使用变压器油作为绝缘介质时，出于(    )原因，一般在电极表面加覆盖层。

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SQL Error: select id,body,options from ***_enewstiku_ where sourceid='69525' and id < 4504376 and id<>'4504376' order by id desc limit 5 SQL Error: select id,body,options from ***_enewstiku_ where sourceid='69525' and id > 4504376 and id<>'4504376' order by id asc limit 5