CAT A/B/C functionality meaning

In nuclear projects, CAT A/B/C functionality describes how an instrumentation and control (I&C) function is graded for its importance to nuclear safety. The grading derives from IEC 61226 (Nuclear power plants – I&C important to safety – Classification of I&C functions). It is not defined in UK or Irish statute or case law, but is widely used in industry practice and referenced in Office for Nuclear Regulation (ONR) guidance in Great Britain. Category A covers the highest‑significance safety functions; Category B covers significant but lower‑risk safety functions; Category C covers functions of the lowest safety significance that are still important to safe operation. The assigned category typically drives regulatory expectations and contract requirements, including design assurance, qualification and software standards, segregation and independence, testing, change control, competence, and record‑keeping. It is routinely used in safety cases, licensing submissions, technical specifications, and procurement and supply chain flow‑down. Usage and meaning are broadly consistent across England & Wales and Scotland (regulated by the ONR). Northern Ireland and Ireland have no nuclear power plants; where the term appears (for example in cross‑border supply, decommissioning or international projects), it carries the same IEC 61226 meaning.