Short answer

Start with the loop powered and the meter in the correct current range. Place the meter in series, not across the two loop terminals. A current measurement made in parallel can create a short circuit and damage equipment.

4–20 mA current loop diagramA 24 volt supply powers a transmitter and returns a 4–20 mA signal to a PLC analog input.TWO-WIRE CURRENT LOOP24 V DCSUPPLYTRANSMITTER4–20 mAPLC AIINPUT+ loopsignalreturn / commonREAD HERE Original diagram · Signal Field Guide / field note troubleshooting

What to look for

First measure the supply voltage at the transmitter terminals. If the supply is absent, the loop cannot produce a meaningful current. If the supply is present, inspect polarity, terminal tightness, and the receiving input's load.

Then move the meter one point at a time. A reading that appears on one side of the transmitter but disappears on the other usually tells you where the circuit opens. Do not replace the transmitter until the loop has been proved continuous.

FIELD CASE / REASONING PATH

When the symptom appears in the plant

An operator reported that a tank level indicator suddenly dropped to 0%, shutting down the transfer pumps. The maintenance technician found the local display blank. Further investigation revealed that the 24V DC loop power supply had blown a fuse due to a crushed conduit that shorted the two-wire transmitter cable to the grounded pipe. The 0mA reading correctly indicated a complete loop failure rather than an actual empty tank.

DIAGNOSTIC SEQUENCE

Five checks before replacement

  1. Check the loop power supply with a multimeter to ensure it is delivering 24V DC across the loop.
  2. Inspect all field wiring, terminal blocks, and splices for loose connections, broken wires, or visible damage.
  3. Disconnect the field device and use a loop calibrator to inject 4-20mA directly into the PLC input to isolate whether the issue is in the field or the cabinet.
  4. Verify the integrity of the transmitter by checking its local display (if equipped) or testing it with a bench power supply.
  5. Check the PLC analog input card for blown internal fuses or diagnostic error flags in the control software.
PRACTICAL NOTES

A reading of 0mA always means a broken circuit or dead power supply; a true zero measurement should read 4mA.

Always check the cheapest components first: fuses, loose terminal screws, and quick-disconnect plugs.

In a series loop, one failed device or loose wire takes down the entire signal.

BEFORE YOU REPLACE THE PART

Bench checklist

  • Confirm the supply at the device terminals.
  • Confirm the meter is in the correct mode and position.
  • Confirm the input range matches the signal type.
  • Confirm the reference/common is intentional.
  • Record the reading before changing the wiring.

Reference families

  • ISA-50.1 Standard
  • Instrument Troubleshooting Guides

Always verify the current edition and the exact device manual for the installation.

READ THE PRIMARY MATERIAL

Official references

These links point to standards bodies, industry organizations, or manufacturer documentation. Standards landing pages may require purchase or institutional access.

This note is an educational reference, not a substitute for the equipment manual, site procedures, or qualified electrical work.