← Home

IEC 61508

IEC 61508 is the international foundation standard for functional safety of electrical, electronic, and programmable electronic (E/E/PE) safety-related systems. It defines a complete safety lifecycle, methods for determining acceptable risk, and requirements for designing, implementing, operating, and maintaining safety systems that protect people, equipment, and the environment from hazardous failures.

Topic: Functional Safety & Industrial Networking

Standard: IEC 61508 – Functional Safety of Electrical/Electronic/Programmable Electronic (E/E/PE) Safety-Related Systems

IEC 61508 Functional Safety Overview
Figure – IEC 61508 functional safety lifecycle, Safety Integrity Levels (SIL), and Safety Instrumented Systems (SIS).

Overview

IEC 61508 is the international foundation standard for Functional Safety of electrical, electronic, and programmable electronic systems.

Unlike IEC 62443, which protects systems from intentional cyber threats, IEC 61508 protects people, equipment and the environment from hazardous failures.

The standard defines a complete safety lifecycle, methods for determining acceptable risk, and requirements for designing, implementing, operating and maintaining safety systems.

Many industry-specific safety standards are based on IEC 61508, including:

Purpose

IEC 61508 aims to ensure that:

Why Functional Safety Matters

Industrial plants contain many hazards:

Failures can result in:

Functional safety reduces these risks by ensuring safety systems respond correctly.

Functional Safety vs Cybersecurity

Functional Safety Cybersecurity
Prevents accidental hazards Prevents deliberate attacks
IEC 61508 IEC 62443
Random failures Malicious actors
Equipment failure Unauthorized access
Protects people Protects systems and data
Safety Instrumented Systems (SIS) Firewalls, IDS, Authentication

Modern plants require both.

Cybersecurity failures can compromise safety systems.

Safety systems must therefore also be secured under IEC 62443.

What is Functional Safety?

Functional Safety means:

The automatic protection functions perform correctly whenever required.

Examples include:

The safety function operates automatically without operator intervention.

Functional Safety Lifecycle

IEC 61508 defines a complete lifecycle.

Concept

↓

Hazard & Risk Analysis

↓

Safety Requirements

↓

System Design

↓

Hardware Design

↓

Software Development

↓

Installation

↓

Commissioning

↓

Operation

↓

Maintenance

↓

Modification

↓

Decommissioning

Safety must be managed throughout every stage.

Hazard and Risk Analysis

The first step is identifying hazards.

Questions include:

Common techniques include:

The result determines the required Safety Integrity Level (SIL).

Safety Instrumented System (SIS)

A Safety Instrumented System automatically places the process into a safe state.

Typical architecture:

Process

↓

Sensor

↓

Safety PLC

↓

Final Element

↓

Safe State

Example:

High Pressure

↓

Pressure Transmitter

↓

Safety PLC

↓

Shutdown Valve

↓

Plant Safe

A SIS is independent of the normal control system.

Safety Function

A Safety Instrumented Function (SIF) performs one specific protective action.

Example:

"If pressure exceeds 15 bar, close shutdown valve within 2 seconds."

Every SIF has:

Safety Integrity Level (SIL)

The most recognised concept in IEC 61508.

SIL represents the reliability of a safety function.

Higher SIL means lower probability of dangerous failure.

SIL Risk Reduction
SIL 1 Lowest
SIL 2 Medium
SIL 3 High
SIL 4 Extremely High

Most industrial facilities use:

SIL 4 is rare outside industries such as nuclear and aerospace.

Risk Reduction

Every safety function reduces risk.

Initial Risk

↓

Safety Function

↓

Residual Risk

↓

Acceptable Risk

Higher SIL provides greater risk reduction.

Probability of Failure on Demand (PFD)

For low-demand systems:

SIL PFDavg
SIL 1 10⁻² – <10⁻¹
SIL 2 10⁻³ – <10⁻²
SIL 3 10⁻⁴ – <10⁻³
SIL 4 10⁻⁵ – <10⁻⁴

Lower PFD means higher reliability.

Risk Reduction Factor (RRF)

Risk Reduction Factor is approximately:

SIL RRF
SIL 1 10–100
SIL 2 100–1,000
SIL 3 1,000–10,000
SIL 4 10,000–100,000

Random vs Systematic Failures

IEC 61508 separates failures into two categories.

Random Failures

Examples:

Managed by:

Systematic Failures

Caused by human error.

Examples:

Managed through:

Hardware Fault Tolerance (HFT)

Safety systems often include redundancy.

Examples:

1oo1

Sensor

↓

PLC

↓

Valve

One device only.

1oo2

Sensor A

Sensor B

↓

Safety PLC

↓

Valve

One of two sensors required.

2oo3

Sensor A

Sensor B

Sensor C

↓

Safety PLC

Two sensors must agree.

Provides higher reliability and availability.

Safe Failure Fraction (SFF)

SFF measures how many failures are either:

Higher SFF means better diagnostics.

Example:

90 failures safe

8 detected

2 dangerous

SFF

= (90 + 8) /100

=98%

Diagnostics

Safety systems continuously check for faults.

Examples:

Detected faults place the system into a safe condition.

Proof Testing

Not every failure is automatically detected.

Proof testing periodically confirms:

Examples:

Safety Lifecycle Documentation

IEC 61508 requires documentation including:

Functional Safety Management (FSM)

A management system ensures:

Functional Safety is as much about management as technology.

Software Safety

Software must be developed using rigorous processes.

Requirements include:

Higher SIL requires more rigorous development methods.

Industrial Networking Considerations

Industrial communication can affect safety performance.

Networks carrying safety data should provide:

Examples include:

Functional Safety Communication

Standard Ethernet alone does not provide functional safety.

Safety communication protocols add:

Examples include:

These protocols safely transmit safety data over standard industrial Ethernet while meeting functional safety requirements.

Functional Safety and IEC 62443

Modern safety systems are increasingly connected.

Cyber attacks may:

IEC 61508 ensures the safety system functions correctly, while IEC 62443 protects it from cyber compromise.

Together they provide Safe and Secure industrial operations.

Advantages

Limitations

Best Practices

Relationship to Industrial Networking

IEC 61508 influences the design and operation of industrial networks by requiring communication used for safety functions to be dependable, timely, and fault-tolerant.

Networking considerations include:

Common Applications

Key Takeaways

Relevant Standards

Standard Purpose
IEC 61508 Functional Safety – Generic standard for E/E/PE safety-related systems
IEC 61511 Functional Safety for the Process Industry (SIS)
IEC 62061 Functional Safety of Machinery Control Systems
ISO 13849 Safety of Machinery – Safety-related Parts of Control Systems
IEC 62443 Industrial Automation and Control Systems Cybersecurity
IEC 61784-3 Functional Safety Communication Profiles (e.g., PROFIsafe, CIP Safety, FSoE)
IEC 61800-5-2 Functional Safety of Variable Speed Drives

IEC 61508 Reference

Refer to: IEC 61508 (all parts), particularly:

For industrial networking, IEC 61508 should be read alongside IEC 61784-3 (Functional Safety Communication Profiles) and IEC 62443 to ensure both functional safety and cybersecurity are addressed within modern Industrial Automation and Control Systems (IACS).

Related Topics

Standards References