Abstract: The backup automatic switch device can quickly switch on the backup power when the main power supply is out of operation due to a fault, so as to ensure uninterrupted power supply. As a key characteristic quantity of power system operation status, current plays a crucial role in the startup, blocking, action, and fault protection of backup automatic switch devices. This paper systematically elaborates the mechanism of various current criteria in backup automatic switch devices, including no-current criterion, over-current blocking criterion, overload shedding strategies, and reverse current identification criterion. Aiming at practical problems such as maloperation caused by PT disconnection under light load conditions, single-phase current sampling defects, and reverse power flow from distributed generators, systematic optimization schemes are proposed combined with engineering cases. The research results provide positive reference for the operation, maintenance, engineering modification of backup automatic switch devices and the safe and stable operation of power grids.