Safety Sensors

A safety sensor is an input device that forms part of the safety-related parts of a control system (SRP/CS). It detects a hazardous condition or access to a hazardous area in order to initiate a safety function (e.g. stopping, preventing restart, or preventing hazardous movement).

According to ISO 13849-1:2023, SRP/CS must be designed and integrated in such a way that the safety function can achieve a defined Performance Level (PL). The safety sensor supports this, among other things, through internal fault detection/diagnostics and suitability for use within a safety architecture (Category B, 1, 2, 3, or 4).

Please note: PL (and SIL) always applies to the complete safety function (sensor + logic + actuation), not to the sensor alone.

A standard industrial sensor detects position or presence, but it is not intended or validated for functional safety applications.

A safety sensor is typically:

• Designed for use in safety functions within SRP/CS (ISO 13849-1) or safety systems (IEC 62061).
• Equipped with diagnostics/fault detection (e.g. channel monitoring, short-circuit detection, plausibility checks).
• Intended to transition predictably to a safe state in the event of a fault (fail-safe behavior within the intended function).

Supplied with documentation/data required for the design and calculation of the safety function (e.g. for PL calculations within the complete SRP/CS).

The most commonly used types of safety sensors for door and guard monitoring (according to ISO 14119) are:

Magnetic safety sensors
Contactless and wear-free. Suitable for standard applications with limited tampering risk.

RFID-coded safety sensors
Operate with uniquely coded actuators. Provide a higher coding level and improved protection against tampering.

Safety sensors with OSSD outputs
Equipped with dual-channel electronic safety outputs with fault detection. Suitable for integration into higher safety architectures (e.g. Category 3 or 4 according to ISO 13849-1).

ATEX safety sensors
Suitable for potentially explosive atmospheres in accordance with ATEX 2014/34/EU.

The correct choice depends on the required Performance Level (PLr), the tampering risk, and the environmental conditions.

No. PL applies to the complete safety function, which consists at minimum of:

• Input (sensor)
• Logic (safety relay or safety PLC)
• Output (actuators such as contactors, valves, or drive functions such as STO)

The final PL is determined by the combination of all components as well as the selected architecture (category), diagnostics (DC), and measures against common cause failures (CCF).

A manufacturer may state that a sensor is “suitable for applications up to PL e”, but this is not proof that your safety function automatically achieves PL e.

For sensors used as interlocking devices on guards (e.g. door protection), the primary applicable standard is:

ISO 14119 (latest edition ISO 14119:2024, successor to the 2013 edition) – principles for design/selection and measures to reduce tampering/defeat.

In addition, integration into the safety function must comply with ISO 13849-1:2023 (PL approach) or IEC 62061 (SIL approach), depending on your design methodology.

The required Performance Level (PLr) is determined through the risk assessment (ISO 12100) and the method defined in ISO 13849-1 (including the risk graph).

You assess:

• S (Severity): severity of injury
• F (Frequency/Exposure): frequency and/or duration of exposure
• P (Possibility of avoidance): possibility of avoiding the hazard

The result is a PLr from PL a up to PL e, after which the safety function is designed so that PL ≥ PLr.

OSSD (Output Signal Switching Device) outputs are dual-channel electronic safety outputs that transmit the status of a safety sensor to a safety controller.

A key characteristic of OSSD outputs is that they are designed to detect faults such as short circuits to 24 V, short circuits to 0 V, or cross faults between channels, allowing the output to switch to a safe state and enabling fault detection. OSSD is defined within the IEC 61496 series (ESPE context) and is widely used in contactless safety sensors.

In practice, OSSD outputs enable fast fault detection and reliable signal transmission to a safety relay or safety PLC.

Under EU legislation, safety sensors may be considered safety components when they:

• perform a safety function,
• are independently placed on the market, and
• may endanger the safety of persons in the event of a defect or failure.

This aligns with the framework of the Machinery Directive 2006/42/EC and its successor, the Machinery Regulation (EU) 2023/1230. The regulation was published on 29 June 2023, entered into force on 19 July 2023, and will apply from 20 January 2027.

In such cases, CE marking and an EU Declaration of Conformity are generally required (in accordance with the applicable rules for the product and its role as a safety component).