NAMUR NE43 4 to 20 mA Signal Checker
Check a 4 to 20 mA loop current against the NAMUR NE43 signal levels: valid measurement, saturated over or under range, or a failure alarm. The matching percent and process value are shown.
- Measurement information: 3.8 to 20.5 mA
- Failure information: ≤ 3.6 mA or ≥ 21 mA
- % = (I − 4) ÷ 16 × 100 = −3.125 %
NAMUR NE43 Signal Levels
Saturation (over and under range): 3.8 to 4 mA and 20 to 20.5 mA
Failure signal: ≤ 3.6 mA (low) or ≥ 21 mA (high)
NAMUR recommendation NE43 standardises the signal level for failure information of digital transmitters. The gap between 3.6 and 3.8 mA and between 20.5 and 21 mA separates a saturated measurement from a failure, so the control system can tell them apart. Whether a transmitter fails high or low is configured in the device.
Worked example
A loop reading 3.5 mA is a NE43 failure low signal: the transmitter has detected a fault (for example a broken sensor) and the DCS should raise a fault alarm, not show −3 % of range.
Frequently Asked Questions
What is NAMUR NE43?
A NAMUR recommendation that defines separate current ranges for valid measurement and for failure signals on 4 to 20 mA transmitters.
Should a transmitter fail high or low?
It depends on the loop. Choose the direction that makes the controller or trip act safely, for example fail high on a high level trip.
Why not just use 0 mA for a failure?
A two wire transmitter needs some current to run itself, so it cannot output 0 mA. 3.6 mA is low enough to be clearly abnormal.
Learn more on the blog
4 to 20 mA current loop explained
A practical, field focused guide on instrumentationblog.in that explains the theory behind this calculator and how engineers apply it on real plants.
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