Not only for string combiner box but anywhere as per standard practice, we select fuse/breaker size with the help of the below formula: Fuse/Breaker rating size = Current X 1.2 or 1.3 times Suppose we selected 280Wp module, as per datasheet Isc is 8.68Amps so Selected Fuse = 8.68Amps X 1.25 = 10.85Amps so selected fuse say 12Amps But in case of solar system design this formula many times fail to provide .
The main reason for doing earthing in the electrical network is for the safety. Earth or ground wire is mainly to protect the human from electrical shock. If there is contact between live wire directly to metal case/body of the application or device, the user shall get a shock because the current will flow towards the earth through the user body if that person is in contact with the earth. The green earth wire .