7.8 Relay Protection and Safety Automatic Devices
7.8.1 Main protection and back-up protection devices shall be installed for electric equipment and lines.In case a main protection device or circuit breaker fails to actuate，the fault shall be removed by the back-up protection of the component or the protection device of a neighboring component.
7.8.2 Relay protection devices should be digital and meet the requirements for reliability，selectivity，sensitivity and rapidity.The time differential of a protection device may be 0.3s-0.5s.
7.8.3 The current transformer equipped with various protection devices shall meet the requirements for eliminating protection dead zones and reducing the impact of faults in the current transformer.
7.8.4 The steady-state error of the current transformer used for protection devices shall not be greater than 10%.The current transformers of protection devices and measurement instruments shall not share one secondary winding.(https://www.daowen.com)
7.8.5 The protection device might malfunction due to disconnection of the secondary circuit of voltage transformer，hence a disconnection blocking device shall be installed and signals be sent out.If the disconnection of the secondary circuit would not cause malfunction of the protection device，only a signal device for disconnection of the voltage circuit needs to be installed.
7.8.6 Indication signals should be set up for the protection device，and shall be able to respectively indicate collected electricity quantity，set value for relay protection，and actuation status of protection devices.
7.8.7 For lines with voltage grades at or above 10kV that installed with circuit breakers，auto-reclosing devices should be installed. If there is a power source on the opposite side，a synchronism check or novoltage check auto-reclosing device should be installed.
7.8.8 For a hydropower station with two or more plant service transformers，automatic transfer to reserve supply（ATS）devices shall be installed.
