On quick visual inspection, a 2-wire NAMUR sensor looks the same as a standard 2-wire DC sensor. But do not confuse the wiring connections between these two types of sensors. On the inside, NAMUR sensors have different circuitry than standard 2-wire DC sensors and need special treatment.
Due to approvals required for hazardous environments, intrinsically safe sensor configurations require a NAMUR output sensor and an intrinsic safety barrier. These configurations operate at a lower current and lower voltage to prevent the electrical equipment from causing an explosion.
The specification for NAMUR output sensors is typically 5 mA of current at 8.2 V, while standard 2-wire DC sensors operate at current levels up to 100 mA in the range of 5 … 60 V. Voltage drops and off-state currents are also different when a target is detected.
A PLC typically recognizes a standard 2-wire DC sensor output by a current or voltage drop. This is also true for NAMUR sensors, just at a different current or voltage value. A typical PLC input card is not rated to supply exactly 5 mA of current at 8.2 V to a sensor, which is why a NAMUR sensor must always be connected to an intrinsic safety barrier. The barrier provides the correct power to the sensor and supplies the PLC input card with the correct signal. Supplying a NAMUR sensor with 100 mA of current between 5 … 60 VDC can fry its circuitry.