NAMUR NE 43.

NAMUR NE 43 is the recommendation, from the German user-association NAMUR whose guidance the process industry treats as a de-facto standard, that turns the 4-20 mA live zero into a fault-detection scheme. The principle is simple and consequential. A healthy transmitter drives between 4 and 20 mA, so any current outside a narrow margin of that band, below 3.6 mA or above 21 mA cannot be a real measurement and must be an instrument or wiring fault. The failure it guards against is the silent one. Without NE 43 awareness, a transmitter that loses power falls to 0 mA, which an input card reads as a legitimate bottom-of-scale value. A level transmitter that dies this way reads as empty, and a controller acting on that reading commands maximum fill and overflows the vessel. With NE 43 the card recognizes the under-range current as a fault, drops the value out of control, and raises an instrument alarm distinct from a process alarm. Modern transmitters drive a defined fault current, commonly 3.6 mA low, 21 mA high and most controllers implement NE 43 behaviour as a configurable option, so the detailed design and the I/O list should state per loop whether it is enabled. For an SIS loop the under-range current is treated as a dangerous-detected failure in the SIL verification, where the resulting diagnostic coverage feeds directly into the probability-of-failure-on-demand calculation. The behaviour sits on the analog layer, so it applies equally whether or not HART is overlaid on the same pair.