IEC 62689-2:2016 describes electric phenomena and electric system behaviour during faults, according to the most widely diffused distribution system architecture and to fault typologies, to define the functional requirements for fault passage indicators (FPI) and distribution substation units (DSU) (including their current and/or voltage sensors), which are, respectively, a device or a device/combination of devices and/or of functions able to detect faults and provide indications about their localization. By localization of the fault is meant the fault position with respect to the FPI/DSU installation point on the network (upstream or downstream from the FPI/DSU's location) or the direction of the fault current flowing through the FPI itself. The fault localization may be obtained- directly from the FPI/DSU, or- from a central system using information from more FPIs or DSUs,considering the features and the operating conditions of the electric system where the FPIs/DSUs are installed. This part of IEC 62689 is therefore aimed at helping users in the appropriate choice of FPIs/DSUs (or of a system based on FPI/DSU information) properly operating in their networks, considering adopted solutions and operation rules (defined by tradition and/or depending on possible constraints concerning continuity and quality of voltage supply defined by a national regulator), and also taking into account complexity of the apparatus and consequent cost. This part of IEC 62689 is mainly focused on system behaviour during faults, which is the "core" of FPI/DSU fault detection capability classes described in IEC 62689-1, where all requirements are specified in detail.