Thank you for your question regarding the grounding of generators in a mining area, it is our pleasure to help.
When dealing with a 640kVA generator, you have a number of things to be concerned about regarding grounding. This first thing of course is your requirement to account for Human Safety. A 640kVA generator can have a very significant fault current capable of generating serious life-threatening voltages. Please refer to Title 29 of the Code of Federal Regulations Part 1910.269 (29 CFR 1910.269) for more information. This Federal Law requires that the grounding system be capable of safely handling the hazardous Step & Touch Voltages that will form during a line-to-ground fault. Human Safety will be the primary concern for the grounding system for your generator.
The iron ore in the earth can be both a blessing and a curse. On the upside, it should be easy to achieve a nice low resistance-to-ground. On the down side, it will be harder to control fault current from spreading across the surface of the earth generating a greater Step Voltage problem for personnel simply walking near the generator. You will also almost certainly have a bigger corrosion control problem as well. The iron in the soil will be a big source of galvanic corrosion on the copper grounding system, which will only be enhanced by the Alternating Currents (AC) and the electromagnetic fields put off by the generator . Regular inspection of the grounding system will be crucial as you should expect standard grounding rods to corrode away in less than 5 years.
Please see our previous blog regarding corrosion here:
Ultimately, the grounding system for your generator must be built in compliance with National Electrical Code (NEC) NFPA 70 regulations, which means the grounding system must be tied back to the first service disconnect via buried ground conductors. Your generator you will need to ensure that the system is capable of handling the fault currents without burning open (current loading), that hazardous Step & Touch Voltages are mitigated to below Human Fibrillation Current Thresholds (HFCT), and that the Ground Potential Difference (GPD) is low enough that your data and control lines will function properly.
Here is a previous blog you may find applicable to your situation:
E&S Grounding Solutions is of course more than happy to help you in designing your grounding system. But if not us, please get someone with the proper measurement tools, engineering software and experience to get the job done right. Please feel free to call us directly and one of our engineers will be glad to speak with you about your project free of charge.
The Engineering Team at E&S Grounding Solutions