# IEC 61513 — Nuclear I&C systems important to safety > IEC 61513 is the top-level standard for instrumentation and control (I&C) important to safety in nuclear power plants. It is the nuclear sector application of IEC 61508 functional safety: it sets the general requirements for the overall I&C architecture, then hands off to a family of SC 45A standards that cover categorization, software and hardware. It governs both hard-wired and computer-based systems. - Source : https://industryhub.site/en/standards/iec-61513 - Body : IEC - Family : Nuclear I&C - Status : current - First published : 2001 - Latest revision : 2011 (Edition 2) - Edition : 2.0 - Next revision : Active maintenance - Access : Paid (IEC webstore) - Language : English ## Document structure - **IEC 61513** — Overall I&C — general requirements (this standard) : Requirements and recommendations for the overall I&C architecture of a nuclear power plant, the system lifecycle and the safety case. The top of the IEC SC 45A series. - **IEC 61226** — Categorization of functions : Classifies I&C functions important to safety into categories A, B and C by their safety significance — the decision that drives every later requirement. - **IEC 60880** — Software for Category A functions : Requirements for the software of computer-based systems performing the most safety-significant (Category A) functions — the strictest software regime. - **IEC 62138 / IEC 60987** — Category B/C software and hardware : IEC 62138 covers software for Category B and C functions; IEC 60987 sets hardware design requirements for computer-based systems. Together they complete the implementation layer. ## Key concepts - **The nuclear face of IEC 61508** : IEC 61513 is how the generic functional-safety framework of IEC 61508 is applied to nuclear power plants. It keeps the lifecycle logic but speaks the nuclear vocabulary of categories, defence in depth and licensing. - **Categorization A / B / C** : Per IEC 61226, every I&C function is ranked A (highest safety significance — e.g. reactor trip), B or C. The category fixes how rigorously the implementing system must be designed, qualified and documented. - **Class 1, 2, 3 systems** : Functions of a given category are assigned to systems of a matching safety class. Mixing categories on one system is constrained, because a weaker function must not undermine a stronger one. - **Defence in depth and independence** : Safety rests on independent layers — protection systems separate from control, diversity against common-cause failure, physical and electrical separation. 61513 makes this architecture an explicit requirement, not a hope. - **Hard-wired and computer-based** : I&C important to safety may be conventional hard-wired, computer-based, or a mix. 61513 covers all three, which is why software standards (60880, 62138) sit beneath it. - **The SC 45A series** : 61513 is the apex of a coordinated family — 61226 for categorization, 60880 and 62138 for software, 60987 for hardware, 62645 for cybersecurity — aligned with IAEA safety guidance. ## The safety backbone of a nuclear plant's I&C A nuclear power plant is run, monitored and — when needed — shut down by its instrumentation and control. When that I&C is **important to safety**, it must meet requirements far beyond ordinary process control. **IEC 61513** is the top-level standard that sets those requirements: the overall I&C architecture, the system lifecycle and the safety demonstration for a nuclear plant. ## The nuclear face of IEC 61508 IEC 61513 is the **nuclear sector application of [IEC 61508](/en/standards/iec-61508)**. It keeps the functional-safety lifecycle — define the functions, allocate them to systems, design, verify, validate — but expresses it in the nuclear language of safety categories, defence in depth and regulatory licensing. It applies whether the I&C is conventional hard-wired equipment, computer-based, or a combination of both. ## Categorize, then build to the category The pivotal step is **categorization**. Following IEC 61226, every function important to safety is ranked **A, B or C** by its safety significance — a reactor trip is Category A, a monitoring aid may be C. That category then fixes how rigorously the implementing system must be designed and qualified, and the function is assigned to a system of a matching **safety class (1, 2, 3)**. The principle running through it is **independence**: protection separated from control, diversity against common-cause failure, physical and electrical separation, so a weaker function can never undermine a stronger one. ## Architecture, then the series beneath it 61513 sits at the apex of the coordinated **IEC SC 45A** family and hands the detail to the standards below it: **IEC 61226** for categorization, **IEC 60880** for the software of Category A functions, **IEC 62138** for Category B and C software, **IEC 60987** for hardware, and IEC 62645 for cybersecurity — all aligned with IAEA safety guidance. In practice it also meets [IEC 62443](/en/standards/iec-62443) thinking on control-system security, and the plant's long operating life is managed under [ISO 55000](/en/standards/iso-55000) asset management. ## Applicable industries Nuclear power plant operators and new-build projects, Reactor I&C and protection-system vendors, Nuclear safety regulators and technical support organisations, EPC contractors and qualification laboratories ## References & further reading - [IEC 61513:2011 — nuclear I&C, general requirements (standards store)](https://webstore.ansi.org/standards/iec/iec61513ed2011)