Vortex
18
3.6 Cabling
The cabling of gas detectors, fire detectors and alarms and other output devices is an
important matter.
Cabling to a detector must be in accordance with the recognised standards of the appropriate
authority in the country concerned, and it must meet the electrical requirements of the
detector.
For non-IS devices, Crowcon recommends the use of steel wire armoured (SWA)
cable. Suitable explosion-proof glands must be used.
For IS devices, Crowcon recommends the use of twisted pair cable with overall screen
and sheath. Suitable weatherproof glands must be used. IS devices must be used with
a suitable Zener Barrier or Galvanic Isolator when used in a hazardous area.
For fire detectors, Crowcon recommends the use of twisted-pair cable, screened with
an overall protective sheath that is fireproof, for example, Pirelli FP200 or similar.
Suitable weatherproof glands must be used. The maximum acceptable loop resistance
is 100ohms.
Alternative cabling techniques, such as steel conduit, may be acceptable provided that
appropriate standards are met.
The minimum acceptable supply voltage measured at the detector and the maximum current
drawn by that detector is different for each device. Please refer to the relevant Installation,
Operating and Maintenance Instructions provided with each detector to calculate the
maximum cable distances allowed for different cable types. The maximum cable distance
allowed is dependent upon the installation, for example, whether Zener Barriers or Galvanic
Isolators are required (as for IS devices) or not.
When calculating the maximum cable length for a detector, take the minimum supply voltage
at the Quad Channel Input Module as 18V at 350mA for a 3-wire detector head, 19.8V at
25.8mA for a 2- wire head. Typical cable characteristics are shown in Table 5.
INSTALLATION
Table 5: Cable characteristics
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