PURPOSE: To enable insulation deterioration to be assessed by examining the degradation of heat conductivity with time caused by the cavities produced in the insulation layers and dieing of the insulation layers from the temperature rise through current supply to conductor.

CONSTITUTION: A conductor 4 and an earth insulation 3 are incorporated by having a certain fixed gap in the iron groove 5 provided to an iron core 1. This earth insulation 3 does not contain any cavities within the insulation layers initially and has specified heat conductivity but upon long-term use, it produces cavities between the insulation layers owing to thermal expansion and shrinkage and mechanical stresses, thus resulting in the decreased heat conductivity. As a result, if the same current is supplied to the conductor 4, the conductor temperature is higher where there are cavities. The degrees of containing the cavities in the insulation layers are grasped by making use of this and the conductor temperatures are measured with time, whereby the insulation deterioration may be assessed.