Installation
Issue 5 - June 2005 Page 24 of 64 Pages
Maximum cable resistance for three-wire-transmitters
When using three wire transmitters a short circuit between the 20 mA output and the
zero volt wire at the transmitter must cause a fault indication at the controller. The cable
resistance may not exceed the value given by the following formula:
txr
cable
I
800
R =
where:
R
cable
is the resistance per core from Regard-1 to the transmitter (in ohms),
I
txr
is the transmitter operating current (in mA).
This formula assumes that the three cores of the connecting cable each have the same
resistance.
Example Polytron IR operating current at 24 V is approx. 200 mA. So limit of cable
resistance is
core. per ohms 4
200
800
=
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