Earthing Design for 3MW Load, 500KVA Transformer, 400KW DG, 1950KW Gas Generators

Hello,

1. in our factory the machine connecting load is 3MW. i want to design earthing pits for them.what current should i consider to design the earthing?how to do it practically?

2.in our substation there will be a 500KVA transformer and 400KW diesel generator.how to design earthing system for them?

3.there will be two 1950KW gas generators.how to design their earthing?

please help...