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14
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