PL / SIL Quick Overview
Verification logic & key parameters. This overview explains the verification logic behind the performance levels (PL) and safety integrity levels (SIL) used in industrial safety systems. The key parameters to determine the required performance and safety are outlined, along with common application scenarios and examples.
1) PL and SIL Overview
Performance Levels (PL) and Safety Integrity Levels (SIL) are defined in industrial safety standards to classify the required risk reduction performance. The greater the hazard, the higher the required safety level. Below is an overview of how PL and SIL are calculated:
- PL (Performance Level): Based on ISO 13849-1, PL evaluates the risk reduction of a safety function, with levels ranging from PL a (lowest) to PL e (highest).
- SIL (Safety Integrity Level): Defined in IEC 61508, SIL assesses the system's ability to perform safely over its entire lifecycle, ranging from SIL1 (lowest) to SIL3 (highest).
2) Verification Logic
The verification logic behind PL and SIL is based on a series of steps that ensure the system meets the required safety level. The key parameters for verification include:
- Failure Rate (λ): The rate at which safety components fail. It helps determine the probability of failure per year.
- Diagnostic Coverage (DC): Measures the effectiveness of diagnostic functions that detect failures in the system.
- Common Cause Failures (CCF): Accounts for the potential risk that multiple components may fail due to the same cause, reducing system reliability.
- Probability of Failure on Demand (PFD): Measures the likelihood that a safety system will fail when needed, impacting SIL calculation.
3) Key Parameters for PL and SIL
When determining the required PL or SIL level, the following parameters should be considered:
- Hazardous Event Likelihood: Higher risk scenarios demand higher PL or SIL ratings.
- Consequence of Failure: Systems that could result in severe injury or death require the highest levels of safety integrity.
- System Redundancy: Redundant safety systems can help achieve higher PL or SIL ratings by providing multiple layers of protection.
- Safety Component Reliability: The quality and reliability of individual safety components (sensors, relays, etc.) determine the achievable PL or SIL level.
4) Applications & Scenarios
PL and SIL are applied in a variety of industrial scenarios, especially where human safety is at risk. Some common applications include:
- Press Brakes and Shears: Where high levels of protection are necessary due to the risk of physical injury from moving parts.
- Automated Guided Vehicles (AGVs): SIL3 is often required for AGVs to ensure safe operation in dynamic environments, avoiding collisions with humans or obstacles.
- Robotic Cells: High-risk environments where safety is critical, and PL d/e or SIL3 ensures no harm comes from robotic arm movements.
- Conveyors and Other Automated Systems: SIL2 or higher levels are often necessary for systems that operate in potentially dangerous environments.
5) Compliance and Certification
Ensuring compliance with PL and SIL standards involves regular testing and certification of safety components. The following steps are crucial:
- Component Certification: Ensure all safety components meet the relevant ISO/IEC standards for PL or SIL ratings.
- System Validation: Conduct regular system audits and tests to verify that the safety systems still meet the required levels of protection.
- Documentation: Maintain detailed records of safety assessments, tests, certifications, and any changes made to the safety system.
6) Troubleshooting and Best Practices
Proper troubleshooting and maintenance ensure the safety system continues to meet its PL or SIL requirements. Common issues include:
- Misconfiguration: Incorrect settings or wiring can prevent the safety system from achieving the necessary performance levels.
- Component Failure: Regularly monitor components for signs of wear and tear, and replace faulty parts to maintain the system's safety integrity.
- Calibration Errors: Ensure that all sensors and devices are correctly calibrated to avoid false readings or failures during operation.