The following cable lengths are calculated assuming a 300mA constant
current drive, with a minimum supply of 18 V dc from the control
equipment:
C.S.A.
Resistance
Max. Distance
Max. Distance
(Ohms per km)
(km)
(km)
mm
2
Awg
Cable
Loop
2.0 volt pellistors 2.5 volt pellistors
1.0
17
18.1
36.2
1.47
1.42
1.5
15
12.1
24.2
2.2
2.13
2.5
13
7.4
14.8
3.6
3.5
Table 4: maximum cable distances for typical cables
The acceptable cross sectional area of cable used is 0.5 to 2.5mm
2
(20 to 13awg). The table is provided for guidance only, actual cable
parameters for each application should be used to calculate maximum
cable distances.
2.4 Electrical connections
All connections are made via the screw terminal block mounted on the PCB
in the junction box. The terminals are marked ‘+’, ‘sig’ and ‘’ and correct
polarity should be observed when connecting the detector to control
equipment. This version of
Xgard
operates as part of a 3wire mV
Wheatstone bridge circuit and must be connected to a suitable control card.
Note: The internal grounding terminal (see diagram 8) shall be used as the
grounding means of the
Xgard
gas detector. The external grounding
terminal (see diagram 8) is only a supplemental bonding connection, and is
only to be used where local authorities permit or require such a connection.
To limit radio frequency interference, the junction box and cable armour
should be grounded (earthed) at the control panel. Ensure the earth
connection is provided in a safe area only, so as to avoid earth loops.
Diagram 9:
Xgard
Type 3 electrical connections
28
2. Installation
Xgard Type 3
SIG
SIG
Xgard
Type 3 PCB
Control Panel
Earth
See table 3 for sensor voltage settings
