Troubleshooting a 4-20 mA loop
A loop-powered transmitter, a 24 V supply, and a PLC input reading the wrong thing. Six faults, one meter in milliamps and volts, and the rule that cracks all of them: in a series loop the current is the message, and voltage appears across whatever is open.
Before you start — job brief
A 4-20 mA loop is low-voltage (24 V DC) and generally safe to work live — that is the point of the exercise, since the current is the reading. But the loop may run beside 480 V wiring, and the process it controls is not harmless. Know what the signal drives before you break the loop.
- Know what the loop controls — breaking it can swing a valve or trip a process
- Put the control loop in manual, or notify the operator, before you open it
- To read current, the meter goes in series (break the loop); to read volts, in parallel
- Treat nearby power wiring as live and keep your leads clear of it
Loop · V≡ mode: click two points · mA mode: click one point to break in
The current is the message — read it in series.
Meter
Readings taken
- No readings yet.
Your diagnosis
Why 4-20 mA, and why it fails cleanly
A 4-20 mA loop carries a process value as a current, not a voltage, and that choice is the whole reason it's easy to troubleshoot. Current is the same everywhere in a series loop, so it doesn't drop off over a long cable run the way a voltage signal would, and a broken wire reads a definite zero instead of a plausible-but-wrong number. The live zero at 4 mA is the other half of the trick: a healthy transmitter at its lowest reading still pushes 4 mA, so 0 mA can only mean the loop is broken or unpowered — never a real process value.
So every fault sorts into two questions, and one meter answers both. Put it in milliamps, in series, and read the current — that is the message. Put it in volts, across two points, and read where the voltage lands — that finds the break. The rule underneath is the same one that solves a control circuit: in a series loop, voltage appears across whatever is open.
Read the current first
Break the loop at a test point and read it in series. The number tells you which kind of fault you have before you touch anything else:
- 0 mA — the loop is open or has no power. A real transmitter never sends zero.
- 4 to 20 mA, steady — the loop is healthy; if the reading is still wrong, the problem is calibration or the process, not the wiring.
- Below about 3.8 mA (but not zero) — the transmitter is driving under-range: a fault-low signal per NAMUR NE43, or a process below range.
- Above about 20.5 mA — over-range: a fault-high signal, a failed sensor, or a genuinely high process.
Then use the voltmeter to place the fault
When the current is zero, the voltmeter separates the two causes that look identical on the milliamp range:
- Supply reads 24 V, current is 0. The power is there; the loop is open. Walk the voltmeter along the loop — the full 24 V sits across the break, so wherever the reading jumps to 24 is your open.
- Supply reads 0 V. The loop power supply is dead or its fuse is blown. No wiring fault will fix a loop with no voltage behind it.
And across the 250 Ω sense resistor at the input, Ohm's law ties volts back to current: a healthy 12 mA drops 3 V, a 4 mA zero drops 1 V, a 20 mA full scale drops 5 V. That 1-to-5 V is exactly what the analog card reads.
Field note — the PLC number is not the loop
The nastiest loop faults are the ones where the screen lies. A shorted input or a shorted sense resistor reads 0 on the HMI while a perfectly good 12 mA circulates in the field — the current is flowing, the input just can't see it. And a leak to ground in a wet junction box drains current away, so the transmitter sends 12 mA and the PLC receives 9. The only way to catch either is to stop trusting the displayed value and put a real meter in series. If the current at the transmitter doesn't match the current at the PLC, something in between is stealing it.
When the loop is fine and the reading is still wrong
A steady current in range with a wrong value on the screen is not a loop fault at all — it's calibration, ranging, or the process. Confirm the transmitter's zero and span against a known input, check that the PLC's scaling matches the transmitter's range, and look at the process itself. Chasing wiring on a loop that reads a solid, stable milliamp signal wastes the afternoon; the loop already told you it's healthy.
What normally goes wrong, in order
- Loose or corroded field terminals — the everyday open, found by the 24 V across the break
- Moisture in a junction box — leakage to ground that pulls the reading low
- A tripped or dead loop supply taking a whole rack of signals down at once
- A failed sensor pinning the transmitter over- or under-range
- A shorted input or bridged terminal in the cabinet reading zero on a live loop
Most of these are found in a minute or two once you work in the right order: current first to sort the kind of fault, then voltage to place it. The loop is honest — it fails in ways that point straight at the cause, as long as you read what it's actually doing instead of what the screen claims.