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Safety-Related vs. Safety-Critical: What's the Difference?

Prof. Dr. Mirko Conrad
Article Functional Safety (ISO 26262)

When discussing systems that affect safety, people often use the terms "safety-critical systems" and "safety-related systems." Do they mean the same thing or do they refer to different concepts? What sounds like an easy question is actually a tricky one. So, let's ask Microsoft Copilot:

Q: What is the difference between safety-related and safety-critical?

A (slightly shortened): While both terms deal with risk and protective measures, they differ in severity, consequence, and regulatory requirements. Here's a breakdown:

Safety-Related Systems

Systems that contribute to safety but whose failure usually does not directly cause serious harm.

Role: They support or enhance overall safety, often acting as backups or warnings.

Impact of failure: Might lead to reduced reliability, delays, or minor operational risks—but not life-threatening.

Examples: Fire alarms, Vehicle warning indicators

Safety-Critical Systems

Systems whose failure can lead to severe consequences like injury, death, or major environmental damage.

Role: They are essential to prevent catastrophic outcomes and must meet rigorous safety standards.

Impact of failure: High-risk—may cause loss of life, significant asset damage, or legal liabilities.

Examples: Airplane flight control systems, Medical devices (like pacemakers), Automotive airbag deployment systems

You can think of safety-related as important—but safety-critical as indispensable.

That sounds like a clear distinction. However, when asked about the sources for these definitions, Copilot becomes rather vague:

The definitions and distinctions I shared come from a blend of expert sources and established engineering references.

To obtain some actual, verifiable definitions with references, if any, one needs to press Copilot really hard. By following some of the clues and navigating around the red herrings, you can find the following:

Safety-Critical System (SCS)

A system is referred to as safety-critical when the consequences of its failure can lead to loss of life, or to significant property or environmental damage.

[PR13] Pietrantuono, R., Russo, S. (2013). Introduction to Safety Critical Systems. In: Cotroneo, D. (eds): Innovative Technologies for Dependable OTS-Based Critical Systems. Springer, Milano.

https://doi.org/10.1007/978-88-470-2772-5_2

A definition of safety-related systems (SRS) can be found in the basic safety publication IEC 61508:

Safety-Related System

Designated system that both

  • implements the required safety functions necessary to achieve or maintain a safe state for the equipment under control; and
  • is intended to achieve, on its own or with other E/E/PE safety-related systems and other risk reduction measures, the necessary safety integrity for the required safety functions.

NOTE: The term refers to those systems […] that are intended to achieve, together with the other risk reduction measures, the necessary risk reduction in order to meet the required tolerable risk.

[IEC 61508-4:2010] Functional safety of electrical/electronic/programmable electronic safety-related

systems – Part 4: Definitions and abbreviation, Clause 3.4.1. International Standard, IEC, 2010

According to the insight gained, we can refer to the overall systems whose failure could lead to severe harm (e.g. injuries, death, major environmental damage) as SCSs. Reducing the risk associated with such systems may require the realization of dedicated safety functions (or safety mechanisms as they are referred to in other standards). Designated systems that implement these safety functions with the necessary integrity can then be termed SRSs.

But keep in mind, that safety terminology can vary by domain and standard. So, other application domains may use these two terms in a slightly different way.

For example, the automotive functional safety standard ISO 26262 uses the term safety-related system without actually defining it. However, it defines safety-related element and safety-related function as follows:

Safety-Related Element

Element that has the potential to contribute to the violation of or achievement of a safety goal

Safety-Related Function

Function that has the potential to contribute to the violation of or achievement of a safety goal

[ISO 26262-1:2018] Road vehicles - Functional safety - Part 1: Vocabulary. International Standard, ISO, 2010

In contrast, the term safety-critical does not appear in ISO 26262.

What do you think about this topic? Do you have further definitions or examples?

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Prof. Dr. Mirko Conrad
Managing Director | samoconsult GmbH

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