PURPOSE: To obtain test results under the same condition as that in an actual circuit economically by providing an separate transformer on an arbitrary one phase to supply a voltage verifying the insulation performance through a high voltage winding on the secondary side.

CONSTITUTION: A specified interruption current and a specified recovery voltage are supplied to each phase of a sample switch 25 through transformers 37aW37c from each phase of 3-phase power sources 33aW33c. At the same time, a separate transformer 39 is provided on one phase 33a of the power source and a voltage is supplied to verify the insulation performance between an interrupting section 25a and the ground and between the interrupting section 25a and those 25b and 25c through a high voltage winding 40 on the secondary side. This enables the testing under the same condition as that in an actual circuit thereby affording test results with a high reliability while allowing a small capacity of a test equipment.