At present, in the substation running high, medium sized transformer iron core multipoint earthing of the high accident rate. Under normal circumstances, the big, medium-sized transformer core is through the ground special, with a small sleeve leads the transformer body outside the grounding point. But in operation, due to various reasons, such as the iron precipitate, iron magnetic vibration, in transformer oil structure and other reasons, will result in the other point grounding of transformer core. At this time, because the potential of each parts of the iron core of different, will produce large current in the core, sometimes up to tens of amperes, if not promptly found the core damage will cause the core of local heating or arc grounding.