PURPOSE: To prevent erroneous function of relay due to exciting surge current by storing a maximum current flowing into a ground overcurrent relay under exciting surge state, automatically setting the relay based on thus stored maximum current, and then operating the relay based on thus set value.

CONSTITUTION: A ground overcurrent relay OCGR has function for storing a maximum exciting surge current of a transformer TR and function for automatically operating and setting function set values of the relay based on thus stored maximum exciting surge current. The relay OCGR is further provided with an automatic switching function for operating at manually set values when no exciting surge current flows while operates at automatically set values when the exciting surge current flows. An instantaneous voltage relay 84T makes a decision whether the exciting surge current is flowing or not thus deciding that an exciting surge current state prevails for several seconds when a voltage is applied on the secondary of transformer. An operation signal is delivered from the relay 84T to the ground overcurrent relay OCGR in order to make decision of exciting surge current state.