# 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.

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## Core Idea

Risk is defined as:

Risk = Frequency × Consequence

Safety functions reduce this risk.

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## Key Concepts

### Safety Function
A function that:
- Detects hazardous conditions
- Takes action to prevent harm

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### Safety Integrity Level (SIL)

| SIL | Meaning |
|-----|--------|
| SIL 1 | Low risk reduction |
| SIL 2 | Moderate |
| SIL 3 | High |
| SIL 4 | Very high |

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### 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](PFD_PFH_when_to_use.md)

[PFD_and_PFH](PFD_and_PFH.md)]

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## Safety Lifecycle

1. Hazard analysis  
2. Safety requirements  
3. System design  
4. Hardware design  
5. Software design  
6. Verification & validation  
7. Operation & maintenance  
8. Decommissioning  

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## Hardware Reliability

- FIT = 1e-9 failures/hour  
- PFH = Probability of dangerous failure/hour  
- PFDavg = Probability of failure on demand  

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## Failure Types

### Random Failures
Handled statistically (FIT, PFH)

### Systematic Failures
Handled by process, reviews, and discipline

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## Techniques

- Redundancy (1oo2, 2oo3)
- Diagnostics
- Diversity
- Formal methods (high SIL)

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## Standard Structure

1. General  
2. Hardware  
3. Software  
4. Definitions  
5–7. Guidelines  

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## Related Standards

- EN 61511 (Process)
- EN 62061 (Machinery)
- ISO 26262 (Automotive)
- EN 298 (Burners)

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## Key Takeaways

- Focus on risk reduction
- SIL is a target, not a feature
- Process matters as much as maths
- Lifecycle coverage is essential

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## One-Line Summary

EN 61508 ensures safety systems reduce risk using quantified reliability and disciplined engineering processes.

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