Phase-phase insulation naturally occurs in stations—between buses and from one piece of equipment to another. Therefore the phase-phase strike distance and SSFOR should always be considered in station insulation coordination studies. A phase-phase insulation always exists with a phase-to-ground insulation. Phase-phase flashover can be essentially eliminated if the phase-ground strike distance is sufficiently lowered—or the phase-to-ground flashover can be essentially eliminated if the phase-to-ground strike distance is increased. Properly and theoretically, the SSFOR of the usual insulation system, which is composed of both phase-ground and the phase-phase, should be calculated as a single system. For internal insulations such as in transformers, cables, and Gas-insulated stations, the insulation strength is only a function of the phase-phase voltages and is not a function of the division of this voltage into positive and negative polarities. The density function is that for phase-phase overvoltages.