PURPOSE: To dispense both mounting of high-tension bushings to a transformer tank and enlargement of dimensions of a transformer tank by leading connection leads between both terminals of an oil-immersed arrester and a tap winding into an insulation cylinder respectively via low-tension bushings and by leading them out of the insulation cylinder again via low-tension bushings.

CONSTITUTION: The insulation strengths to ground of low-tension bushings 41, 42, 44 and 45 piercing an insulation cylinder 10 are secured by this insulation cylinder 10. During the operation of an on-load tap changing transformer, the upper terminal of an oil-immersed arrester 1 is connected to the upper terminal of a tap winding 32 by sequential connection with connection lead 52, low- tension bushing 42, connection lead 53, low-tension bushing 41, and connection lead 51, and the lower terminal of the oil-immersed arrester 1 to the lower terminal of the tap winding 32 by sequential connection with connection lead 55, low-tension bushing 45, connection lead 56, low-tension bushing 44, and connection lead 54. When overvoltage exceeding the limit voltage of the oil- immersed arrester 1 develops across the tap winding 32, the oil-immersed arrester 1 begins to operate and electrify for suppression of overvoltage, thereby protecting the insulation of the tap winding 32 and of leads connected to it.