This is the test that determines the healthiness of insulation A high voltage at the power frequency is applied on one of the windings with the other being shorted together and earthed along with the tank. HV test voltage varies for VTs used in Earthed and Un-earthed systems. In earthed system, one end of the primary of the PT is connected to the earth through an earth link. whereas in the unearthed system, primary is not earthed. In case of earthed system, the high voltage applied on the primary side will be only 3KV irrespective of its rated voltage. In case of Un-earthed system, the High voltage is applied on the primary side with secondary and tank are earthed together.
For eg: If the test is done on the 33KV side of a PT, 70KV is applied on the winding with the secondary shorted and connected to earth. This voltage is maintained at the terminals for one minutes, during which there shall not be any flash over or collapse of voltage.