(57) An on-load tap changer of the present invention comprises a switch 18 including an
arc interrupting unit 20, a vessel 16 containing therein the switch, an electrically
insulating oil disposed within the vessel for electrically insulating the switch,
and a closed oil passage loop 43 for circulating the insulating oil therealong. The
loop includes a conduit 38 having an inlet at the bottom portion of the interior of
the vessel, the conduit upwardly extending inside of the vessel and exiting from the
upper portion of the vessel, extending through a position lower than the vessel, and
connecting to an upper portion of the vessel, the conduit having formed therein a
hole 44 at a position higher than the arc interrupting unit, the hole being inside
of the vessel. A float valve 46 is provided in the vessel for closing the hole in
the conduit when the level of the insulating oil within the vessel is at or higher
than the hole and opening the hole when the level of the insulating oil within the
vessel is lower than the hole.
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