It's not always easy to check faults in individual devices in a current-loop
system due to the inaccessibility of these devices from the loop starting point.
Take, for instance, long-distance 4-to-20-mA current loops, which generally
consist of insulated twisted-pair conductors. A transducer connected to the
loop receives power from the loop power supply (24 V) and conducts a 4-to-20-mA
current that is proportional to the signal typically derived from the detector
or some other device.
Two different situations may cause common faults in the current loop. One
type of fault involves loop breakage, which leads to no current in the loop.
The other circumstance is the loss of insulation, which causes current to leak
into the ground path.
Detecting loop breakage is a fairly easy task. Simply set a comparator limit
just below 4 mA. When loop current falls below this level, the comparator generates
an error signal indicating line breakage. On the other hand, leakage through
the ground path may remain totally undetected because the signal current plus
the leakage current error may fall within the 4-to-20-mA limit. This type of
fault can be detected with the circuit wiring shown (see
the figure).
A simple technique monitors the flow of current on both lines of the loop.
The differential voltage drop (V1) across resistor R1
in a fault-free loop equals the differential voltage drop across R2.
Any difference in V1 and V2 is caused by the leakage current
through some undesired secondary path, which obviously must be corrected.
