How PL and SIL Can Work Together in Industrial Safety Systems
Introduction
As industrial systems become increasingly complex, ensuring functional safety is no longer optional—it’s essential. Engineers and safety practitioners are often tasked with designing safety functions that comply with both IEC 61508/IEC 61511 for SIL and ISO 13849 for PL. While these two frameworks stem from different origins—process safety and machine safety respectively—they often intersect in real-world applications.
So, how can PL (Performance Level) and SIL (Safety Integrity Level) work together in a unified safety strategy?
With over 30 years in the field of automation, safety system implementation, and control integration, I’ll break down both standards, compare them, and explain how to bridge the gap to design compliant, robust safety systems across disciplines.
🧩 Understanding the Basics: SIL vs. PL
✅ SIL (Safety Integrity Level)
- Defined by: IEC 61508 / IEC 61511
- Application: Process industries (e.g., chemical, oil & gas, pharmaceuticals)
- Quantitative Measurement: Based on Probability of Dangerous Failure per Hour (PFH)
- Levels: SIL 1 (lowest) to SIL 4 (highest integrity)
SIL addresses functional safety for systems with electronic/electrical/programmable electronic (E/E/PE) elements. It is highly analytical and risk-based, requiring detailed probability calculations.
✅ PL (Performance Level)
- Defined by: ISO 13849-1
- Application: Machinery safety (e.g., robotics, packaging, automotive assembly)
- Quantitative & Qualitative: Based on Mean Time to Dangerous Failure (MTTFd), architecture, diagnostics, and common cause failure
- Levels: PL a (lowest) to PL e (highest integrity)
PL is more design-focused, suitable for discrete systems and simpler safety functions commonly found in manufacturing.
🧠 Why Do We Have Two Standards?
| Aspect | SIL (IEC 61508/61511) | PL (ISO 13849-1) |
|---|---|---|
| Target Industry | Process, petrochemical, oil & gas | Discrete manufacturing, machinery |
| Complexity Level | Complex (multi-loop, continuous) | Simpler (machinery, modular) |
| Data Focus | Statistical PFH, failure data | MTTFd, diagnostic coverage |
| Validation Method | Probabilistic modeling, fault trees | Block diagrams, semi-quantitative |
| Risk Assessment Tool | LOPA, risk graphs | Risk estimation graphs |
🎯 While different in methodology, both aim to reduce risk to acceptable levels through structured safety design.
⚙️ When Do PL and SIL Coexist?
In many industrial environments, particularly those implementing IT/OT convergence, both SIL and PL-rated devices may exist within the same facility or even the same safety function. Common cases include:
- Chemical plants with machine cells (robotic arms, packaging lines)
- Automotive assembly lines with explosive paint zones
- Pharmaceutical packaging using both process control (SIL) and machine safety (PL)
- Food and beverage lines with automated conveyors and thermal processes
🔄 How PL and SIL Can Work Together
✅ 1. Common Safety Function Across Zones
Imagine a scenario where a robotic palletizer (PL e) must work in tandem with a pressurized vessel (SIL 2). The overall safety function (emergency shutdown) must:
- Include a safety-rated PLC (dual SIL/PL certified)
- Monitor machine stops (via PL e-rated safety relays)
- Engage process shutdown logic (via SIL 2-rated interlocks)
Here, a hybrid safety architecture ensures compliance with both sets of standards.
✅ 2. Unified Risk Reduction Approach
While the methods of calculating risk are different, the risk reduction goal is similar:
- Reduce initial risk to a tolerable level
- Use layers of protection (mechanical, electronic, procedural)
- Meet safety targets through redundancy and diagnostics
Some manufacturers use tools like SISTEMA (PL) or exSILentia (SIL) to map requirements and even cross-verify systems.
✅ 3. Component Certification Overlap
Many manufacturers now produce dual-certified devices:
- Safety PLCs (e.g., Siemens S7-1500F, Rockwell GuardLogix)
- Safety relays (e.g., PILZ, SICK, Omron)
- Sensors and actuators rated for both SIL and PL compliance
These components allow for interoperability, simplifying system design and documentation.
📈 Mapping PL to SIL: Is It Possible?
While direct conversion isn’t always possible due to different assessment methods, general equivalence can be observed:
| PL Level | Approximate SIL Equivalent |
|---|---|
| PL a | Not applicable (below SIL 1) |
| PL b | SIL 1 (lower bound) |
| PL c | SIL 1 |
| PL d | SIL 2 |
| PL e | SIL 3 |
⚠️ Note: These are rough mappings and must not be used as a substitute for full safety analysis.
🛠️ Design Best Practices When Using SIL and PL Together
✅ 1. Start With a Unified Risk Assessment
- Use HAZOP, LOPA, or ISO 12100 depending on process/machinery focus
- Define all hazardous events, consequences, and tolerable risk thresholds
- Identify whether the safety function requires PL, SIL, or both
✅ 2. Use Dual-Certified Components Where Possible
- Reduces design complexity
- Simplifies documentation and validation
- Enhances consistency in safety strategy
✅ 3. Use Software Tools to Verify Designs
- SISTEMA (PL): Validate MTTFd, DCavg, and CCF values
- exSILentia (SIL): Perform SIL verification and lifecycle documentation
- Some tools now support both standards in hybrid projects.
✅ 4. Document Safety Lifecycle Properly
- Use the V-model for system design and validation
- Maintain traceability from hazard analysis to validation testing
- Comply with IEC 62061, which aligns more closely with both SIL and PL
📋 Interactive Checklist: Are You Managing SIL and PL Together Effectively?
✅ Have you defined safety functions needing SIL vs. PL?
✅ Are you using dual-certified hardware where applicable?
✅ Are your PLCs and sensors compliant with target safety levels?
✅ Have you used SISTEMA or equivalent tools for validation?
✅ Is your documentation compliant with both standards?
Scoring:
- 5 Yes: Excellent — your safety architecture is well-integrated
- 3–4 Yes: Good — review alignment and improve documentation
- 0–2 Yes: Review entire safety lifecycle for compliance gaps
🧠 Final Thoughts: SIL and PL – Not Competing, but Complementing
PL and SIL are not mutually exclusive—they are designed for different applications, but often must coexist in modern manufacturing facilities. By understanding their core differences and how to integrate them, engineers can create flexible, reliable, and fully compliant safety systems.
🔑 Key Takeaways:
- SIL is used in process safety, while PL applies to machine safety.
- Both aim to reduce risk through structured, validated safety functions.
- In many facilities, both standards apply simultaneously—plan accordingly.
- Using dual-certified components and unified risk assessments simplifies compliance.
- Use proper tools and documentation frameworks to bridge the standards.