Solar Power Plant : GND Impedance

1) For Solar PV Roof Top Mounted system , normally in DC distribution boxes MCCB /MCB is prefereed .Is it the right practice ?? How can such protecton works in case of ground fault between positive and ground ??

2) For 1 MWp Solar PV system what is the acceptable leakage current ? PCU manufacturers provide value which can be easily manipulated .They provide a long band .. whereby they remain on safer side ?

Regards

Ashish