The basic impulse insulation level (BIL) of the transformers and reactors is 1800 kV and for circuit breakers-2300 kV-with
switching surge and impulse protection for reactors and transformers provided for by lightning arresters on each unit .Bus and disconnecting switch insulation is 2050-kV BIL [180-inches (457-cm) nominal insulator height], and the disconnecting switching open-gap switching-surge withstand is 1750-kV crest line to ground. Rigorous testing of the insulation system was done under special AEP requirements for rainfall, contamination, and switching surges to be sure that safety and reliability aspects were met (for a detailed listing of equipment rating refer to Table I).