IEC 61508 Functional Safety

IEC 61508 Functional Safety Standard

The standard applicable to any industry

The standard applicable to any industry

The Core Functional Safety Standard

IEC 61508 is the granddaddy of all functional safety standards and is applicable to all industries. Where there are no industry-specific functional safety standards for products, IEC 61508 can be applied. 

According to the IEC, 61508 provides functional safety standards for the lifecycle of electrical, electronic, or programmable electronic (E/E/PE) systems and products. It addresses those parts of a device or system that perform automated safety functions including, for example, sensors, control logic, actuators, and micro-processors. It provides a rigorous quantitative approach to risk reduction and can be applied across many industries.

IEC 61508 is a risk-based standard – meaning that the risk of hazardous operational situations is qualitatively assessed, and safety measures are defined to avoid or control systematic failures and to detect or control random hardware failures or mitigate their effects.

The standard helps determine Safety Integrity Levels (SIL). There are four SILs: SIL1, SIL2, SIL3 and SIL4, the risk of failure becoming greater with each respective SIL. Conducting a risk assessment defines which SIL is required. A SIL is determined by examining the Systematic Capability , Architecture Constraints, and the Probability of Dangerous Failure. The table below shows the SIL classification by either probability of failure on demand, or probability of failure per hour.


Low Demand Mode:
Average Probability of Failure on Demand

High Demand or Continuous Mode:
Probability of Dangerous Failure per Hour

1 ≥ 10−2 to < 10−1   ≥ 10−6 to < 10−5
2  ≥ 10−3 to < 10−2   ≥ 10−7 to < 10−6
3 ≥ 10−4 to < 10−3  ≥ 10−8 to < 10−7 (1 dangerous failure in 1140 years)
4 10−5 to < 10−4    ≥ 10−9 to < 10−8

(Side note: SILs with regards to IEC 61508 are different from Safety Integrity Levels in other standards and care should be taken to not confuse them)

A product, process, or system that has been certified as IEC 61508 compliant has demonstrated that it has satisfied all the requirements of the standard.

Industry specific variants

IEC 61508 (General Functional Safety Standard)

Just a few of the industry-specific functional safety standards

As products, processes, or systems increased in complexity, associated functional safety requirements increased in step. It became apparent to many that requirements for IEC 61508 didn’t always adequality address industry-specific challenges. Industry-specific functional safety standards are now commonplace in many other industries, for example:

  • Industrial
  • Transportation
  • Buildings
  • Medical devices
  • Household appliances
  • Energy systems and providers


Functional Safety in a Nutshell

Functional Safety in a Nutshell

A compact overview of the functional safety regulation landscape

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