What is NAMUR Sensor?
What is a NAMUR sensor?
A NAMUR sensor communicates its detection state through a low-current, two-wire interface. Power and state information share the same pair. Rather than supplying a conventional 24 V switching output, the sensor changes the current evaluated by a switch amplifier or a dedicated NAMUR input. The interface is covered by IEC 60947-5-6. [1]
The name comes from a German user organisation founded in 1949 for measurement and control in the chemical industry. Its remit later broadened to process-industry automation. In this context, NAMUR identifies an interface convention, not a sensor brand. [2]
Two wires do not establish compatibility. A conventional two-wire DC switch, a NAMUR detector and a loop-powered 4–20 mA transmitter can all have two conductors while requiring different receiving circuits. Check the output specification, not just the terminal count. [3]
Does it need a separate amplifier?
A standard 24 V digital input is not a NAMUR input. A PNP/NPN interface has a different electrical arrangement. One common path is NAMUR sensor → switch amplifier → PLC input. The amplifier supplies the field circuit and presents a suitable relay or transistor output to the controller. [9]
A separate box is not always necessary. A dedicated NAMUR input module can provide the interface directly; Siemens ET 200SP DI 8×NAMUR HF is one example. That answers the electrical interface question, not the separate questions of hazardous-area approval or suitability for a safety function. [7]
An ordinary analog channel is not an automatic substitute. Being able to read a small current does not prove that the channel provides the correct sensor supply, line diagnostics or required protection. Likewise, adding a resistor to a general-purpose supply does not create a certified intrinsic-safety barrier.
What 8.2 V and the current figures mean
8.2 V DC with approximately 1 kΩ source resistance is the reference supply condition shown in NAMUR documentation, including the Pepperl+Fuchs NJ8-18GK-N-10M data sheet. It does not mean that the loaded sensor terminals always sit at 8.2 V or that a fixed 5 mA must be forced through the device. Voltage is specified in volts; the state-related current is specified in milliamps. [4]
The interface is separate from the sensing method. When considering inductive, capacitive or photoelectric sensing, verify the output option as well. The example below uses an NC characteristic, with current falling when the target is detected. Other operating directions exist, so read NO/NC behaviour alongside any output inversion in the amplifier. [3]
Normal operation uses two current regions. For example, Contrinex specifies ≤1 mA / ≥2.2 mA for its DW-AD-605-M12. These are not promises that every instrument will read exactly 1.0 or 2.2 mA. A NAMUR switching signal should not be scaled as if it were a calibrated 4–20 mA measurement of position or percentage. [5]
Receiver thresholds are a separate specification. The Pepperl+Fuchs KFD2-SR2-Ex2.W lists a switching point within 1.2–2.1 mA and approximately 0.2 mA switching hysteresis. The transition region is not an extra process state. Interpret readings there using the actual sensor characteristic and receiver limits. [6]
A valid low current is not a broken wire
A detected target can produce low but non-zero current. An open loop instead drives current towards zero. The same amplifier specifies I ≤0.1 mA for lead breakage and I >6 mA for short circuit detection. These are that product’s diagnostic thresholds, not universal settings to copy into another input module. [6]
Line monitoring must be enabled and used. A zero-current observation alone cannot distinguish every possible cause, such as a broken connection or loss of supply. More importantly, detecting a fault does not by itself place the process in a safe state. The controller must not silently treat invalid field information as an ordinary “target clear” condition.
Four states, shown separately
The teaching sequence uses 3.0 mA, 0.7 mA, 0 mA and 7.0 mA. It assumes an NC sensor characteristic and enabled line monitoring with the example receiver limits above. The figures are selected for the demonstration; they are not live readings.
Start plays the sequence from the beginning; Stop freezes the displayed frame. The stages represent target clear, target detected, open loop and short circuit. This is not an installation drawing or an intrinsic-safety assessment. Do not reproduce the illustration by deliberately shorting a live field circuit.
Three separate questions: NAMUR, Ex i and SIL
NAMUR concerns the signal interface. Ex i concerns intrinsic safety against ignition. SIL concerns the integrity of a safety function. Low operating current alone does not give a sensor hazardous-area approval. Under the ATEX framework, equipment must suit the relevant area classification. [8]
An intrinsically safe loop needs compatible documentation for the field device and associated apparatus. Check voltage, current, power, cable capacitance and inductance together with group, temperature limits and any special conditions. Two devices carrying Ex markings do not automatically form an acceptable combination. [8]
Wire-break detection is not a SIL certificate. A safety function under IEC 61508 involves the complete sensor, interface, logic and final-element chain. Use the approved combinations, safety manuals and validation requirements. NAMUR is not the only sensing approach available for hazardous locations; the protection concept and application determine the selection. [3]
Replacing a sensor: check the entire signal path
Match the complete specification, not the barrel diameter. An M12 or M18 housing can contain different output versions. Compare detection method, target distance, mounting arrangement, operating direction and electrical interface. A listing in a two-wire and NAMUR category is not enough to establish that an individual product has a NAMUR output.
Follow the pulse through every device. For speed monitoring or an encoder signal, the receiver’s counting capability matters as much as the sensor. The relay amplifier used above is rated at 10 Hz; a different transistor-output model, KFD2-ST3-Ex1.LB, specifies 5 kHz. A faster sensor does not remove a slower interface bottleneck. [6] [9]
Review the installation details. Confirm pin assignments and cable connections, polarity, cable length and suitable mounting accessories. In an Ex circuit, changing a cable can change the documented loop conditions. Make wiring changes under safe isolation; hazardous-area checks require appropriate test equipment and qualified personnel. [8]
Which standard edition is being referenced?
IEC 60947-5-6:1999 addresses the DC interface between proximity sensors and switching amplifiers. Its European adoption is EN 60947-5-6:2000. Both remain listed in the IEC and DIN catalogues checked on 6 October 2026; an old publication date is not, by itself, evidence of withdrawal. The interface standard does not replace Ex or functional-safety requirements. [1] [10]