What is a 4-20 mA signal, and why does it start at 4 mA, not 0?
A 4-20 mA signal carries a measured value as current, a common analog standard. The live zero at 4 mA means 0 percent of range, so a broken wire (0 mA) can be told apart from a real minimum reading. Current is also unaffected by voltage drop along the cable.
Live zero: why 4 mA?
An analog signal must separate a real minimum reading from a wiring or transmitter fault. In a 4-20 mA loop, 4 mA means zero percent of range. This live zero is what lets 0 mA signal a fault. Two-wire transmitters draw their power from the same loop, so the minimum current must stay at 4 mA for them to run. Current is the same at every point in a series loop, so cable voltage drop does not change the reading. NAMUR NE 43 recommends current levels outside the measuring range for fault indication; check the current edition of the standard for exact limits.
Example: a broken wire reads as zero
Suppose a tank level transmitter is set to a 0-2 m range (hypothetical). The readings follow the scaling formula:
- 4 mA: (4 − 4) / 16 × 2 = 0 m
- 12 mA: (12 − 4) / 16 × 2 = 1.0 m
- 0 mA (wire broken): (0 − 4) / 16 × 2 = −0.5 m
The value −0.5 m lies outside the tank's measuring range, so it is not a physical level. Treat it as a wiring or transmitter fault and raise an alarm. Reading 0 mA as an empty tank would give false confidence.
Common mistakes
The most frequent mistake is treating 0 mA as a valid measurement. A current well below 4 mA should be handled as a separate fault state in the control logic. The second mistake is scaling the signal over 0-20 mA: then 4 mA reads as 20 percent and the whole measurement shifts. The third is skipping the check of loop resistance and supply voltage. Loop resistance and the minimum supply voltage must be checked together, and the operating point should stay within the transmitter's data sheet limits.
Follow-up questions
Is 0 mA on a loop always a fault?
Usually, yes. Normal operation does not produce currents below 4 mA, so 0 mA usually means a broken wire, a supply failure or a transmitter fault. Still, check how the device reports faults, because some instruments send a different current level when they detect a problem.
Does cable length limit a 4-20 mA signal?
Yes, within limits. A current signal tolerates voltage drop along a cable, but as loop resistance grows, the transmitter needs more supply voltage. Below its working range, the reading becomes wrong. Judge length, wire size, noise and routing together, and take the exact limit from the device data sheet.
Sources
Example numbers are hypothetical, not measured field results.