This part of IEC 62689 describes electric phenomena and electric ystem behaviour during faults, ccording to the most widely diffused distribution system architecture and o fault typologies, to define the functional requirements for fault assage indicators (FPI) and distribution substation units (DSU) (including their current and/or voltage sensors), which are, espectively, a device or a evice/combination of devices and/or of functions able to detect faults and provide indications about their localization.