Thank you for your question regarding electrical grounding on top of granite rock, it is our pleasure to help.
This is one of the biggest questions we ever get, how do you ground on top of a rock mountain? We usually answer by pointing out that the International Space Station is grounded, and without the aid of a ground wire trailing down to earth! It should also be noted that airplanes, helicopters and ships out at sea are all grounded without a connection to earth. How is this possible?
When your site is located on top of a granite mountain, you can be relatively assured that the resistance-to-ground of your electrodes will be very high. This tells a Grounding Engineer that the Ground Potential Difference (GPD) of the system is going to be even more important than usual.
When electrical energy enters the site, the resistance will be very high, and thus the voltage will be very high as well. Imagine a 5,000 amp fault entering a site with a 100-ohm resistance-to-ground. Ohms law tells us that the voltage or Ground Potential Rise (GPR) of the site will be as high as 500,000 volts! If your site has a difference of potential from any given point to any other point within the system of 2-ohms, the site could see a voltage differential of 10,000 volts! If that difference in potential is 1-ohm, then you could see 5,000-volts forming across your site! If the difference in potential is only 0.1-ohms, then your site would only experience 500 volts. This should demonstrate the importance of a low Ground Potential Differential (GPD) in high resistance grounding systems.
One can imagine that as fault currents enter a grounding grid, the voltage will rise at some gradient across the system. The actual voltage that the system rises is not the primary concern, it’s the current or amperage that can flow across people or equipment that is our concern. So, like a bird landing on a power line which may have high voltage, the bird will be safe as long as no current flows through the bird itself. Here is some additional info:
Now with all that said, it is still advisable to provide an effective low-impedance path to earth for your grounding system. Many times in cases such as this one, the power for the site is being brought in via overhead power lines (power poles). You may wish to add a 4/0 AWG or greater copper conductor on the overhead pole system, so as to ensure that there is a ground path from your site back to the source. This will ensure that OCPD (Over Current Protection Devices) such as fuses and circuit breakers will function properly. It can also give you an opportunity to install a grounding electrode for your site in better soil conditions. This will be true even if you install an isolation transformer at your site.
The problems your site is facing are very complex. You really need to get a grounding expert involved so that the site will be safe for personnel and equipment, and so that it functions properly. E&S Grounding Solutions is of course more than happy to help you resolve all the problems at this site. But if not us, please get someone with the proper engineering experience and tools to get the job done right.
Feel free to contact us directly at 310-318-7151 and someone will be glad to discuss your project with you free of charge.
The Engineering Team at E&S Grounding Solutions
Photo credit: http://www.flickr.com/photos/12943203@N04/3685584725/sizes/l/in/photostream/