4-20 mA instrumentation and controls usually support a signal range slightly below 4 mA and above 20 mA. For transmitters, current values below 4 mA and above 20 mA are used to signal a fault such as a thermocouple burnout, Signal cable broken, Cable Low/High Resistance, Transmitter Failure or other sensor failure.
The transmitter can be configured for failure indication low or high. Unfortunately manufacturers use different signal levels to indicate failure which prevents tight analog signal integration and interpretation in single loop controllers, control systems, and safety systems.
Transmitters 4-20mA Current Failure Alarm Limits
Some transmitters may use 3.75 mA while others may use 3.6 mA or less. Some transmitter may use 21.75 mA or more while other use 23 mA. This inconsistency of signal levels for failure indication makes it difficult to take full advantage of the failure information in control strategies.
The NAMUR NE43 “Standardization of the Signal Level for the Failure Information of Digital Transmitters” recommendation was created to standardize failure indication from transmitters and interpretation in control systems to enable better analog integration.
NE43 defines 3.8 to 20.5 mA as a valid (‘Good’) measurement value where 3.8 to 4 and 20 to 20.5 mA indicates saturation. A signal of <3.6 mA or >21 mA indicates a transmitter failure (‘Bad’).
By using transmitters and systems that both conform to the NE43 recommendation, it is possible to flag faults to the operators and control strategies.
However, note that all device errors, severe and trivial, are flagged the same way so the operator cannot tell the difference and that if any error occurs it is flagged and the measurement value is not provided.