5
95-8546
17.3
FALSE ALARM SOURCES
UV:
The UV sensor is solar blind to the ultraviolet
component of solar radiation. However, it may
respond to sources of UV besides fire, such as arc
flash, electric arc welding, grinding metal, lightning,
high voltage corona, x-rays, and gamma radiation.
NOTE
Radiation generated by false alarm sources such
as periodic lightning or sparks in the area may be
effectively ignored by the detector using the arc
rejection feature or time delay.
IR:
The detector has been designed to ignore steady
state infrared sources that do not have a flicker
frequency characteristic of a fire, however, it
should be noted that if these steady state infrared
sources are hot enough to emit adequate amounts
of infrared radiation in the response range of the
IR sensor and if this radiation becomes interrupted
from the view of the detector in a pattern
characteristic of a flickering flame, the IR sensor
can respond.
Any object having a temperature greater than
0° Kelvin (–273°C) emits infrared radiation. The
hotter the object, the greater the intensity of the
emitted radiation. The closer the infrared source is
to the detector, the greater the potential for a false
alarm. The IR sensor can respond to IR radiation
sources that can meet the amplitude and flicker
requirements of the detector such as vibrating hot
objects.
Although the detector is designed to reduce
false actuations, certain combinations of ambient
radiation must be avoided. For example, if IR
radiation with an intensity that exceeds the fire
threshold of the IR sensor should reach the detector
as a flickering signal, and if at the same time
an electric arc welding signal also reaches the
detector, an alarm output will be generated.
FACTORS INHIBITING DETECTOR RESPONSE
Windows
Glass and Plexiglas windows significantly attenuate
radiation and must not be located between the detector
and a potential flame source. If the window cannot be
eliminated or the detector location changed, contact
Det-Tronics for recommendations regarding window
materials that will not attenuate radiation.
Obstructions
Radiation must be able to reach the detector in order
for it to respond. Care must be taken to keep physical
obstructions out of the line of view of the detector. In
addition, UV or IR absorbing gases or vapors must not
be allowed to accumulate between the detector and
the protected hazard. See Table 3 for a list of these
substances.
Smoke
Smoke will absorb radiation. If accumulations of dense
smoke can be expected to precede the presence of a flame,
then detectors that are used in enclosed areas should be
mounted on the wall approximately 3 feet (0.9 m) from the
ceiling where the accumulation of smoke is reduced.
Detector Viewing Windows
It is important to keep the detector viewing windows as
free of contaminants as possible in order to maintain
maximum sensitivity. Commonly encountered substances
that can significantly attenuate UV and/or IR radiation
include, but are certainly not limited to, the following:
–
Silicones
–
Oils and greases
–
Dust and dirt buildup
–
Paint overspray
–
Water and ice
