User’s Guide ______________________________________________________________________
38 ___________________________________________________________________ M010056EN-I
Calculation of the maximum cable length from barrier to transmitter:
If assuming that the operating voltage would be ≥ 24 V, the maximum
acceptable voltage drop U
drop
in cables is:
U
drop
= U
S
- U
min
U
drop
= 14 V - 12 V = 2 V
We also know that:
I
out
= 20 mA
and that total resistance of the cable R
cabletot
is cable resistance R
cable
multiplied with the total maximum length of the cables l
max
:
R
cabletot
= R
cable
× l
max
From these facts the following equation can be formed:
U
drop
= R
cabletot
× I
out
2 = 2 × 0.085 Ω/m × l
max
× 20 mA
l
max
= 2 V / (20 mA × 2 × 0.085 Ω/m)
l
max
= 588 m = 1930 ft, maximum cable length.
Cable resistance (as an example) R
cable
= 0.085 Ω/m/core (2 ×
0.085 Ω/m/pair)
NOTE
If longer cable length is required, use of the galvanic isolators is
recommended if possible.
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