11
2.2 Mounting
Xgard
should be installed at the designated location with the sensor
pointing down. This ensures that dust or water will not collect on the
sensor and stop gas entering the cell. The mounting detail is shown in
Diagram 2. Care should be taken when installing the detector to avoid
damaging the painted surface of the junction box and sensor retainer.
2.3 Cabling requirement
Cabling to
Xgard
must be in accordance with the recognised standards
of the appropriate authority in the country concerned and meet the
electrical requirements of the detector.
Crowcon recommend the use of 2core screened cable with a minimum
crosssectional area of 0.5 mm
2
(20awg). Suitable weatherproof cable glands
must be used. Intrinsically Safe cables should be identified by some means,
for example coloured blue. Alternative cabling techniques such as steel
conduit may be acceptable provided appropriate standards are met.
Xgard
requires a dc supply of 830 V and is loop powered (if mounted in
a hazardous area, do not apply a voltage higher than the maximum rating
of the Zener barrier, typically 28 V). Ensure there is a minimum of 8 V at
the detector, taking into account the voltage drop due to cable resistance,
the Zener barrier (if fitted) and the sense resistance of the control panel to
which it is connected.
For example, a nominal dc supply at the control panel of 24 V has a
guaranteed minimum supply of 19.5 V. The circuit may demand up to 20
mA. Given a sense resistor in the control panel of 232 Ohms the
maximum voltage drop allowed due to cable resistance is 6.8 V. The
maximum loop resistance allowed is 340 Ohms (approx.).
A 1.5 mm
2
cable will typically allow cable runs up to 14 km. Table 1 below
shows the maximum cable distances given typical cable parameters.
C.S.A.
Resistance
Max. Distance
Max. Distance with 330 ž
(Ohms per km)
(km)
Zener barrier km
mm
2
Awg
Cable
Loop
1.0
17
18.1
36.2
9.4
0.35
1.5
15
12.1
24.2
14
0.5
2.5
13
7.4
14.8
23
0.85
Table 1: Maximum cable distances for typical cables
The acceptable cross sectional area of cable used is 0.5 to 2.5 mm
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. Installation
Xgard Type 1
