Why NAMUR NE 43 matters more than it looks

A 4-20 mA loop is the workhorse of industrial instrumentation since the 1970s. Linear, simple, dirt-cheap to wire. But it has a fatal flaw : if the sensor fails by outputting 0 mA (wire broken) or 20 mA (sensor stuck at maximum), the control system has no way to distinguish “sensor failed” from “process variable is at the extreme of the range”.

A pressure transmitter on a steam line that fails high (20 mA) would be interpreted by the DCS as “pressure is at scale max” — an alarm condition, operator alerted. But a pressure transmitter that fails low (0 mA) would be interpreted as “pressure is at zero” — a normal-looking measurement, no alarm, operator unaware that the protection is offline.

This silent failure mode caused incidents. NAMUR NE 43 (1994) fixed it by reserving the out-of-range bands:

mA |  Status                            | Band
---+-----------------------------------+------------------
0  |                                    |
   |  ▓▓▓ Wire break / sensor failure ▓▓▓ |  < 3.6 mA (Fault LOW)
3.6|                                    |
   |  ░░░ Underrange (saturation low) ░░░ |  3.6 - 4.0 mA
4.0|                                    |
   |                                    |
   |  ■ Normal measurement (4-20 mA)  ■ |  4.0 - 20.0 mA  ← actual PV
   |                                    |
20.0|                                    |
   |  ░░░ Overrange (saturation high)░░░ |  20.0 - 21.0 mA
21.0|                                    |
   |  ▓▓▓ Wire break / sensor failure ▓▓▓ |  > 21 mA (Fault HIGH)

The DCS/PLC sees a current below 3.6 mA → it knows it’s a fault, not a measurement. Goes into “BAD QUALITY” or “STATUS FAULT” state. Operator alerted. Safety logic can take fail-safe action.

Why “NE” (recommendation) became universal

NAMUR is technically just a user association. NE 43 is a recommendation, not a regulatory requirement. So why does every transmitter sold today comply ?

Buyer power. NAMUR’s member companies (Bayer, BASF, Evonik, Henkel, Linde, etc.) buy billions of euros of instrumentation per year. When they collectively specify “must comply with NAMUR NE 43” in every procurement contract, every vendor either complies or loses the bid. After ~20 years of this, NE 43 became the de facto specification for any reputable 4-20 mA transmitter.

It’s the same dynamic with NE 21 (EMC), NE 43 (failure signaling), NE 53 (software change documentation), NE 130 (calibration / prior use), and so on. The NAMUR recommendations effectively define the floor of what counts as “industrial grade” instrumentation.

Implementation details

On the transmitter side

Every modern smart transmitter has a configuration parameter for failure direction :

• “Fail Low” (typical default) : on diagnostic fault, output drives to ≈ 3.6 mA
• “Fail High” : on diagnostic fault, output drives to ≈ 22 mA
• “Hold last value” : not NE 43 compliant, avoid in safety-critical loops

The choice depends on the fail-safe direction of the safety function. For a high-pressure trip, “fail high” forces the trip to activate even on sensor failure → fail-safe. For a low-flow trip, “fail low” forces the trip → fail-safe.

On the DCS / safety logic side

The control system must explicitly handle the diagnostic bands :

IF current < 3.6 mA OR current > 21 mA THEN
   pv_quality = "BAD"
   alarm = "Sensor failure on tag PT-101"
ELSE IF current >= 3.6 AND current < 4.0 THEN
   pv_quality = "UNCERTAIN"
   alarm = "PT-101 underrange"
   pv_value = range_min  // saturate
ELSE IF current > 20.0 AND current <= 21.0 THEN
   pv_quality = "UNCERTAIN"
   alarm = "PT-101 overrange"
   pv_value = range_max
ELSE  // 4.0-20.0
   pv_quality = "GOOD"
   pv_value = scale(current, range_min, range_max)

Most modern DCS (Emerson DeltaV, Siemens PCS 7, Honeywell Experion, ABB 800xA, Yokogawa CENTUM) handle this natively when the I/O card is set to “NAMUR mode”. Safety PLCs (Pilz PNOZ, HIMA HIMax, Siemens S7-1500F) typically require this mode in the safety I/O configuration.

Companion NAMUR recommendations worth knowing

• NE 21 : EMC requirements for process instrumentation
• NE 31 : Test and qualification of plant communication systems
• NE 53 : Software change documentation for field devices (the famous version-tracking requirement)
• NE 79 : Functional safety in process control — process industry interpretation of IEC 61511
• NE 107 : Self-monitoring and diagnosis of field devices (extends NE 43 to status flags via HART / Fieldbus)
• NE 130 : Functional and reliability of asset management for field devices (Prior Use statistics)

Verdict in our calculator

The 4-20 mA scaling calculator implements the NAMUR NE 43 bands : when you enter a current value, the calculator displays the corresponding diagnostic status alongside the scaled engineering value.

For Liquiphant-style tuning fork frequency sensors, see also NAMUR frequency status which applies the same diagnostic philosophy to frequency outputs.