Overview of SIL and PL
**Safety Integrity Level (SIL)** from IEC 62061 quantifies risk reduction in electrical/electronic/programmable electronic safety-related systems using probability of dangerous failure per hour (PFHd), ranging from SIL 1 (≥10⁻⁶ to <10⁻⁵) to SIL 3 (≥10⁻⁸ to <10⁻⁷).[1][2] **Performance Level (PL)** from ISO 13849-1 assesses mechanical safety controls via average probability of dangerous failure per hour, from PL a (≥10⁻⁵ to <10⁻⁴) to PL e (≥10⁻⁸ to <10⁻⁷).[1][4] Both metrics overlap in PFHd calculations, enabling direct comparisons (e.g., PL d ≈ SIL 2).[1][5]
Key Factors and Considerations
Select SIL for complex electrical systems with subsystems (architectures A-D); use PL for simpler mechanical/hybrid setups with categories B-5.[4][6] Consider application: SIL suits continuous/high-demand (e.g., robotics, SIL 2-3); PL fits machine tools with exposure-based risks.[3][6] Factors include hazard severity (Se: 1-4 for SIL, S1-S2 for PL), frequency/exposure (Fr/F), avoidance possibility (P/A), and diagnostic coverage (DC).[2][6] MTTFd links both: high MTTFd boosts PL/SIL.[5] Costs rise sharply for higher levels (SIL 3+ demands redundancy).[3][8]
| Metric | Standard | Range | Key Metric |
|--------|----------|-------|------------|
| **SIL** | IEC 62061 | 1-3 | PFHd (1/h) |
| **PL** | ISO 13849 | a-e | PFDavg (1/h) |
Technical Specifications to Compare
- **Probability Bands**: PL b (≥3×10⁻⁶ to <10⁻⁵) aligns with SIL 1; PL e matches SIL 3 upper end.[1]
- **Architecture**: SIL uses A-D with fault tolerance; PL uses B-5 with DC (e.g., 90-99% for high PL).[6]
- **Validation**: Both require PFHd via tools like SISTEMA; SIL emphasizes PFDavg for low-demand.[1][3]
- **Risk Graph**: PL uses qualitative flowchart (severity, frequency, avoidance); SIL scores Fr, Pr, Av for class.[2][6]
Step-by-Step Selection Methodology
1. **Risk Assessment**: Evaluate hazards using SIL risk matrix (Se + probability) or PL graph (S, F, P).[2][6]
2. **Determine Target**: Map to required SIL/PL (e.g., high severity + frequent exposure → SIL 2/PL d).[1][4]
3. **Design Architecture**: Select category (PL: B-5; SIL: A-D) with redundancy/DC to meet MTTFd/PFHd.[5][6]
4. **Calculate Metrics**: Use SISTEMA for PFHd/PFDavg; verify λD, T1 (proof test).[1][3]
5. **Validate/Verify**: Test DC, fault tolerance; document compliance.[3]
Example: For λ=1×10⁻⁵/h, T=8760h, PFDavg=(λ×T)/2=0.0438 → SIL 2.[3]
Common Mistakes to Avoid
- Ignoring overlap: Assuming no SIL-PL conversion; always cross-check via PFHd tables.[1][5]
- Overlooking DC: Low DC drops PL/SIL despite high MTTFd.[6]
- Skipping validation: Relying on vendor claims without SISTEMA.[1]
- Mismatched standards: Using PL for electrical-heavy systems or vice versa.[4]
- Underestimating costs: Targeting SIL 3 without redesigning process risks.[8]
Industry Best Practices
Use both standards complementarily: PL for machinery subsystems, SIL for overall functions.[2] Employ SISTEMA libraries from manufacturers for accurate modeling.[1] Conduct iterative risk assessments pre-design.[2] Prioritize SIL 1-2 for most automation (e.g., food processing); reserve SIL 3 for critical (e.g., energy).[3][7] Document mappings (PL d = SIL 2) for audits. Train on tools like risk graphs; integrate with PHA for intrinsic risk reduction.[6][8] (512 words)