Vortex Manual Field Devices
Issue 7 December 2009 41
FIELD DEVICES
• Consider the process conditions. Gases that are normally heavier than air, but are released from a
process line that is at a high temperature and/or under pressure, may rise rather than fall.
The placement of the sensors should be decided following the advice of experts with specialist
knowledge of gas dispersion, experts with knowledge of the process plant system and equipment
involved, and safety and engineering personnel. The agreement reached on the locations of sensors
should be recorded. Crowcon is pleased to assist in the selection and siting of gas detectors.
5.3 Fire detectors
Only one fire loop may be fitted to each Quad Channel Input Module, and this can only be connected
to the first input channel. Up to 20 detectors, for example, Apollo Series 60, may be used on the same
loop at any time. The fire loop can also support other switched devices such as manual call points.
Devices on the loop may be mixed provided electrical characteristics are compatible and fire
regulations allow.
All fire loops require a 1.8Kohm end-of-line resistor to be fitted (R
t
in Figure 13). Simple switch
devices such as manual call points require a 470ohm series resistor (R
m
.in Figure 13).
A fire detector circuit can be represented by an equivalent circuit of a switch operating in series with a
sensor resistor (R
s
in Figure 13).
Figure 13: Representation of a fire detection circuit
With no detectors in alarm (all switches open) a small current flows in the circuit through the end-of-
line resistor (R
t
). If a short circuit or open circuit cable fault occurs, the current in the loop increases or
decreases. The Quad Channel Input Module detects this and a fault alarm is raised.
R
m
R
t
R
s
To quad
channel
module
fire
sensor
input
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