2.2 KiB
2.2 KiB
EN 61508 — Functional Safety of E/E/PE Systems
Overview
EN 61508 is the generic functional safety standard for Electrical / Electronic / Programmable Electronic (E/E/PE) systems.
It provides a framework to ensure systems reduce risk to an acceptable level.
Core Idea
Risk is defined as:
Risk = Frequency × Consequence
Safety functions reduce this risk.
Key Concepts
Safety Function
A function that:
- Detects hazardous conditions
- Takes action to prevent harm
Safety Integrity Level (SIL)
| SIL | Meaning |
|---|---|
| SIL 1 | Low risk reduction |
| SIL 2 | Moderate |
| SIL 3 | High |
| SIL 4 | Very high |
Probability of Failure
Low Demand Mode (PFDavg)
| SIL | Range |
|---|---|
| SIL 1 | 1e-2 to 1e-1 |
| SIL 2 | 1e-3 to 1e-2 |
| SIL 3 | 1e-4 to 1e-3 |
| SIL 4 | 1e-5 to 1e-4 |
High/Continuous Mode (PFH)
| SIL | Failures/hour |
|---|---|
| SIL 1 | 1e-6 to 1e-5 |
| SIL 2 | 1e-7 to 1e-6 |
| SIL 3 | 1e-8 to 1e-7 |
| SIL 4 | 1e-9 to 1e-8 |
| PFD_PFH_when_to_use |
Safety Lifecycle
- Hazard analysis
- Safety requirements
- System design
- Hardware design
- Software design
- Verification & validation
- Operation & maintenance
- Decommissioning
Hardware Reliability
- FIT = 1e-9 failures/hour
- PFH = Probability of dangerous failure/hour
- PFDavg = Probability of failure on demand
Failure Types
Random Failures
Handled statistically (FIT, PFH)
Systematic Failures
Handled by process, reviews, and discipline
Techniques
- Redundancy (1oo2, 2oo3)
- Diagnostics
- Diversity
- Formal methods (high SIL)
Standard Structure
- General
- Hardware
- Software
- Definitions
5–7. Guidelines
Related Standards
- EN 61511 (Process)
- EN 62061 (Machinery)
- ISO 26262 (Automotive)
- EN 298 (Burners)
Key Takeaways
- Focus on risk reduction
- SIL is a target, not a feature
- Process matters as much as maths
- Lifecycle coverage is essential
One-Line Summary
EN 61508 ensures safety systems reduce risk using quantified reliability and disciplined engineering processes.