I — Theoretical studies and laboratory tests

TESTS show that when a surge is applied to the primary of a distribution transformer that has a secondary ground, the isolated case will assume a low electrostatic potential above ground so that about from 80 to 95 per cent of the surge voltage will appear from primary lead to case. For simplicity in the considerations that follow, it will be assumed that the case potential under these conditions is the same as that of the secondary neutral.