This International Standard provides recommendations for the design and implementation of electrical interlocks used actively or passively to prevent unsafe conditions or to ensure specific safe conditions and states during the operation of nuclear power plants. Safety system interlocks are covered by standards for safety systems. This standard gives design, reliability and test criteria arising from the consideration of interlocks important to safety in accordance with IEC 61226. It uses categories A, B and C as defined by that document. It takes into account the interlock safety significance and functions, and the role of the operator in some interlocks. Guidance is given on redundancy and on diversity of equipment for implementing interlocks, where high availability or integrity is involved, or where common mode failure may limit reliability. The use of computer-based equipment for interlock functions is discussed, and recommendations for diversity are given. Requirements for software and for verification and validation are given by reference to IEC 60880. The provision of test facilities is discussed together with self-testing and self-monitoring methods. The system of interlocks for a specific function includes the sensors of plant state (e.g., measuring devices and limit switches), the interlock and control power supplies, the control and instrumentation equipment providing the particular logic function for prevention or limitation of operation, and the cables, electromechanical features, key control and administrative control associated with the function.