The object of this standard is to specify the minimum requirements for functional and performance evaluation of (longitudinal) differential protection designed for the detection of faults in ac motors, generators and transformers. This standard also defines how to document and publish performance test results. This standard covers the differential protection function whose operating characteristic can be defined on a bias-differential plane and includes specification of the protection function, measurement characteristics, compensation of energizing quantities, additional restraint or blocking methods (for over-excitation and magnetising inrush), starting and time delay characteristics. The standard also covers unrestrained differential protection functions traditionally combined with the restrained (biased) differential element to form a single differential relay. The standard defines the influencing factors that affect the accuracy under steady state conditions and performance characteristics during dynamic conditions. The test methodologies for verifying performance characteristics and accuracy are also included in this standard. The differential protection functions covered by this standard are as follows: This standard does not specify the functional description of additional features often associated with biased differential relays such as CT supervision (CTS), switch onto fault (SOTF) and detection of geo-magnetically induced currents (GIC). This standard does not cover differential relays designed for bus bar protection (including high impedance differential protection) or line protection. Additionally, this standard does not explicitly cover generator incomplete longitudinal differential, generator split-phase transverse differential, self-balancing or magnetic balanced protection scheme, differential protection of phase-shifting transformers, directional restricted earth fault protection, railway transformers, convertor transformers and reactors but the principles covered by this standard may be extended to provide guidance for these applications. The general requirements for measuring relays and protection equipment are defined in IEC 60255-1.