Abstract: Taishan CEPR is the third- generation advanced pressure water reactor. According to the redundancy requirements of safety consumers, the auxiliary power supply system in the nuclear island sets up as 4 independent trains. Different cross-connection between four trains can make sure that the safety consumers work as the redundancy requirement, and provide lighting and power for maintenance, which is the condition for improving the maintenance efficiency. Mechanical interlocking is the method to manage the switch-over between the cross-connection. "Special condition" can make sure the consistence between cross-connection and CEPR separation rule and operation technical specification.