Live Zero Concept
The use of 4 mA as live zero is one of the most important engineering decisions in industrial instrumentation. It allows fault detection without additional wires or digital protocols.
4–20 mA Current Spectrum
The green zone represents valid process measurement. Red/yellow zones indicate faults.
Why 4 mA Instead of 0 mA?
- 4 mA = valid minimum measurement (LRV)
- 0 mA = broken wire or dead transmitter
- Power for 2-wire transmitters flows even at zero process
- Fault detection is built into the signal itself
In older 0–20 mA systems, zero measurement and cable failure look identical. 4–20 mA eliminates that ambiguity.
NAMUR NE43 Signal Levels
| Current | Meaning |
|---|---|
| < 3.6 mA | Sensor fault / underrange (Fail Low) |
| 4.0 mA | Lower range value (Valid) |
| 20.0 mA | Upper range value (Valid) |
| > 21.0 mA | Electronics fault / overrange (Fail High) |
NAMUR NE43 standardizes how transmitters communicate fault conditions to PLC/DCS systems.
Fail-Low vs Fail-High Configuration
- Fail-Low: Transmitter drives current < 3.6 mA on fault
- Fail-High: Transmitter drives current > 21 mA on fault
Field Rule
Any reading below 3.6 mA or above 21 mA must be treated as a fault, not a process condition.
Related Blog Articles
Why 4–20 mA Uses Live Zero Instead of 0 mA
Engineering reasoning behind live zero and built-in fault detection.
5 Common 4–20 mA Mistakes in Field Work
Real-world errors that lead to wrong signal interpretation.