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The maximum loop resistance in the field cable is calculated as follows:
R loop = (V controller – V detector min) / I detector
Example;
The controller or power supply is supplying a nominal 24VDC (V controller), the detector
minimum allowable voltage is 12VDC (V detector min), therefore the maximum allowable
voltage drop between the controller and detector is 12VDC; this means a voltage drop of 6V in
each core (V+ core and V- core). Minimum power consumption of the detector without any
optional module is 1W. The current required to drive the detector at the minimum voltage is (I
= P / V): 1.0 / 12 = 85mA (I detector).
Maximum power consumption of the detector when optional relay modules installed and all
relays are active is 5.0W. The current required to drive the detector at the minimum voltage is
(I = P / V): 5.0 / 12 = 416.6mA (I detector). So, the maximum field cable loop resistance (R
loop) = 12 / 0.41 = 29 Ohms, or 15 Ohms per core, (allowing for component variations, losses,
etc.).
The following tables show the maximum cable distances between the controller and
transmitter assuming a voltage drop of 6V in each core and for different cable parameters.
The tables are examples only and actual cable parameters and source power supply voltage
for the application should be used to calculate the maximum cable distance allowed at the
installation site. Typical cable data for detector with relay module
Cable size
(cross sectional area)
0.5mm2
1.0mm2
1.5mm2
2.0mm2
2.5mm2
PRS-UM-PC3-EN-Rev.04-06.2020
Diagram 4: Field cabling
Cable type
nearest equivalent
20AWG
17AWG
16AWG
14AWG
13AWG
Table 3: Typical cable details and maximum distance for cabling
Cable resistance
Maximum Cable length (L)
Ω/km
36.8 Ω/km
19.5 Ω/km
12.7 Ω/km
10.1 Ω/km
8.0 Ω/km
Meters
407
769
1181
1485
1875
10
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