The instrumentation transformers 6a to 6d and 7a to 7f for measuring the voltage of the main circuit are arranged within the container 15b located in the midst of the containers 15a to 15c. In the present invention, the containers 15a and 15b are linked by the insulating tube 21a and the primary-side lead lines 49 and 53 connected electrically to the primary-side terminal of the instrumentation transformers 6a and 7a are made routed from the container 15b through the insulating tube 21a to the container 15a, and the containers 15b and 15c are linked by the insulating tube 21b and the primary-side lead lines 51 and 55 connected electrically to the primary-side terminal of the instrumentation transformers 6d and 7e are made routed from the container 15b through the insulating tube 21b to the container 15c.