NAMUR NE43 is a recommendation published by the NAMUR organization that standardizes the fault signaling of 4-20 mA analog instruments. It defines specific current levels outside the normal 4-20 mA measurement range to indicate sensor failures or diagnostic events. This allows control systems (like a PLC or DCS) to distinguish between a valid process measurement (e.g., 4.0 mA) and a fault condition (e.g., a broken wire).

The primary problem NE43 solves is ambiguity in fault detection within a standard 4-20 mA loop. Without it, a control system can't easily differentiate between:

• A true zero process measurement (4.0 mA).
• A broken wire or power loss to the transmitter (which would result in 0 mA).
• A measurement that is slightly out of its calibrated range.

NE43 creates clear "guard bands" above and below the 4-20 mA range, reserving these levels exclusively for fault and status information, thus eliminating this ambiguity.

NAMUR NE43 extends the standard 4-20 mA range to define clear operational and fault zones. The key levels are:

• Fault (Undershoot): <= 3.6 mA (Indicates a hardware failure, sensor break, etc.)
• Valid Measurement Range (Underrange): 3.8 mA to 4.0 mA (Indicates the process value is below the 0% calibration point but the sensor is healthy).
• Normal Operating Range: 4.0 mA to 20.0 mA (Represents 0% to 100% of the calibrated process measurement).
• Valid Measurement Range (Overrange): 20.0 mA to 20.5 mA (Indicates the process value is above the 100% calibration point but the sensor is healthy).
• Fault (Overshoot): >= 21.0 mA (Indicates a hardware failure, sensor short, etc.)

The "live zero" is the 4.0 mA signal that represents the 0% point of the measurement range in a 4-20 mA loop. Its key benefit is that a signal of 0 mA unequivocally indicates a fault (like a power loss or broken wire), because even at the lowest possible measurement, a healthy loop still has 4.0 mA of current flowing.

NAMUR NE43 enhances this by creating a buffer zone below 4.0 mA. Instead of dropping straight to 0 mA on a fault, an NE43-compliant transmitter will drop to a specific, controlled level (e.g., 3.6 mA). This provides a more robust and intentional fault signal than just a complete loss of signal.

Both are fault states, but they are driven by different failure modes and user configurations:

• Undershoot (Fail-Low): This is when the transmitter outputs a current of <= 3.6 mA. It is typically configured to signal failures like a sensor wire break, corrosion, or internal electronic faults. This is the more common fail-safe direction.
• Overshoot (Fail-High): This is when the transmitter outputs a current of >= 21.0 mA. It can be configured to signal the same types of failures as the undershoot condition. The choice between fail-low and fail-high depends on the process safety philosophy—what state is safer for the process to be in upon a sensor failure (e.g., close a valve vs. open a valve).

This is the core purpose of the standard. Here’s the distinction:

• True Zero Measurement: A healthy, NE43-compliant transmitter will output exactly 4.0 mA. The control system interprets this as a valid 0% process value.
• Wire Break: A wire break causes a complete loss of loop power, resulting in 0 mA. However, an NE43-compliant transmitter, upon detecting an internal sensor fault (like a thermocouple burnout), will actively drive the current to a controlled fault level, such as <= 3.6 mA (fail-low) or >= 21.0 mA (fail-high).

The control system is programmed to see any value below 3.8 mA or above 20.5 mA as a fault, clearly distinguishing it from the valid 4.0 mA signal.

NE43 provides distinct levels to differentiate these two important states:

• Saturation / Out-of-Range: This indicates the sensor is still healthy and working, but the process value has exceeded the calibrated measurement range (0-100%). The transmitter will output a signal between 3.8 mA - 4.0 mA (underrange) or 20.0 mA - 20.5 mA (overrange). This tells the operator the measurement is valid but outside the expected norms.
• Fault Condition: This indicates the sensor or transmitter itself has failed. The measurement is no longer reliable. The transmitter signals this by driving the current to the dedicated fault levels: <= 3.6 mA or >= 21.0 mA. This prompts immediate maintenance action.

• Improved Diagnostics: It provides a clear, unambiguous signal for instrument faults, separating them from out-of-range measurements.
• Enhanced Process Safety: By allowing for predictable fail-safe behavior (fail-low or fail-high), it enables control systems to take appropriate automated action during a sensor failure, preventing dangerous conditions.
• Standardization: It creates a uniform standard across different instrument vendors, simplifying the configuration of control systems and reducing engineering effort.
• Reduced Downtime: Maintenance teams can quickly identify the root cause of a problem. An NE43 alarm points directly to an instrument fault, rather than an unusual process condition, speeding up troubleshooting.

No, it is not a mandatory or legally binding standard. It is a recommendation (the 'NE' in NAMUR NE stands for 'Normenempfehlung', German for 'standard recommendation'). However, it has become a de-facto industry standard for process automation and is widely adopted by virtually all major instrument manufacturers and end-users due to its clear benefits in safety and reliability.

The analog input (AI) cards and control logic in a modern PLC or DCS are configured to recognize the NE43 ranges:

• The AI card is configured with detection limits, for example, a low-low limit of 3.7 mA and a high-high limit of 20.8 mA.
• If the incoming signal crosses these limits (e.g., drops to 3.6 mA), the AI card's channel driver flags a "Bad" or "Uncertain" status for the process variable.
• The control logic (function blocks) will then use this status bit to ignore the faulty process value in any control calculations (e.g., placing a PID loop in manual).
• Simultaneously, an alarm is generated in the HMI/operator station, specifically indicating an "Instrument Fault" or "Hardware Failure," directing maintenance to the correct device.

NAMUR is a German acronym for "Normenarbeitsgemeinschaft für Mess- und Regeltechnik in der chemischen Industrie," which translates to the "User Association of Automation Technology in Process Industries."

It is an international association of user companies (primarily from the chemical, pharmaceutical, and petrochemical sectors) that works to define requirements, develop recommendations, and standardize automation technologies to improve efficiency, safety, and reliability in process plants.

Yes, but it's not ideal and requires careful consideration. A non-NE43 transmitter might simply output 0 mA on failure.

Implications:

• If a non-NE43 transmitter fails (outputting 0 mA) on a channel configured for NE43 (expecting <= 3.6 mA), the control system will correctly interpret it as a fault. The desired outcome is achieved, but it's not a controlled fault signal.
• The main issue is the lack of overrange/underrange information. A non-NE43 transmitter might just clamp its output at 4 mA or 20 mA when the process exceeds its range. The control system would not see the valuable 3.8 mA or 20.5 mA signals, losing the ability to distinguish between a saturated measurement and one at the exact range limit.
• In summary, you lose the diagnostic richness of the NE43 standard.

This setting determines the transmitter's output current during a fault. The choice is based on process safety.

• Fail-Safe Low (Undershoot): The transmitter outputs <= 3.6 mA. The control system is programmed to take the "safer" action for a low signal.
  • Example: A temperature transmitter on a reactor coolant line. If it fails, you want the coolant valve to open fully to prevent overheating. If the valve is "fail-open" (opens on loss of signal), a fail-low setting is appropriate. The 3.6 mA signal would cause the controller to drive its output to 0%, opening the valve.
• Fail-Safe High (Overshoot): The transmitter outputs >= 21.0 mA. The control system is programmed to take the "safer" action for a high signal.
  • Example: A level transmitter in a tank that is filling. If the level sensor fails, you want to immediately stop the inflow to prevent an overflow. The fail-high signal (21 mA) would be interpreted as a high-level alarm, triggering the control logic to close the inlet valve.

NE43 significantly streamlines maintenance workflows:

• Clear Problem Identification: An alarm generated by an NE43 fault level (e.g., "AI Channel Fault") immediately tells technicians that the problem lies with the instrument or its wiring, not the process itself. This prevents them from wasting time investigating process upsets.
• Reduced "No Fault Found": It reduces instances where a technician is sent to the field for a suspected instrument failure, only to find the process was genuinely at 0% or 100%.
• Proactive Maintenance: Some advanced diagnostics can be mapped to the NE43 fault levels. For example, a pH sensor nearing the end of its life could trigger a fault signal before it fails completely, allowing for planned replacement instead of a reactive shutdown.

This buffer zone, sometimes called a "deadband," is crucial for preventing false alarms. It provides a clear separation between the lowest valid signal and the highest fault signal.

• It accounts for minor inaccuracies, drift, or noise in the analog-to-digital converters (ADCs) of the control system's input card.
• Without this buffer, a signal that is very close to 3.8 mA could fluctuate slightly due to electrical noise and intermittently trigger the fault threshold of 3.79 mA, leading to nuisance alarms.
• The buffer ensures that only a deliberate, sustained drop to the fault level (<= 3.6 mA) is registered as a genuine hardware failure.

Scenario: A feed flow transmitter (calibrated 0-100 GPM) controls a critical chemical injection pump in a reactor. The control loop is tuned to maintain a flow of 50 GPM (12 mA).

• Without NE43: The flow transmitter's sensor gets corroded and fails. Its output drops to 4 mA. The control system sees this as a valid 0 GPM flow. The PID controller reacts by ramping the injection pump to 100% to try and restore the flow to 50 GPM. This results in a massive overdose of the chemical, potentially ruining the batch, causing a dangerous reaction, and forcing a complete plant shutdown for cleanup.
• With NE43: The same sensor corrosion occurs. The NE43-compliant transmitter detects the internal failure and drives its output to 3.6 mA (fail-low). The control system immediately flags the input as "Bad," puts the PID loop into manual mode (holding the pump at its last good value or a safe state), and generates an "Instrument Fault" alarm. The overdose is completely avoided. An operator can safely intervene while maintenance replaces the faulty transmitter.

While overwhelmingly beneficial, there are minor considerations:

• Legacy Equipment: It may not be supported by older instruments or legacy control system I/O cards, which might not be configurable to detect the extended ranges. This can complicate retrofits.
• Configuration Overhead: Both the transmitter (for fail-safe direction) and the control system's analog input channel must be correctly configured to use the standard. Misconfiguration can lead to the system not recognizing the fault signals.
• Single Fault Indication: The analog signal itself can only indicate one thing: "There is a fault." It cannot provide details on the *type* of fault. For more detailed diagnostics, a digital communication protocol like HART or Fieldbus is needed.

NE43 and HART are complementary technologies that often work together.

• NE43 is the "What": It provides the immediate, high-level alert that a fault has occurred via the analog 4-20 mA signal. It's a simple, fast, and robust "something is wrong" indicator.
• HART is the "Why": The HART (Highway Addressable Remote Transducer) protocol uses a digital signal superimposed on the same 4-20 mA wiring. Once the control system gets the NE43 fault alarm, a technician can use a HART communicator or asset management software to digitally interrogate the device and get specific diagnostic details, such as "Sensor wire break," "Electronics failure," or "Device requires maintenance."

In short, NE43 raises the alarm, and HART provides the detailed diagnostic report.

The core of the NE43 recommendation is the standardization around the <= 3.6 mA and >= 21.0 mA values. Most compliant transmitters will be fixed to these levels for their fault signaling to ensure interoperability.

However, the detection thresholds in the PLC or DCS are almost always customizable. A user could, for example, set the fault detection threshold at 3.7 mA instead of the implied 3.8 mA. While possible, it is highly recommended to stick to the standard levels to maintain consistency and avoid confusion during operations and maintenance.

The loop power supply must be robust enough to handle the full range of NE43 signals. Specifically:

• It must provide enough voltage to drive the highest required current (>= 21.0 mA) through the total loop resistance (wiring + instrument + input card resistance).
• If the power supply is inadequate, the transmitter might not be able to physically generate the 21.0+ mA required for a fail-high signal, even if it's trying to. This could lead to the fault signal not being properly transmitted or detected.
• Therefore, during loop design, engineers must perform a voltage drop calculation to ensure the power supply is sufficient for the worst-case (fail-high) scenario.