IEC 62855 provides the electrotechnical engineering guidelines for analysis of AC and DC electrical power systems in nuclear power plants (NPPs) in order to demonstrate that the power sources and the distribution systems have the capability for safe operation and shut down of the NPP, bringing it to a controlled state after an anticipated operational occurrence or accident conditions and finally reaching a safe state. The analytical studies discussed in this document provide assurance that the design bases are satisfied to meet their functional requirements under the conditions produced by the applicable design basis events. The studies provide assurance that the electrical power system is capable of supporting safety functions during all required plant conditions. NOTE – The safety functions are described in IAEA Specific Safety Requirements SSR-2/1 related to the design of the nuclear power plants.. Analytical studies validate the robustness and adequacy of design margins and demonstrate the capability of electrical power systems to support plant operation for normal, abnormal, degraded and accident conditions. The analyses are used to verify that the electrical power system can withstand minor disturbances and that the consequences of major disturbances or failures do not degrade the capability of the electrical power systems to support safe shutdown of the plant and maintain the plant in shutdown condition. The analyses are performed with one or more of • simulation tools (software and hardware) that have been verified and validated, • hand calculations, and • tests. This document provides guidance on the types of analyses required to demonstrate that the plant's auxiliary power system can perform the required safety functions. This document does not provide specific details on how the analysis should be conducted. This document does not cover digital controllers (such as controllers for rectifiers, inverters, sequencers and electrica ...