People have asked me about setting fault alarms in level transmitter analog signals at 2mA or 3mA levels. What they typically don’t understand is that a two-wire transmitter uses the electrical current below 3.6 mA for its own power and operation. So, a 2.0 mA or 3.0 mA fault indication just isn’t possible. At these low currents, there wouldn’t be enough power to generate the fault indication signal and to keep the transmitter functioning properly.
NAMUR, an international association of process instrumentation user companies, made a recommendation, known as NE 43, to promote a standardization of the signal level for failure information. You’ll see Compliant to NAMUR NE 43 on the specs of most digital transmitters available today.
The goal of NE 43 is to set a basis for proactively using transmitter failure signals in process control strategies. Using these failure signals (A), instrument faults are separated from process measurements (M). And since the faults get signaled at an early stage, the team can design maintenance strategies that keep the instruments in operation and increase productivity.
NAMUR NE 43 uses the 3.8 to 20.5 mA signal range for measurement information, with ≥21 mA or ≤3.6 mA to indicate diagnostic failures. With that information, it’s easier to detect a fail high condition on a level transmitter, for example, that clearly tells you whether you have a full tank or a failed instrument.