Instrumentation
4-20 mA underrange and overrange troubleshooting
Separate valid extended measurement, NAMUR-style fault indications, open-loop behavior, saturation, and PLC clamping when a 4-20 mA value leaves its nominal span.
- Product
- Analog transmitters and PLC inputs
- Level
- Field triage
- Read time
- 10 min
- Reviewed
- 2026-07-15
What to establish before troubleshooting
Nominal 4-20 mA is not the same as every possible current the transmitter or input can report. Devices may provide limited underrange/overrange measurement and separate fault currents, while an open circuit may collapse toward 0 mA.
Scaling, clamping, and alarming should remain separate decisions: preserve the raw signal, calculate the engineering value, and then assign quality based on the documented device thresholds.
Abbreviated worked example
Interpret 3.6 mA against a 4-20 mA span
A 0-100 percent transmitter produces 3.6 mA and the simple linear formula is evaluated without clamping.
- 1Normalize: (3.6 - 4) / 16 = -0.025.
- 2Scale to 0-100 percent: -0.025 x 100.
Result: The mathematical value is -2.5 percent, but the operational meaning must come from the transmitter fault specification.
Caution: Do not label 3.6 mA as a valid -2.5 percent process value unless the device documentation and site policy explicitly allow it.
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