NAMUR NE 43
NAMUR NE 43 is a recommendation from the European user-association NAMUR that defines fault-state behaviour for 4-20 mA process transmitters. Below 3.6 mA or above 21 mA signals an instrument or wiring fault rather than a legitimate measurement. PLCs and DCS configured to NAMUR NE 43 detect these out-of-band currents and raise an instrument-fault alarm distinct from process-variable alarms.
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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.
Why NE 43 matters.
Without NE 43 awareness, a transmitter that loses power, current drops to zero reads as 4 mA at the AI card, which the PLC interprets as a legitimate zero-scale measurement. Real-world consequence. A level transmitter dies, the PLC sees zero level, the level controller commands maximum filling, the vessel overfills. NE 43 fixes this. The AI card reads 0 mA, recognizes the under-range condition, and raises an instrument-fault alarm rather than feeding the bogus measurement to the controller.
Implementation in practice.
Modern transmitters drive 3.6 mA on diagnostic-detected fault, 21 mA on over-range fault. The AI card configuration on the PLC or DCS interprets these as faults and either drops the value out of regulatory control or raises an alarm. Most major DCS and PLC platforms have NE 43-compliant AI card behaviour as a configurable option. The I/O list and detailed-design specifications should call out whether NE 43 fault detection is enabled per loop. SIS-classified transmitters universally support NE 43. Legacy non-smart transmitters do not.
Frequently asked.
Is NE 43 mandatory.
No, it's a recommendation. In practice it's universal in modern process plants because the alternative, silent transmitter failures going undetected is unacceptable. SIS-classified loops effectively require it for fault-detection coverage.
Does NE 43 apply to HART instruments.
Yes. The NE 43 fault-current behaviour is on the underlying 4-20 mA layer, independent of whether HART is overlaid on the same wires. HART adds digital diagnostics on top. NE 43 catches the failures that the analog layer can detect.
How does NE 43 fault detection affect SIS-classified loops.
For a SIS transmitter, the NE 43 under-range current, below 3.6 mA must be configured as a dangerous detected failure in the SIL verification calculation. The SIS logic solver monitors the AI channel. A current below 3.6 mA transitions the channel to a defined safe state, typically voting as a trip or raising a maintenance bypass alarm rather than being ignored. This diagnostic coverage is a direct contributor to the PFD calculation for the safety function.