Abstract: In process of abnormal situation analysis of a substation in Taizhou area, it was found that when the measurement and control device D25 manufactured by certain company was matched with line protection, in which the CZX-12A operational relay box was used, if the relay in measurement and control box acted abnormally, the automatic recloser of the line protection should be latched. In this situation, if a single phase earthing fault occurred, the switch should trip single phase, and the recloser should not recluse, so the single earthing fault may translate into multiphases short circuit fault, and the equipment damage in fault point may be enhanced. This is a hidden danger for safety operation of power grid. The abovementioned phenomenon was analyzed, and relevant improvement scheme was proposed.

Key words: measurement and control device, automatic recloser, single phase earthing fault