What are common causes of a 4-20 mA loop failure and how would you diagnose them?

• A. Open circuit, short circuit, wiring faults, transmitter failure, power supply issues; diagnose by measuring loop current, checking supply voltage, inspecting wiring, and substituting a known-good transmitter.
• B. Only calibration drift.
• C. Valve stuck.
• D. Sensor type mismatch.

Answer: (C)

In a 4-20 mA loop, a failure is usually due to problems that interrupt or corrupt the electrical path or the transmitter’s ability to modulate current. The most common causes are an open circuit (no return path for current), a short circuit (too low impedance that drags the loop out of spec), wiring faults (loose, damaged, or corroded connections), transmitter failure (the device that sets the current is not functioning), and power supply issues (insufficient voltage or a failing supply). To diagnose, start by measuring the loop current with a meter to see if current is present and whether it falls within 4-20 mA. Then verify the supply voltage at the transmitter to ensure the loop has enough headroom. Inspect the wiring for continuity, loose connections, or damage, and check for any signs of corrosion or grounding issues. If possible, substitute a known-good transmitter to determine whether the problem is with the transmitter itself. If the loop current is absent or out of range, focus on the power path and wiring; if a specific component causes the issue, isolating that component (for example, with a test resistor or loop tester) helps pinpoint the fault. Calibration drift or a valve stuck are more about the process signal or load behavior rather than the loop’s electrical integrity, and a sensor type mismatch would typically affect the reading rather than collapse the loop current.