UL 60947-5-2: Proximity Switches

UL 60947-5-2

UL 60947-5-2: Proximity switches

Choosing the right sensor and connection for non-contact detection

UL 60947-5-2 addresses the construction and performance of proximity switches used in industrial control circuits. Part of the UL 60947 series, it combines an IEC base text with North American national differences. When selecting a proximity sensor, consider the target, electrical output and installation conditions alongside the standard reference.

Editor’s note: Compliance with UL 60947-5-2 does not automatically give a sensor a SIL or PL rating. A standard sensor counting products and a sensing system assessed for safely stopping hazardous motion have different requirements. “UL approved” alone is insufficient as a selection criterion for personnel protection.

Which sensors are covered?

The publication covers inductive, capacitive, ultrasonic and photoelectric sensors, as well as non-mechanical magnetic proximity switches. Select the sensing principle for the target: metal parts, non-metallic materials, sound-reflecting surfaces and objects affecting a light path require different consideration.

The scope addresses self-contained products with semiconductor switching elements and expressly excludes proximity switches with analogue outputs. Its circuit rated-voltage limits are 250 V AC at 50/60 Hz or 300 V DC. These are scope limits, not permission to apply a higher voltage to a sensor rated for 24 V DC.

Why check UL and IEC editions separately?

A publisher check on 17 October 2026 identifies the fourth edition of UL 60947-5-2, dated 31 March 2022, as active. It is based on IEC 60947-5-2:2007 and Amendment 1:2012. The IEC fourth edition published in 2019 contains separate technical revisions. “Fourth edition” on both publications does not mean their contents are identical.

Sensing distance is more than one number

Sn is the rated operating distance; it does not by itself account for manufacturing tolerances and every external influence. Sr is the effective distance measured under specified conditions; Sa identifies the range in which operation is assured under stated conditions. Target material and size can change the result. Do not treat the catalogue distance as an installation gap that will work under every machine operating condition.

For example, a distance specified for an inductive sensor with a steel target may not apply to a smaller aluminium part. Check flush and non-flush mounting, surrounding metal and spacing between adjacent sensors. Verify the final setting with the actual target under the expected operating conditions.

PNP, NPN and two-wire connections

In three-wire DC models, PNP and NPN outputs differ in the direction of current through the load. A PNP output sources current from the positive supply, while an NPN output sinks load current towards 0 V. Normally open and normally closed are separate operating functions, available with either output type. Match the sensor to the PLC input card’s wiring diagram.

With two-wire sensors, off-state leakage current and on-state voltage drop can affect whether the load switches correctly. A sensitive input may fail to return to its expected off state when the sensor is inactive. Matching supply voltages alone does not establish compatibility between a sensor and an input. NAMUR sensors can also use two wires, but their interface requires a suitable switching amplifier and must not be confused with a conventional PNP/NPN connection.

Hysteresis, repeatability and high-speed counting

Hysteresis is the difference between the switching points as a target approaches and moves away. Repeatability describes the consistency of successive detections under the same conditions. Read these characteristics alongside switching frequency and response times; they describe different aspects of performance.

Worked example: If 10 mm-long parts move at 1 m/s with a 10 mm gap between them, the ideal geometric target and gap times are each 10 ms, giving 50 pulses per second. This calculation does not complete sensor selection. Assess detection geometry, output delays, PLC input filtering and the method used to read the signal together. Use a high-speed counter input where required: an ordinary input can still miss short pulses from a fast sensor.

Electrical performance and environmental conditions

Assessment considers markings, insulation, rated characteristics and performance under the specified operating conditions. Electromagnetic noise immunity cannot compensate for every installation error. Route sensor cables with suitable separation from power cables and arrange the supply and shielding according to the manufacturer’s instructions.

An IP rating under IEC 60529 is separate from shock, vibration, chemical resistance and temperature limits. Confirm short-circuit and reverse-polarity protection in the specific model’s data. General operating limits can differ from temperature or supply conditions stated for UL approval. Take test voltages and acceptance criteria from the applicable edition rather than assigning one universal test value to every sensor.

What are the limits for machine safety?

A proximity switch’s normal operating performance does not establish safe behaviour under faults. IEC 60947-5-3 adds requirements for proximity devices with defined behaviour under fault conditions. The safety-related control system is assessed under ISO 13849 or IEC 62061 as appropriate to the application. Establishing device suitability and validating the complete safety function are separate stages.

Protective equipment that directly detects people also has its own applicable standards. For a guard, press or hazardous-motion application, sensing distance and a UL 60947-5-2 reference alone are insufficient to select a safety solution.

Documentation checks and maintenance

When reviewing UL markings, match the manufacturer, complete model code, certification record and application conditions. UL Listed and UL Recognized Component are different forms of evaluation. A Recognized component’s conditions of acceptability, such as enclosure, wiring or temperature requirements, must be maintained in the end product. Evaluating a sensor does not certify the complete machine.

During maintenance, inspect for loose connections, displaced targets, accumulated metal chips and cable damage. For intermittent detection, first compare the supply, target distance, mounting and input state. A changing sensor LED does not by itself show that the PLC is counting every pulse correctly.