Why Industrial Sensors Start at 4mA, Not Zero: The Logic Behind 4-20mA

The 4-20mA standard, developed in the 1950s, is the analog signaling protocol that underpins most industrial sensor data worldwide, converting physical measurements into electrical currents carried through copper wire. A sensor configured for a range such as 0–200°C maps 4mA to the bottom of that range and 20mA to the top, with all values scaled linearly in between. The deliberate choice to start at 4mA rather than zero means a current reading of 0mA unambiguously signals a fault — such as a broken wire or dead transmitter — rather than a valid low measurement. The NAMUR NE43 convention formalises this by defining the normal operating band as 3.8–20.5mA, with readings outside 3.6–21.0mA treated as failure states. Developers consuming this data must know their transmitter's configured burnout direction, since a failing sensor can otherwise be misread as a genuine process condition like overheating.