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MAN0530 Issue 15 - 07/17
Searchline Excel
2104M0506
3. INSTALLATION AND OPERATION
3.4 POWER SUPPLY
3.4.1 General
The Searchline Excel gas detector requires a power supply from the controller of between
18 VDC and 32 VDC. Ensure that a minimum supply of 18 VDC is measured at the detector,
taking into account the voltage drop due to cable resistance.
The maximum loop resistance in the field cable is calculated as follows (R = V/I)
R loop = (V controller – V detector min) / I detector
Example 1
The controller is supplying a nominal 24 VDC (V controller), the detector minimum allowable
voltage is 18 VDC (V detector min), therefore the maximum allowable voltage drop
between the controller and detector is 6 V, which means a voltage drop of 3 V in each core
(+ve core and –ve core).
+21 V
+24 V
Signal
Excel
4 – 20 mA
Controller
18 V
Rx
(min)
Field Cable
0 V
3 V
(wrt Controller 0 V)
Example 2
Assume that the power consumption of an Excel receiver (Rx) is 8.0 W. The current required
to drive the receiver at the minimum voltage (P / V = I) is 8.0 / 18 = 444 mA (I detector).
So, the maximum field cable loop resistance (R loop) would be 6 / 0.44 = 13.5, and 13.5 / 2 =
6.75 Ω per core, (allowing for component variations, losses, etc.).
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