PL vs. SIL: What’s the Difference? A Functional Safety Guide for Industrial Applications
Introduction
Functional safety is non-negotiable in today’s industrial landscape. As machines and systems become more complex and integrated, so too does the need for rigorous safety evaluations. Two terms often encountered in safety system design are Performance Level (PL) and Safety Integrity Level (SIL).
Although both aim to quantify risk reduction in safety systems, they are not interchangeable. Each belongs to a different standard, applies to different industries, and uses distinct evaluation methods.
With over 30 years of experience in industrial automation, safety lifecycle management, and engineering risk assessment, I’ll walk you through the key differences, where each applies, and how to choose the right standard for your application.
🧠 What Are PL and SIL?
| Term | Stands For | Defined In | Application Scope |
|---|---|---|---|
| PL | Performance Level | ISO 13849-1 | Machinery safety, discrete automation |
| SIL | Safety Integrity Level | IEC 61508 / IEC 61511 | Process industry, functional safety, E/E/PE systems |
🏗️ Understanding SIL (Safety Integrity Level)
🔹 Defined In:
- IEC 61508 – Functional safety of electrical/electronic/programmable systems
- IEC 61511 – Functional safety in the process industry
🔹 Purpose:
SIL defines the required risk reduction level for Safety Instrumented Functions (SIFs) within E/E/PE systems (electrical/electronic/programmable electronic).
🔹 SIL Levels:
| SIL Level | Risk Reduction Factor (RRF) | Probability of Failure on Demand (PFD) |
|---|---|---|
| SIL 1 | 10 – 100 | 1.0 × 10⁻¹ to 1.0 × 10⁻² |
| SIL 2 | 100 – 1,000 | 1.0 × 10⁻² to 1.0 × 10⁻³ |
| SIL 3 | 1,000 – 10,000 | 1.0 × 10⁻³ to 1.0 × 10⁻⁴ |
| SIL 4 | 10,000 – 100,000 (rarely used) | 1.0 × 10⁻⁴ to 1.0 × 10⁻⁵ |
⚠️ SIL 1–3 are most common in industrial systems. SIL 4 is often too costly or impractical.
🔹 Key SIL Concepts:
- SIF (Safety Instrumented Function): A specific safety task
- SIL Verification: Uses PFDavg, diagnostic coverage, and redundancy analysis
- Tools: Fault Tree Analysis (FTA), Layer of Protection Analysis (LOPA)
⚙️ Understanding PL (Performance Level)
🔸 Defined In:
- ISO 13849-1 – Safety of machinery – Control systems
🔸 Purpose:
PL quantifies the ability of a machine’s safety-related control system to perform a safety function under foreseeable conditions.
🔸 PL Levels:
| Performance Level (PL) | Probability of Dangerous Failure per Hour (PFHd) |
|---|---|
| PLa | ≥10⁻⁵ to <10⁻⁴ |
| PLb | ≥3×10⁻⁶ to <10⁻⁵ |
| PLc | ≥10⁻⁶ to <3×10⁻⁶ |
| PLd | ≥10⁻⁷ to <10⁻⁶ |
| PLe | ≥10⁻⁸ to <10⁻⁷ |
🔸 Key PL Concepts:
- Based on MTTFd (Mean Time to Dangerous Failure), DCavg (Diagnostic Coverage), and CCF (Common Cause Failures)
- Uses a risk graph or decision table for PL selection
- Implemented in machinery controls, robots, conveyors, pick-and-place machines
🤖 PL is more intuitive for mechanical and electrical machinery designers.
🔬 PL vs. SIL: A Side-by-Side Comparison
| Feature | SIL (IEC 61508/61511) | PL (ISO 13849-1) |
|---|---|---|
| Focus Area | Process automation, complex control | Machinery, discrete automation |
| Target Systems | SIFs, E/E/PE safety functions | Electromechanical safety controls |
| Quantitative Metric | PFDavg (low demand) | PFHd (per hour) |
| Assessment Approach | Detailed calculation + FTA/LOPA | Risk graph, semi-quantitative |
| Risk Reduction Range | Up to 10,000× (SIL 3) | Up to 1,000× (PLe) |
| Tools Used | ExSILentia, SILcet, SISTEMA | SISTEMA, risk graphs |
| Complexity Level | High (esp. SIL 2 & 3) | Moderate |
| Regulation | IEC standards | ISO standards |
🏭 Where to Use PL vs. SIL
| Industry/Application | Use PL | Use SIL |
|---|---|---|
| Factory Machinery | ✔️ Yes | ❌ Not typically required |
| Process Industry (Oil & Gas) | ❌ | ✔️ Yes (IEC 61511 mandatory) |
| Pharmaceutical Packing Line | ✔️ Yes | ✔️ (if integrated with DCS/PLC safety) |
| Standalone Machine with E-stop | ✔️ Yes | ❌ |
| SIS for Compressor Shutdown | ❌ | ✔️ Yes (SIL 2 or 3) |
🛠️ Interactive Assessment: Which Do You Need—SIL or PL?
Answer Yes or No:
✅ Is your system part of a process plant or continuous operation (e.g., oil, chemical, gas)?
✅ Do you design machine-level safety systems like light curtains, e-stops, safety PLCs?
✅ Are you dealing with catastrophic process hazards or environmental risks?
✅ Is your system standalone with electromechanical actuators?
Scoring:
- Mostly Yes to Q1 & Q3: → Use SIL-based assessment
- Mostly Yes to Q2 & Q4: → Use PL-based assessment
📘 How PL and SIL Can Work Together
In hybrid systems, both PL and SIL may apply:
Example:
A robot arm in a packaging plant may use PLd-rated light curtains, while the connected DCS may enforce a SIL 2 SIF for emergency shutdown.
Many OEMs now certify components for both SIL and PL. For example:
- Safety PLCs: Siemens S7-1500F (PL e / SIL 3)
- Relays/Switches: Pilz, Schmersal, Rockwell (dual-certified)
⚙️ Use SISTEMA software to analyze PL compliance and align with SIL-level systems when needed.
📋 Summary Table
| Criteria | SIL | PL |
|---|---|---|
| Defined In | IEC 61508 / IEC 61511 | ISO 13849-1 |
| Use In | Process industry | Machinery and discrete automation |
| Typical Tools | ExSILentia, PHA-Pro | SISTEMA, risk graph |
| Common in | Chemical plants, refineries | Packaging lines, assembly machines |
| Risk Metric | PFDavg (per demand) | PFHd (per hour) |
✅ Conclusion
Both PL and SIL aim to reduce risk and protect people, equipment, and the environment. While PL dominates the machinery sector, SIL governs process safety. Understanding the differences helps you:
- Comply with the right safety standard
- Choose appropriate certified components
- Ensure end-to-end risk reduction
- Pass safety audits with confidence
🔍 The key takeaway? Choose the right standard based on your system’s function—not just convenience.
🔑 Key Takeaways:
- SIL is suited for process safety and functional logic systems.
- PL is best for machine safety and discrete automation.
- Both standards are compatible but apply to different risk environments.
- Selecting the correct level ensures safe and compliant operations.