Troubleshooting Guides · Free Tool
4 to 20 mA Loop Fault Finder
Choose the current loop symptom and get likely causes in the wiring, power supply, barrier, I/O card or configuration, with checks and fixes.
0 mALow currentNoisy loopHART not talking
Symptom0 mA, loop completely dead
Most likely causeOpen circuit: broken wire, loose terminal, open fuse or link
First checkMeasure voltage at each terminal point from supply to field
Causes in order of likelihood
| Cause | How to check | Fix |
|---|---|---|
| Open circuit: broken wire, loose terminal, open fuse or link | Measure voltage at each terminal point from supply to field | Repair the break; replace fuse |
| Power supply or I/O channel failed | Measure supply voltage at the marshalling | Repair supply, replace I/O channel or card |
| Barrier or isolator failed or fused | Measure across the barrier | Replace the barrier |
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Before You Start
- Work under a permit, inform the control room and put the loop in manual or bypass the trip before touching a live instrument.
- Measure the current in series or with a clamp mA meter, and the voltage at the transmitter terminals.
- Work from the field towards the control room: transmitter, junction box, marshalling, barrier, I/O card.
- Use the loop drawing to identify every terminal and device in the loop.
4 to 20 mA Loop Fault Finder: All Symptoms
0 mA, loop completely dead
| Likely cause | How to check | Fix |
|---|---|---|
| Open circuit: broken wire, loose terminal, open fuse or link | Measure voltage at each terminal point from supply to field | Repair the break; replace fuse |
| Power supply or I/O channel failed | Measure supply voltage at the marshalling | Repair supply, replace I/O channel or card |
| Barrier or isolator failed or fused | Measure across the barrier | Replace the barrier |
Current lower than expected or stuck below 20 mA
| Likely cause | How to check | Fix |
|---|---|---|
| Too much loop resistance (long cable, barrier, receiver) | Measure terminal voltage at the transmitter at high output | Reduce resistance or raise supply voltage |
| Leakage current to earth from wet cable | Insulation test (instrument disconnected) | Dry or replace the cable, fix glands |
| Transmitter range or saturation setting | Check URV and saturation limits | Correct configuration |
Current stuck at a fixed value
| Likely cause | How to check | Fix |
|---|---|---|
| Transmitter in fixed current or loop test mode | Check HART status | Exit fixed current mode |
| Process isolated | Check valves and impulse lines | Restore the process connection |
| Transmitter in failure mode (3.6 or 21 mA) | Read diagnostics | Fix the reported fault |
Noisy or fluctuating signal
| Likely cause | How to check | Fix |
|---|---|---|
| Screen earthed at both ends (earth loop) | Check screen terminations | Earth the screen at the control room end only |
| Cable run beside power cables or VFD outputs | Check routing | Segregate and use screened cable |
| Intermittent connection | Wiggle test while monitoring | Retighten or replace terminals |
Correct current but wrong value in DCS
| Likely cause | How to check | Fix |
|---|---|---|
| Scaling (LRV, URV) mismatch | Compare transmitter range with DCS scaling | Align the scaling |
| Wrong channel or swapped pairs | Inject a known current and watch the tag | Correct the marshalling |
| Square root or characterisation applied wrongly | Check DCS block settings | Correct the function block |
HART communication fails
| Likely cause | How to check | Fix |
|---|---|---|
| Loop resistance below 230 Ω | Measure resistance between supply and transmitter | Add a 250 Ω resistor in the loop |
| Barrier or I/O card not HART compatible | Check device data sheets | Use HART compatible barriers or I/O |
| Noise or filter capacitors on the loop | Check for added capacitors or long cable | Remove capacitors, reduce cable capacitance |
Key insight: measure the voltage at the transmitter terminals at 20 mA: if it is below the transmitter minimum, the loop has too much resistance, no matter how healthy each part looks.
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Frequently Asked Questions
What does 0 mA in a 4 to 20 mA loop mean?
An open circuit or no power; a working transmitter never draws less than about 3.6 mA.
How do I measure loop current without breaking the loop?
Use a clamp mA meter, or the test terminals or diode test points provided on many transmitters.
Why does HART need 250 Ω?
The HART signal is a current modulation that must develop a voltage across a resistance of at least 230 Ω to be read.
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