This publication covers relevant aspects of open phase conditions (OPC) initiated in the transmission systems or on-site plant electrical systems. It provides details on the methods that can be used to identify vulnerability to OPC in the existing electrical protective schemes. The technical guidance provided will help to enhance the safety of nuclear power plants and to address vulnerability to OPC in the plant electrical systems. The publication will serve as a useful guidance, with a focus on electrical power systems, for all personnel involved in the design, manufacture, qualification, operation, maintenance, management and licensing of nuclear facilities.