This chapter aims to present switching overvoltage insulation coordination methods for substations. As for transmission lines, switching overvoltages only become important for systems whose nominal system voltage is equal to or greater than 345 kV. The switching surges at the end of the line are those that impinge on the station insulation—or the station insulation is the insulation at the end of the line. Since the time to crest of the Switching overvoltages (SOV) are large compared with the travel time for these distances, the shape and magnitude of the SOVs are essentially constant throughout the station. Provided the SOVs are sufficiently high so that at least one arrester operates, the phase-phase SOVs are reduced, thus reducing the required clearance and BSL. The BIL/BSL of post insulators or of equipment is based not only on switching surges but also on lightning and contamination. In addition, BILs and BSLs are standardized numbers that sometimes greatly exceed those required.