5
between the detector and the sources of potential fires. Small concentrations of these types of gases may not
absorb enough UV radiation to cause a problem, but when higher concentrations of these gases are present the
detectors may become blind as not enough ultra-violet radiation can reach them to activate an alarm. Moving
detectors closer to the probable source of fire and increasing the sensitivity of the detector can help to overcome
this problem in some cases. For a list of some common UV absorbing chemical vapours see
Appendix B
.
False Alarms
Arc Welding, long duration lightning flashes, cracked mercury lamps, cracked halogen lamps, and X-rays may
cause false alarms and should be avoided.
NOTE:
Ultraviolet detectors are sensitive to arc welding and if this type of radiation can be expected, nuisance
alarms must be controlled through proper application including careful positioning and shielding of
the detectors. Some applications may require a UV/IR system.
CAUTION:
Caution must be exercised if the detection system is turned off, as the hazardous area will not be
protected.
.
Lightning strikes can trigger the detector but this can be solved by pointing the sensor cone of vision below the
horizon and setting the time delay to 7 seconds.
Cracked mercury or halogen lamps in the vicinity of the fire detector should be replaced as soon as they are
discovered.
UV radiation other than that produced by an actual fire is referred to as “background UV”. An example of a high
level of background UV could be a flare stack situated outside of a building. The UV radiation produced by this
flare may be detected when a door to the building is opened. Windows or other reflective surfaces may also cause
unusually high levels of UV radiation to enter the building from the flare. In a situation like this, the fire
detection system response must be carefully checked and the sensitivity level adjusted high enough, so that this
“background UV” will not cause false alarms.
UV fire detectors respond to radiation other than ultraviolet. X-rays can activate the detector and are often used
in industrial inspection. It may be necessary to disable the system when X-ray is conducted nearby.
Position and Density of Detectors
The UVU-140-A & UVU-140-AR fire detectors have a nominal 120 degree cone of vision when measured in
accordance to NFPA 72 guidelines. In an application such as a loading dock with a ceiling height of 25 feet (7.5
metres) where it is desired to have complete detector coverage at floor level and a detector is mounted 2 feet (0.6
metres) from the ceiling and pointed straight down, the distance from the detector to the designated level would
be 23 feet (7 metres). The detector has a 120 degree cone of vision and would cover a circular area 80 feet (24
metres) in diameter at floor level. A sketch of the area to be covered will indicate the number of detectors
required to monitor the area. Detectors should be placed as close as practical to the expected fire hazard.
NOTE:
Do not mount UV detectors close to the ceiling of enclosed buildings if smoke might accumulate
before the break-out of flame. It is preferable to mount the detectors on walls a few feet (about 1
