Overcurrent protection of pole top distribution transformers that have tank mounted lightning arresters

When protecting distribution transformers, most protection engineers in the utility industry have recognized the importance of placing arresters either on the transformer tank or under oil within the transformer tank. This method greatly reduces arrester lead length and the associated increase in discharge voltage thereby improving transformer failure rates. To obtain the shortest lead lengths possible, utilities have moved from arrester installations on the pole or cross arm to arresters mounted on the transformer tank. By moving the arrester on the tank (or under oil), the lead lengths are shortened as much as possible. Utility experience has proven that this method of arrester protection will decrease the number of transformer failures due to lightning. Another benefit of moving the arrester to the load side of the fuse is that the fuse can also protect the arrester