This International Standard specifies the performance and the functional characteristics of the electrical supply systems required for the instrumentation and control (l&C) systems important to safety of a nuclear power plant. Guidance is also given on the possible use of these supplies for other l&C systems. These supplies should be fed from primary sources of suitable redundancy and reliability, so that the safety and functional objectives of the l&C system can be adequately achieved. This standard defines the methods of application of IAEA Safety Guide NS-G-1.3. The specific design requirements for the components of the l&C power supply system are covered by IEC standards and standards listed in the references and are otherwise outside the scope of this standard. The scope of the l&C power supply system covered by this standard does not apply to supplies to certain equipment within an l&C system, which may need specific and special quality or reliability of power supplies, such as closer tolerances of frequency or voltage or of interruption time for loss of supply shorter than the period required by the normal tolerances of IEC standards relevant to power plants.