NAMUR Proximity Sensors Explained: The 8.2 V Interface and Intrinsic Safety

A NAMUR proximity sensor is a two-wire inductive switch that does not switch a load at all. It varies the current it draws from an 8.2 V supply — below 1.2 mA when damped, above 2.1 mA when undamped. An isolating amplifier outside the hazardous area reads that current, and because the levels are unambiguous it also detects cable break and short circuit.

The problem NAMUR solves

An ordinary PNP proximity switch tells you one of two things: target present, or target absent. It has no way of telling you a third thing that matters far more in a refinery or a pharmaceutical plant — that the sensor itself, or its cable, has failed.

If a rodent chews through the cable of a standard NO sensor, the input reads “no target”. If the sensor’s output transistor fails open, the input reads “no target”. A valve that has not moved and a valve whose feedback cable is severed look identical to the control system.

NAMUR removes that ambiguity by encoding the sensor state as a current level rather than a switched voltage, and by leaving deliberate gaps between the levels. Any current outside the two valid bands is a fault, and the amplifier says so on a separate output.

The current bands in full

Sensor current	Meaning	Amplifier response
< 0.15 mA	Open circuit — cable break or disconnected sensor	Fault output operates; signal output goes to its defined safe state
0.15 – 1.2 mA	Damped — target present in the sensing field	Signal output switches
1.2 – 2.1 mA	Transition band — no defined state	Output holds its last valid state
2.1 – 6 mA	Undamped — no target present	Signal output in its rest state
> 6 mA	Short circuit in the field wiring or sensor	Fault output operates

Note that “damped” means the target has entered the field and damped the oscillator — which corresponds to the lower current. This trips people up, because intuition says more target should mean more current. It is the opposite.

Whether the amplifier’s output reads as “on” or “off” for a given sensor state is set on the amplifier, not the sensor, so the same NAMUR sensor can serve an NO or NC function depending on how the loop is configured.

Why this makes intrinsic safety straightforward

Intrinsic safety works by limiting the electrical energy available in the hazardous area to below the level that can ignite the gas or dust present. A NAMUR sensor operates on 8.2 V at a few milliamps — a few tens of milliwatts. There is simply not enough stored or available energy in the loop to produce an igniting spark or hot surface, even under fault conditions.

That makes NAMUR loops naturally suited to Ex ia certification, the highest intrinsic-safety protection level, which permits use in Zone 0 (gas present continuously) and Zone 20 (dust). The isolating amplifier sits in the safe area and acts as the barrier: it limits voltage and current into the field circuit and provides galvanic isolation between the hazardous-area loop and the control system.

The division of responsibility is worth being clear about. The sensor alone is not a safety device. Certification applies to the combination of the certified sensor, the certified amplifier or barrier, and field wiring that meets the stated cable parameters (capacitance and inductance limits). Substituting an uncertified amplifier voids the whole loop.

Where NAMUR sensors are used

• Oil, gas and petrochemical — valve position feedback on manifolds, pump and compressor monitoring, tank farm equipment.
• Pharmaceutical — solvent handling areas, reactor and centrifuge interlocks, where Zone 1 classification is routine.
• Chemical processing — anywhere flammable vapour is a normal part of operation.
• Grain, sugar, flour and pigment handling — dust atmospheres in Zone 20 and 21, where dust-cloud ignition is the hazard.
• Paint and coating plant — spray booths and solvent stores.
• Safety-related duties in non-hazardous areas, purely for the line-monitoring benefit.

That last case is often overlooked. Plenty of installations use NAMUR sensors with no explosion risk at all, simply because knowing the difference between “no target” and “broken cable” is worth having on a critical interlock.

Specifying a NAMUR loop

1. Establish the zone and gas or dust group. This determines the required equipment category and the certification the sensor and amplifier must carry. Get it from the site’s hazardous area classification drawing, not from assumption.
2. Choose the sensor. Thread size and rated operating distance are selected exactly as for a standard inductive sensor, including the reduction factor for the target material and the assured-distance rule. NAMUR changes the electrical interface, not the physics of detection.
3. Choose the amplifier. One channel per sensor. Confirm the entity parameters (U_o, I_o, P_o, C_o, L_o) are compatible with the sensor and the cable run.
4. Check the cable. Total capacitance and inductance of the field wiring must stay within the amplifier’s C_o and L_o limits — this is what caps the permitted cable length. Use the cable manufacturer’s per-metre figures.
5. Decide the fault action. What should the plant do when the amplifier reports a line fault? Alarm only, or trip? Design it deliberately rather than leaving the fault contact unwired, which is depressingly common.
6. Keep intrinsically safe wiring segregated and identified in light blue, in its own trunking, terminated on marked IS terminals.

NAMUR compared with a standard three-wire sensor

	NAMUR (2-wire)	Standard PNP/NPN (3-wire)
Supply	8.2 V from amplifier	10–30 V DC
Output	Current level, 0.15–6 mA	Switched voltage, up to ~200 mA
Drives a load directly	No — needs an amplifier	Yes
Detects cable break	Yes	No
Detects short circuit	Yes	Only via output protection, not reported
Suitable for Zone 0 / 20	Yes, with certified barrier	No
Cost per point	Higher — amplifier channel required	Lower
Wiring	Two cores, segregated IS routing	Three cores, standard routing

If your application needs neither hazardous-area approval nor line monitoring, a standard three-wire sensor is simpler and cheaper — see our comparison of PNP and NPN outputs and of two, three and four-wire wiring schemes.

Frequently asked questions

Can I connect a NAMUR sensor directly to a PLC input?

No. A NAMUR sensor has no output stage — it only modulates its own supply current, and at levels a standard 24 V digital input cannot interpret. It requires a NAMUR isolating amplifier or a dedicated NAMUR-capable input module. Connecting one to a normal 24 V input will not work and may damage the sensor.

Does a NAMUR sensor make my installation intrinsically safe on its own?

No. Intrinsic safety is a property of the complete loop: certified sensor, certified associated apparatus (the amplifier or barrier), and field wiring within the stated capacitance and inductance limits. All three must be right, and the documentation must show the entity parameters match.

What is the difference between damped and undamped?

Damped means a target is present in the sensing field and is damping the oscillator — the sensor then draws less than 1.2 mA. Undamped means no target is present and the sensor draws more than 2.1 mA. The lower current corresponds to target present, which is the reverse of most people’s intuition.

Why is the supply exactly 8.2 V?

It is the value fixed by the NAMUR recommendation and carried into IEC 60947-5-6, chosen to sit comfortably below the ignition-energy limits for intrinsically safe circuits while still giving the sensor enough headroom to run its oscillator reliably. The amplifier supplies it through a defined internal resistance, so the sensor and amplifier form a consistent, interchangeable pair across manufacturers.

Can NAMUR sensors be used outside hazardous areas?

Yes, and it is a sound choice for critical interlocks. You give up the ability to drive a load directly and pay for an amplifier channel, and in return the control system can distinguish a genuine “no target” from a broken cable or a shorted line. On a safety-related position feedback that distinction is often worth the extra cost on its own.