Troubleshooting Current Loops using standard milliammeter to measure loop current

The most fundamental diagnostic method for troubleshooting 4-20 mA analog current
loops is to measure current and/or voltage at different points in the circuit. Several types of test
instruments are available for this purpose.

Since the signal of interest is represented by an electric current in an instrumentation current “loop”
circuit, the obvious tool to use for troubleshooting is a multimeter capable of accurately measuring
DC milliamperes. Unfortunately, though, there is a major disadvantage to the use of a milliammeter:
the circuit must be “broken” at some point to connect the meter in series with the current, and
this means the current will fall to 0 mA until the meter is connected (then fall to 0 mA when
the meter is removed from the circuit). Interrupting the current means interrupting the flow of
information conveyed by that current, be it a process measurement or a command signal to a final
control element. This will have adverse effects on a control system unless certain preparatory steps
are taken.
Before “breaking the loop” to connect your meter, one must first warn all appropriate personnel
that the signal will be interrupted at least twice, falling to a value of -25% each time. If the signal to
be interrupted is coming from a process transmitter to a controller, the controller should be placed
in Manual mode so it will not cause an upset in the process (by moving the final control element in
response to the sudden loss of PV signal). Also, process alarms should be temporarily disabled so
they do not cause panic. If this current signal also drives process shutdown alarms, these should be
temporarily disabled so that nothing shuts down upon interruption of the signal.
If the current signal to be interrupted is a command signal from a controller to a final control
element, the final control element either needs to be manually overridden so as to hold a fixed setting
while the signal varies, or it needs to be bypasses completely by some other device(s). If the final
control element is a control valve, this typically takes the form of opening a bypass valve and closing
at least one block valve:

Since the manually-operated bypass valve now performs the job that the automatic control valve
used to, a human operator must remain posted at the bypass valve to carefully throttle it and
maintain control of the process.
Block and bypass valves for a large gas flow control valve may be seen in the following photograph:

In consideration of the labor necessary to safely interrupt the current signal to a control valve in
a live process, we see that the seemingly simple task of connecting a milliammeter in series with a
4-20 mA current signal is not as easy as it may first appear.