Earthing Questions

1)What is potential gradient control as it applies to earthing?I am involved in the earthing of a power station and in my RFP(request for proposal) it was clearly stated that all locations where dangerous surface voltage gradients may occur a potential gradient control system should be provided.this will be applicable to 1)Transformer areas 2)sub-stations 3)MV and LV switchgear rooms 4)Turbine/Generator floor.

2)Adequate corrosion protection shall be provided when conductors leave concrete.What can i apply to prevent corrosion.

3)Can u please help me with methods of measuring earth resistance,respectively the earth voltage,the touch n step voltage.

Thank you very much for your anticipated positive response.