How to test a 4–20 mA loop with a multimeter
How to measure 4–20 mA loop current (in series, at the test terminals or across 250 Ω) and read 0 mA, negative current and the NAMUR NE 43 limits.
A 4–20 mA loop carries the transmitter measurement to the PLC or control system as a current. Measuring that current shows whether the problem is in the instrument, the cables or the input card.
Before touching the loop, get the work permit and tell operations. The value goes to the control system: opening or changing the loop can raise an alarm, trip an interlock or move a valve. In hazardous areas, use only certified instruments and follow the plant procedure.
Set up the multimeter
- Select direct current (mA DC) on the milliamp range.
- Red lead in the mA socket and black in COM. Many multimeters have a socket just for current.
- Never touch a voltage source with the multimeter set to current: it becomes a short circuit and blows the fuse.
Ways to measure
- In series, opening the loop: release one wire from a terminal and connect the multimeter between the wire and the terminal. The current flows through the multimeter and the reading is direct.
- At the transmitter test terminals: many models have TEST terminals. With the multimeter on mA connected to them, the current shows without opening the loop. Check the model manual.
- As the voltage across the input resistor: if the card reads the current across a 250 Ω resistor, measure its voltage in V DC. 1 V corresponds to 4 mA and 5 V to 20 mA, without opening anything.
- With a milliamp clamp meter: it measures the current through the magnetic field of the cable, without disconnecting.
How to read the result
| Reading | What it means |
|---|---|
| 0 mA or almost zero | No current: open loop, no supply, blown fuse or loose wire. |
| Negative value | Multimeter leads or wiring reversed. A 4–20 mA loop does not produce negative current. |
| 4 to 20 mA | Normal measurement. Convert it to the process unit with the configured endpoints (LRV and URV). |
| Below 4 or above 20 mA | Out of range: saturation, failure or configuration. Check the alarm configured in the transmitter. |
NAMUR NE 43 limits
When the transmitter is configured for the NAMUR NE 43 recommendation, the current also reports failures:
| Current | Per NE 43 |
|---|---|
| ≤ 3.6 mA | Failure (low signal) |
| 3.6 to 3.8 mA | Undefined: neither measurement nor failure |
| 3.8 to 4 mA | Measurement, process below 0% |
| 4 to 20 mA | Measurement |
| 20 to 20.5 mA | Measurement, process above 100% |
| 20.5 to 21 mA | Undefined: neither measurement nor failure |
| ≥ 21 mA | Failure (high signal) |
If the current is wrong
- Measure the voltage at the transmitter terminals and compare it with the minimum in the manual. The supply must also cover the drop across the loop resistors and barriers.
- Compare the current with the value the PLC shows. If the current is right and the PLC is wrong, the problem is in the input or the configured scaling.
- Look for splices, loose terminals, moisture and damaged cables.
When you finish, close the loop, check that the value is back to normal in the system and tell operations.