Have you ever wondered how to effectively choose a safety relay to not only meet regulatory requirements but also provide real protection for workers and machinery? With a myriad of standards, safety levels, and manufacturers, making the right decision can be challenging. Below, you’ll find guidance to help you select the appropriate solution, focusing on the standards prevalent in the machinery and process engineering industries.
Table of Contents
Why is a Safety Relay So Important?
A safety relay can serve as the heart of a protective system in a machine or production line, especially in less complex systems. Its role is to detect signals from safety sensors (such as emergency stop switches and light curtains) and respond swiftly to unwanted events. In more complex applications, PLC controllers with safety outputs often perform this function. However, if the basic component of the safety system fails, the consequences can be severe, ranging from workplace accidents to financial losses due to downtime and repairs.
Standard PN-EN ISO 13849-1 and Performance Levels (PL)
How Does PL Influence the Choice of Safety Relay?
The PN-EN ISO 13849-1 standard defines the safety requirements for machinery from the perspective of ensuring appropriate Performance Levels (PL). This means that every safety system should be designed to:
- Achieve a defined PL level (from PL a to PL e), with the specific level depending on the conducted risk assessment and the type of hazards. Lower levels (PL a or PL b) are used in systems with low risk and less frequent hazards, while higher levels (PL d or PL e) are necessary in applications where there is a high likelihood of accidents or potentially severe consequences.
- Consider reliability and fault tolerance factors (MTTFd, DC – Diagnostic Coverage, CCF – Common Cause Failures), which help determine if the safety system can achieve the required PL or SIL. MTTFd is the mean time to dangerous failure, DC describes the effectiveness of detecting potential failures, and CCF minimizes the risk of events resulting from the same cause (e.g., environmental conditions). By analyzing these parameters, designers can assess the actual level of reliability and make necessary decisions to ensure safety.
Choosing a safety relay involves determining the target PL level you want to achieve. Relays that must provide PL e will be designed and tested more rigorously than those intended for lower requirements.
What to Check in the Relay Documentation?
- MTTFd (Mean Time To Dangerous Failure): The higher the value, the better. However, this is not the only factor determining the safety level; other parameters such as DC (Diagnostic Coverage) and CCF (Common Cause Failures) must also be considered to comprehensively assess system reliability.
- System Architecture: Does the relay have redundancy (e.g., multiple safety channels)? This can achieve a higher level of reliability required for the appropriate PL or SIL. Redundancy minimizes the effects of a potential failure of one channel and increases the overall safety of the system.
- Certificates and Declarations of Conformity: For safety relays marketed as ready-made devices or components, the manufacturer must issue the appropriate declaration of conformity with relevant regulations (e.g., the Machinery Directive or Regulation 2023/1230, if it concerns a safety component). If the product operates within voltage ranges covered by the Low Voltage Directive (LVD), compliance with its requirements must also be ensured. In most cases, the EMC Directive must also be considered, especially when the device can generate or be exposed to electromagnetic interference. If the entire system is purely pneumatic or hydraulic, without electrical or electronic components, LVD or EMC obligations may not apply.
- Application of PL in Pneumatic and Hydraulic Systems: It’s important to remember that the PN-EN ISO 13849-1 standard (and the resulting PL levels) can be applied not only to electrical systems but also to those based on pneumatics or hydraulics if they perform safety functions. This means that even valves or actuators can be included in the risk analysis to achieve the required PL level. However, it should be noted that in practice, the maximum reliability level for solutions based on this standard usually reaches SIL 3 (if comparing PL e with SIL 3 according to PN-EN 62061). Furthermore, in extreme cases, when three safety elements with a declared PLe level are in series, the total level may drop to PLd, resulting from the assumptions in the standard regarding risk summation and possible failure causes.
Standard PN-EN 62061 and Safety Integrity Levels (SIL)
Where is SIL Applied?
The second key standard (PN-EN 62061) is based on the concept of Safety Integrity Level (SIL) and is one of the sector-specific standards based on IEC 61508, which forms the basis for functional safety systems. SIL levels are also defined in other standards, such as IEC 61511 (for industrial processes), IEC 61513 (for the nuclear industry), and IEC 62061 (for machinery). A safety relay can be certified, for example, for SIL 2 or SIL 3. From a designer’s perspective, it is important to:
- Know whether a specific application requires meeting certain SIL levels, depending on normative requirements and risk level.
- Check which system parameters must be considered in the risk analysis (including the probability of dangerous failures, PFH calculations – Probability of Failure per Hour).
- Consider testing frequency requirements – according to IEC 61508, IEC 61511, IEC 61513, and IEC 62061, for SIL 2 systems, safety relays require periodic testing at least once a year, while for SIL 3 – even once a month. This is due to the need to meet specific PFH levels (Probability of Failure per Hour), which for SIL 2 are in the range of 10⁻⁶ – 10⁻⁷, and for SIL 3 in the range of 10⁻⁷ – 10⁻⁸. The lower the permissible failure probability, the more frequent tests are required to verify the reliability and integrity of the safety function. High test frequency for SIL 3 minimizes the risk of unnoticed damage that could result in a critical error in the safety system.
In practice, situations arise where a relay with a specific SIL is needed because the entire system must comply with the requirements of EU Regulation 2023/1230 (new regulations replacing the Machinery Directive). Therefore, it is crucial that the selection of the relay considers both the risk level and the test schedule, which ensure the system’s effectiveness in long-term operation.
How to Choose a Safety Relay
- Define Application Requirements
- Does the machine fall under PN-EN ISO 13849-1 or PN-EN 62061?
- If using the PL approach, determine the required safety level from PL a to PL e.
- If using the SIL approach, determine the target level from SIL 1 to SIL 3, according to IEC 62061.
- Analyze Risk
- For PL: Determine the safety category (B, 1, 2, 3, 4) and the required PL level.
- For SIL: Determine the SIL level based on risk assessment and PFH indicator.
- In both cases, consider key reliability parameters such as MTTFd, DC, and CCF.
- Check Certificates and Documentation
- Ensure that the relay complies with the appropriate standard (PN-EN ISO 13849-1 for PL, PN-EN 62061 for SIL).
- For PL: Verify MTTFd and DC data to confirm compliance with the requirements for the specified PL level.
- For SIL: Check the declared PFH values and the component’s compliance with the required SIL level.
- Consider Additional Features
- Future expansion capability.
- For PL: Does the system meet the requirements for the given category and PL? Does it have redundancy?
- For SIL: Does the system have diagnostic mechanisms and provide a specified error detection capability?
- Conduct Tests and Verification
- For PL: Perform compliance analysis with the required PL level through functional tests.
- For SIL: Verify reliability level through periodic tests and PFH assessment.
- Ensure that the documentation includes the required validation procedures.
This approach allows for the selection of a safety relay that complies with both PL and SIL standards, depending on the specific application and regulatory requirements.
FAQ: Safety Relay – How to Choose?
Yes, safety relays are relatively simple devices dedicated to performing safety functions in specific applications. In more complex systems, PLC controllers with safety functions are used, allowing for more advanced diagnostics and flexible management of protective functions. Relays are often chosen where a quick and independent response to a threat is required.
For the Performance Level (PL) approach, use PN-EN ISO 13849-1, where key parameters are MTTFd, DC, and CCF. If the system requires Safety Integrity Level (SIL), use PN-EN 62061, which is based on PFH (Probability of Failure per Hour) analysis and system architecture.
Not always. Safety relays are designed for a specific level of reliability. PL e does not always correspond to SIL 3, and three elements with PL e in series can lower the level to PL d. Proper system architecture selection is crucial.
One of the most common mistakes is not considering the required PL category or SIL level, which can result in insufficient system protection. Another issue is the lack of compatibility analysis with the rest of the control system or choosing a relay with too low a reliability level.
Yes, if they are safety components, they must have a EU/EC conformity declaration, in accordance with Regulation 2023/1230. Additionally, they may be subject to the Low Voltage Directive (LVD) and EMC, if they include electrical elements generating interference.