PL / SIL Quick Overview
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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
The machine safety standards index distinguishes design, positioning and product standards. ISO 13849-1:2023 provides a design and integration method for high-demand and continuous operation; it does not prescribe every application's required PL.
Performance Levels (PL) and Safety Integrity Levels (SIL) are defined in industrial safety standards to classify the required risk reduction performance. The required target follows the full risk assessment and applicable requirements, not severity alone. Below is an overview of the two frameworks:
- PL (Performance Level): Based on ISO 13849-1, PL describes the ability of safety-related control parts to perform a safety function under foreseeable conditions, 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 SIL 1 to SIL 4 in that general framework. Machinery IEC 62061 addresses SIL 1 to SIL 3.
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, expressed per unit time. Use the correct dangerous-failure data and mission assumptions; a rate is not itself a probability 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): PFDavg applies to low-demand functions under the appropriate framework. Machinery high-demand/continuous calculations generally use PFH/PFHd per hour; do not substitute PFDavg.
3) Key Parameters for PL and SIL
Separate the required target derived from risk assessment from the achieved level demonstrated by the design. Consider:
- 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 evaluation together with exposure, avoidance and applicable machine requirements; severity alone does not select a level.
- 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): Specify each safety function and determine its target from the applicable truck requirements and risk assessment; there is no universal SIL 3 rule.
- Robotic Cells: High-risk environments where safeguarding and required control integrity must be evaluated for each function. A PL/SIL label cannot guarantee no injury.
- Conveyors and Other Automated Systems: Identify access, trapping, transfer and restart hazards before setting a target; the equipment name does not select SIL 2.
5) Compliance and Certification
- Define the safety function, safe state, response time and required PLr/SIL.
- Record device/subsystem safety data, operating limits and the actual circuit architecture.
- Verify the achieved level with the chosen standard's calculation and systematic requirements.
- Validate actual fault response, restart and stopping behavior; retain evidence and control later changes.
IEC 62061:2021 covers machinery safety-control design, integration and validation; IEC also lists Amendment 2:2026. Confirm the edition and adoption applicable to the project.
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.
PL and SIL questions
What is the difference between PL and SIL?
PL and SIL are integrity frameworks for safety functions, not separate risk-reduction and reliability product tiers. ISO 13849-1 uses PL a–e; machinery IEC 62061 uses SIL 1–3. Each requires the applicable quantitative and systematic evidence for the complete function, not a direct exchange of labels.
When is SIL required over PL?
PL is not a lower-safety alternative to SIL. Select the applicable machinery design framework and required target from the risk assessment, relevant machine standard and project requirements. ISO 13849-1 and IEC 62061 both address safety-related machinery controls.
How do I ensure compliance with PL and SIL requirements?
Define the required integrity for each safety function, then verify the design, architecture, reliability calculations, diagnostics, common-cause and systematic measures using the applicable framework. Validate the implemented function and its fault responses, keep records, and perform the maintenance and periodic tests specified by the validated design. Periodic component tests alone do not establish compliance.