4-20mA Loop Troubleshooting

Understanding the 4-20mA Current Loop

The 4-20mA current loop is the most widely used analog signaling standard in industrial instrumentation. A transmitter converts a process variable (pressure, temperature, level, flow) into a proportional current signal where 4 mA represents the low end of the measurement range (zero or LRV) and 20 mA represents the high end (span or URV). The beauty of a current loop is that the signal is immune to voltage drop across wire resistance, making it reliable over long cable runs up to several thousand feet.

When a 4-20mA loop fails, the symptom at the PLC or indicator is typically one of four conditions: reading pegged at 0% (below 4 mA — usually an open circuit), reading pegged at 100% (above 20 mA — usually a short circuit or failed transmitter), reading offset from the correct value (calibration error or loop resistance problem), or erratic fluctuating readings (noise, intermittent connection, or failing transmitter).

Loop Types: 2-Wire vs. 4-Wire

2-Wire (Loop-Powered) Transmitters

The transmitter is powered by the same two wires that carry the signal. The PLC or loop power supply provides 24 VDC, and the transmitter modulates the current drawn from the loop to represent the measurement. Total loop resistance (including the PLC input impedance) must stay within the transmitter's drive capability, typically 250-600 ohms at 24 VDC.

4-Wire Transmitters

The transmitter has a separate power supply (24 VDC or 120 VAC) and sources the 4-20mA signal independently. The signal wires connect to the PLC analog input. 4-wire transmitters can drive higher loop resistance because they have a dedicated power source.

Systematic Troubleshooting Steps

Step 1: Measure Loop Current

• Set your multimeter to DC milliamps and connect it in series with the loop (break the loop at the PLC terminal and insert the meter)
• Alternatively, use a clamp-on milliamp meter (Fluke 771/772) to measure without breaking the circuit
• Compare the measured current to the expected value based on the actual process condition
• If current reads 0 mA, there is an open circuit in the loop. If it reads above 21 mA, there is a short circuit or failed transmitter

Step 2: Check Loop Power Supply

• Measure DC voltage at the PLC analog input terminals with the transmitter connected. You should see the supply voltage minus the voltage drop across the transmitter (typically 10-15 VDC across the PLC input resistor at midrange).
• If voltage is 0 VDC, the loop power supply has failed or the fuse is blown
• If voltage equals the full supply voltage (24 VDC), the loop is open — no current is flowing
• Check for blown fuses in the loop power supply. Many loop-powered analog input cards have per-channel fuses.

Step 3: Inspect Wiring

• Check for loose, corroded, or damaged terminal connections at the transmitter, junction boxes, marshalling panels, and PLC terminals
• Look for damaged cable insulation, especially where cables enter conduit fittings or are exposed to mechanical damage
• Verify wire gauge is appropriate for the cable run length. For runs over 1,000 feet, 18 AWG minimum is recommended to keep total loop resistance within limits.
• Measure conductor resistance end-to-end. Each conductor should read low ohms (typically under 50 ohms for runs under 2,000 feet). High or infinite resistance indicates a broken conductor.

Step 4: Test the Transmitter

• Zero and span check: Apply known process conditions (or use a calibration standard) at 0%, 50%, and 100% of range. Measure the output current at each point.
• HART communication: If the transmitter supports HART, connect a HART communicator to read diagnostics, check configuration, and verify the measured process variable matches the output current.
• Substitution test: If a spare transmitter is available, swap it to determine if the original transmitter has failed

Common Failure Modes

• Transmitter reads 3.6 mA (below 4 mA): The transmitter is signaling a fault condition. Most transmitters drive the output below 3.8 mA to indicate a sensor failure (broken thermocouple, plugged impulse line, etc.)
• Transmitter reads 21.5+ mA (above 20 mA): Saturated output indicates the process variable exceeds the configured range, or the transmitter has an internal failure driving the output high
• Readings drift over time: Sensor degradation, reference junction compensation failure (thermocouples), or diaphragm fouling (pressure transmitters)
• Intermittent dropouts: Loose terminal connections, broken conductor that makes contact intermittently, or moisture in the junction box causing intermittent leakage paths

Loop Resistance Calculation

For 2-wire transmitters, verify the total loop resistance does not exceed the transmitter's drive capability:

• Total loop resistance = PLC input impedance + wire resistance + any other series components (barriers, isolators)
• Typical PLC input impedance: 250 ohms (this also provides the voltage signal the A/D converter reads: 1-5 VDC from 4-20 mA across 250 ohms)
• Maximum loop resistance: (Supply voltage - Minimum transmitter operating voltage) / 0.020 A
• Example: 24 VDC supply, 12V minimum transmitter voltage = (24-12) / 0.020 = 600 ohms maximum total loop resistance