidity, excessive air flow

rates, or the normal presence of combustion particles

in the air streams surrounding the detectors.

• Improper installation can occur when detectors and

their wiring are not protected from interference from

induced currents and noise in adjacent wiring systems,

radio-frequency transmissions, and other types of elec-

tromagnetic effects.

• Inadequate maintenance can result in the accumula-

tion of dust and dirt on the detector’s sensing cham-

bers over a period of time.

• Seasonal effects such as the reactivation of a building

heating system after an extended summer shutdown

can cause alarms.

• Building maintenance issues, such as accidental trig-

gering of a detector’s magnetic test switch, or the

introduction of plaster dust from drywall repairs into a

detector’s sensing chamber can cause unwanted

alarms.

• Induced current effects from lightning storms can

cause alarms.

• Infestation from insects small enough to enter the

detector’s sensing chamber.

•Vandalism or mischievous acts — detectors set off as a

prank have been found to be a problem in dormitories.

If an alarm occurs and a fire does not exist, the alarm

should be silenced, the problem unit located, and the

alarm system controls reset so that the effectiveness of the

detection system is restored.

Make sure that all the detectors in the zone or pinpointed

device(s) that show an alarm are checked before deciding

that it is a false alarm. If a fire does exist, more than one

detector may be in the alarm state, although no signs of fire

may be evident in the vicinity of the first activated detec-

tor. The fire could be overlooked.

Maintain an Alarm Log

The next step for all alarms should be entry of a report into

an Alarm Log. A typical Alarm Log is shown in Appendix

2 (page 19). Such a log serves immediate and long-term

purposes.

The Alarm Log indicates which individuals responded to

the alarm and whether or not they took appropriate action.

Periodic review of the cumulative Alarm Log can help those

responsible for the detection system discern patterns in the

reported alarms. Generally, several months (or even years)

of data may be necessary before patterns begin to emerge.

In a worst case example, a pattern of repeated alarms or

small fires in a particular area may indicate a serious defi-

ciency in safety practices that should be promptly correct-

ed. In less obvious cases, patterns are indicated by repeat-

ed alarms in the same or adjacent zones with similar prob-

able causes, or repeated alarms in the same zone that

occur at about the same time of day, or time of year.

Effects of Location or Environment

Check for the effects of location and environment. Review

the information in this guide Where To Place Detectors and

Where NOT to Place Detectors to determine whether the

detector’s location or its environment is potentially causing

the unwanted alarms. Also, refer to the installation manu-

al for further information.

One often overlooked source of problems is the placement

of detectors where air streams carry smoke or chemical

fumes from some areas of an installation past detectors in

other areas unrelated to the source of the contaminants.

Diagnosing problems of this kind requires that air move-

ments into the problem area, especially near the ceiling, be

carefully checked and their sources be determined.

Experienced heating, ventilating, and air conditioning

(HVAC) engineers or contractors usually have the training

and specialized equipment (flow meters, etc.) to conduct

such a study. In very difficult cases, a full-scale smoke test

may be required to solve the problem.

Conversely, strong air streams near air inlet or supply

ducts, etc. can also prevent a detector from signaling an

alarm when a fire is present by blowing smoke away from

the detector heads.

Inspect Detector for Dirt and Review Maintenance

If the Alarm Log indicated that after several months or a

year with a fairly stable alarm rate, there is a gradual

increase in the frequency of unwanted alarms, this is usu-

ally an indication that the detectors in the system should

be cleaned.

Section 6

Troubleshooting

Techniques

1 2 ... 10 11 12 13 14 15 16 17 18 19 20 21

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