PURPOSE: To facilitate polarity comfirming test by providing a wiring for polarity confirming test penetrating through an iron core along its inside and by pulling out at least one end of said wiring with insulating it from a tank.

CONSTITUTION: Both ends of a polarity test wiring 8 arranged so as to penetrate through inside of one or plural BCT cores 5a are connected to a terminal 9 built in a terminal box 6 attached to a transformer tank 1. The terminal 9 has an insulated terminal structure and is insulated from the transformer tank 1. Such structure enables to perform the polarity confirming test of the BCT5 easily from the outside without any operation of a main circuit in case of that there are plural BCT iron cores provided.