Glossary · Process instrumentation and analytical measurement
4–20 mA current loop
Also known as: 4-20 mA signal, Current loop
German: 4–20-mA-Stromschleife
In process instrumentation, a 4–20 mA current loop is an analog signal circuit in which a measured or set value is represented by a DC current between 4 and 20 milliamperes; the live zero at 4 mA allows wire break detection and powering of two-wire devices.
- Process instrumentation
- Standards
- Process industry
In one sentence
A 4–20 mA current loop transmits an analog value as a current from 4 to 20 mA, robust over long cables and able to detect a broken wire.
Example
A pressure transmitter in the field sends 4 mA at 0 bar and 20 mA at 10 bar to the analog input of the control system over a 300 m cable.
How it applies
- Engineering: The current is the same everywhere in the loop, so cable resistance does not change the value as long as the supply voltage is sufficient. HART superimposes digital communication on the analog signal.
- Diagnostics: Currents below 4 mA or above 20 mA indicate faults. NAMUR NE 43 defines the ranges for measurement information and failure signals so that the control system can distinguish them.
- Commissioning: A loop check verifies wiring, scaling and the value displayed in the control system, usually with a loop calibrator.
- Documentation: Loop diagrams show every device, terminal and power source in the loop. Keep them up to date; they are the main reference for troubleshooting.
Current loop vs. voltage signal
A current loop is robust over long distances and detects breaks. A 0–10 V signal suits short connections and cannot distinguish a broken wire from zero.