Parts of an installation and its plant (including computer systems) which: a.) the failure of which could cause or contribute substantially to a major accident or, b.) the purpose of which is to prevent, or limit the effect, of a major accident.
Note: for installations covered by the Safety Case Regulations 2015, these are now termed Safety and Environmental Critical Elements (SECEs).
In simple terms, Safety Critical Elements are the essential components which prevent or limit the effects of a major accident. The failure of an SCE could cause or contribute to a major accident. Typical examples could include: lifeboats, fire and gas detectors, deluge systems or emergency shutdown valves.