REGARD 8-Input 4-20 Module & 8-Channel Display Card (ATEX Version)
Issue 2. October 2011
14
WARNING – when using 3-wire transmitters
, in order to ensure that a short circuit
between the 4-20mA output and the zero volt line at the transmitter causes a fault
indication on the display card, the cable resistance must be kept below the values shown
in the following table.
For example:
the transmitter takes typically 80mA,
under range fault trip has been set to 3.5mA on the display card,
then the cable core resistance must be kept below 12 ohms.
The formula used to generate the values in the table is
fault
tran
fault
cable
I
I
I
R
×
−
×
<
2
255
where:
cable
R
is the resistance in ohms of one core of the cable running from the input module
to the transmitter,
fault
I
is the under range fault current in mA set at the display card,
tran
I
is the current supply in mA from the input module to the transmitter.
This formula assumes that the three cores of the connecting cable each have the same
resistance.
Maximum cable resistance per core (in ohms) for fault currents of:
Transmitter
current (mA)
3.5mA 3.0mA 2.5mA 2.0mA
40 27.0 22.5 18.2 14.2
50 20.8 17.4 14.2 11.1
60 16.8 14.2 11.6 9.1
70 14.2 12.0 9.8 7.7
80 12.2 10.3 8.5 6.7
90 10.8 9.1 7.5 5.9
100 9.6 8.1 6.7 5.3
110 8.7 7.4 6.1 4.8
120 7.9 6.7 5.5 4.4
130 7.3 6.2 5.1 4.0
140 6.7 5.7 4.7 3.8
150 6.2 5.3 4.4 3.5
