The function 87N in the SIPROTEC 5 devices is a sensitive ground fault differential protection. 2 (latest version 2022) have been a fundamental basis of describing functions in the electricity industry. Of course the generic purpose of such "nomenclature/semantic" standards is to eliminate ambiguity and arbitrary definitions that are not. In order to grasp the essence and functionality of restricted earth fault protection, it is expedient to start with the function and the challenges of classic phase differential protection. The differential protection has the decisive advantage of absolute and definitive selectivity, since the. In electric power systems and industrial automation, ANSI Device Numbers can be used to identify equipment and devices in a system such as relays, circuit breakers, or instruments. The device numbers are enumerated in ANSI / IEEE Standard C37.