Dräger REGARD 39 0 Series Instructions For Use Manual page 66

Installation & commissioning
5.7.3 4–20 mA three-wire transmitter
– Only connect transmitters with a 4–20 mA source output. REGARD
cannot be used with a three-wire transmitter with a 4–20 mA sink output (if
necessary, use a sink-source converter).
– The module can supply a three-wire transmitter with up to 400 mA.
– Use a cable in which all 3 wires have the same resistance.
– To ensure that a short circuit between the 4–20 mA signal output and the 0 V
duct at the transmitter causes a fault indication at the control unit, the cable
resistance must not exceed the value given by the following formula:
Rcable =
Rcable is the maximum resistance per wire from the control unit to the transmitter
(in Ω).
Ifault is the alarm threshold for the fault (in mA).
Itxr is the transmitter operating current (in mA).
Example with a transmitter with an operating current of 100 mA and an alarm
threshold for the fault of 3.2 mA.
Rcable =
250 x 3.2
= 8 Ω per wire
100
250 x Ifault
Itxr
®
3900
®
REGARD 3900