JoeGuardian Automation

NAMUR Sensors Explained

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Operation, Wiring, Diagnostics, and Troubleshooting

Introduction

If you work around hazardous area instrumentation, safety barriers, or process automation systems, you will eventually encounter a device called a NAMUR sensor.

Many technicians initially assume a NAMUR sensor is simply another proximity sensor.

It is not.

A NAMUR sensor operates using a completely different principle than traditional PNP or NPN proximity sensors and is specifically designed for use with intrinsic safety barriers and hazardous locations.

Understanding NAMUR sensors is essential for troubleshooting:

• Pepperl+Fuchs barriers
• MTL barriers
• Intrinsically safe circuits
• Process instrumentation systems
• Hazardous area applications

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What Is a NAMUR Sensor?

A NAMUR sensor is a low-energy proximity sensor designed to operate in intrinsically safe circuits.

Unlike traditional sensors that switch voltage, a NAMUR sensor changes the amount of current it draws from the barrier.

The barrier monitors this current and determines whether the sensor is ON or OFF.

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Why Was NAMUR Developed?

In hazardous areas, the goal is to minimize electrical energy.

Traditional proximity sensors require:

24 VDC
Several milliamps

NAMUR sensors require significantly less energy.

Benefits:

• Reduced ignition risk
• Better intrinsic safety compatibility
• Simpler hazardous area approvals
• Superior fault diagnostics

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How a Standard Proximity Sensor Works

PNP Sensor

+24V
  │
Sensor
  │
  └──► PLC Input

Sensor outputs voltage.

PLC detects voltage.

Simple.

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How a NAMUR Sensor Works

NAMUR Sensor

Barrier
   │
   │
NAMUR Sensor
   │
   │
Barrier

The sensor does NOT switch voltage.

Instead:

Target Present
↓
Current Changes
↓
Barrier Detects Change
↓
PLC Receives Signal

The barrier interprets the sensor current.

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Internal Principle

A NAMUR sensor behaves like a variable load.

No Target

Current ≈ 2.1 mA

Target Present

Current ≈ 1.2 mA

The exact values vary slightly by manufacturer.

The barrier monitors this current continuously.

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Signal Interpretation

The barrier converts current into a digital state.

Example:

Sensor Current	Barrier Interpretation
> 2 mA	ON
< 1.5 mA	OFF

The barrier then energizes:

• Relay output
• Transistor output
• PLC signal

depending on the module type.

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Why Use a Barrier?

A NAMUR sensor cannot connect directly to a PLC input.

A PLC expects:

24VDC

A NAMUR sensor provides:

Current Change

Therefore:

NAMUR Sensor
      ↓
Barrier
      ↓
PLC

The barrier performs signal conversion and intrinsic safety protection.

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Typical Wiring

Hazardous Area

NAMUR Sensor
      ↓
Intrinsic Safety Barrier

Safe Area

Barrier Output
      ↓
PLC Input

Complete circuit:

Hazardous Area
      │
NAMUR Sensor
      │
Safety Barrier
      │
PLC Input Card
      │
Safe Area

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NAMUR vs PNP vs NPN

Feature	NAMUR	PNP	NPN
Output Type	Current Change	Voltage Source	Current Sink
Typical Voltage	8 VDC	24 VDC	24 VDC
Hazardous Areas	Excellent	Limited	Limited
Requires Barrier	Yes	Usually No	Usually No
Line Fault Detection	Excellent	Limited	Limited

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Advantages of NAMUR Sensors

Intrinsic Safety Compatible

Ideal for:

• Zone 0
• Zone 1
• Zone 2

applications.

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Low Power Consumption

Typically operates below:

10 V
15 mA

making ignition extremely unlikely.

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Superior Diagnostics

The barrier can detect:

Open Wire

Cable Break

Short Circuit

Conductors Touching

Sensor Failure

Internal Sensor Fault

Many modern barriers immediately report these conditions.

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Understanding Barrier LEDs

Most switch amplifiers provide:

Green LED

Power OK

Yellow LED

Channel Active

Red LED

Fault

Understanding these LEDs is often the fastest troubleshooting method.

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Common Troubleshooting Scenario #1

Sensor Not Detected

Symptoms:

Machine Fault
PLC Input OFF

Check:

Step 1

Verify barrier power.

Green LED should be ON.

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Step 2

Verify sensor target.

Move metal target into sensing range.

Observe yellow LED.

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Step 3

Measure sensor current.

Typical readings:

≈1.2 mA
≈2.1 mA

depending on target state.

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Step 4

Check PLC input.

Barrier output should change state.

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Common Troubleshooting Scenario #2

Flashing Red LED

Typically indicates:

Line Fault

Possible causes:

• Broken wire
• Loose terminal
• Damaged cable
• Failed sensor

Always inspect field wiring first.

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Common Troubleshooting Scenario #3

Solid Red LED

Typically indicates:

Internal Barrier Fault

Possible causes:

• Electronics failure
• Internal component damage

Module replacement may be required.

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Practical Multimeter Test

One of the simplest field tests:

Measure Barrier Supply

Verify:

24 VDC

at the barrier.

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Measure Sensor Circuit

Many NAMUR barriers provide approximately:

8 VDC

to the sensor.

If you measure:

0 V

and power is present,

the barrier or channel may be damaged.

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The Jumper Test

Technicians frequently use this method.

Disconnect sensor wiring.

Install temporary jumper across sensor terminals.

Observe:

Yellow LED
Relay Output
PLC Input

Results:

Changes State

Barrier likely good.

Sensor or field wiring suspect.

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No Change

Barrier or channel suspect.

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Real Industrial Example

Imagine a NAMUR proximity sensor monitoring valve position.

Operator reports:

Valve Open Feedback Lost

Investigation:

Green LED

ON

Red LED

Flashing

Yellow LED

OFF

Diagnosis:

Broken cable

Repair cable.

Fault clears.

System returns to service.

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Common Mistakes

Mistake 1

Connecting a NAMUR sensor directly to a PLC input.

It will not work correctly.

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Mistake 2

Treating NAMUR like a PNP sensor.

The operating principle is completely different.

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Mistake 3

Ignoring line fault alarms.

Many barriers are telling you exactly where the problem exists.

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Mistake 4

Replacing the barrier before checking the sensor.

Field wiring problems are far more common than barrier failures.

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Technician Troubleshooting Flow

Power
 ↓
Barrier LEDs
 ↓
Field Wiring
 ↓
Sensor
 ↓
Barrier Output
 ↓
PLC Input

Following this sequence systematically can reduce troubleshooting time dramatically.

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Final Thoughts

NAMUR sensors are one of the most common sensor technologies used in hazardous area instrumentation.

Although they appear similar to conventional proximity sensors, they operate using current changes rather than voltage switching and always require a compatible switch amplifier or intrinsic safety barrier.

For automation technicians, learning NAMUR technology is a critical step toward understanding:

• Intrinsic Safety
• Hazardous Area Instrumentation
• Safety Barriers
• Process Control Systems
• Advanced Field Troubleshooting

Mastering NAMUR sensors makes troubleshooting intrinsically safe systems significantly easier and helps bridge the gap between traditional automation and process instrumentation.