Thank you for your question regarding electrical grounding via deep-earth shafts. It is our pleasure to help.
When constructing any facility that will have deep-earth shafts, it is only logical to want to take advantage of the situation and get some excellent electrical grounding for free! This is exactly what any good engineer will want to do in your situation, and the answer couldn’t be simpler or easier.
Whether it is a deep shaft, a well, a piling, or any other ‘concrete-encased rebar structure’, cheap and easy grounding is possible. Basically, all one needs to do is add copper conductors to the steel rebar structure prior to the application of concrete, and you get instant deep grounding.
Let’s walk through a typical scenario: The contractors have just finished drilling out a very deep hole and are beginning to put together the pieces of the steel rebar structure that will be lowered into the hole and back filled with concrete. You instruct the contractors to attach a bare copper conductor to the steel rebar using either exothermic welding and/or irreversible compression fittings (often structural engineers will oppose exothermic welding as there is no guarantee that the heat will not compromise the integrity of the steel. Compression fittings are more than acceptable.) at various intervals along the length of the rebar structure. As they lower the steel structure down into the hole, you ensure that a large “pig-tail” of copper conductor is left above-grade so that you can connect to it later. The hole is back-filled with concrete and your free deep grounding is done. As long as the concrete is in direct contact with the earth (and not protected by some plastic barrier), you will have a very effective grounding electrode.
Our chief reason for adding the copper (versus using only steel) is to ensure that any electrical fault will not damage the concrete. The water in the concrete can be turned into steam if the electrical fault heats the steel hot enough. Copper is at least 12 times more conductive than steel and at least 250 times less magnetic. The copper will not only conduct electrical energy better (with less heat), but it will also remove electrical fields better, thus reducing harmonics, transients and other stray currents found in structural steel (which could interfere with your communication systems). Also, the copper ensures connectivity between the steel rebar pieces, which can sometimes get lost with all the transitions from section to section and particularly when the shaft is connected to grade-level structural items.
Now in your case, the facility is handling some impressive amounts of power. Your main concern will be to ensure that under fault conditions the electrical energy is distributed across enough copper conductors to handle the current without getting hot. This should be easy in your case as you may be able to utilize more than one deep-earth shaft. But if this is a concern, the current distribution can certainly be analyzed with a simple computer model. You would need to merely model in the steel rebar structure, add the copper component, and simulate the fault in the local soil conditions. The fault should include the line-to-ground short-circuit fault current, the X/R Ratio (or Zero-Sequence Impedance), and the Clearing Time. A simple current distribution analysis will tell you what size copper wire should be used and ensure that the system is capable. A study like this can also be used for improving your data-line and server room communications by analyzing the Ground Potential Difference (GPD) across your facility, a key factor for high-tech computer signaling systems. Do you know what grounding standard your project will be using for the computer server rooms?
You may also want to consult the Lightning Protection people on this project, as they may also want to utilize deep grounding. In England and much of Europe, the Lightning Protection Standard of choice is the BS EN 62305:2006 standard. This comprehensive standard has specific resistance requirements for steel structures utilized in grounding (another reason to add copper) that may apply to this situation. Here is a link with more info: http://www.esgroundingsolutions.com/about-electrical-grounding/lightning-protection-systems-nfpa-780.php
If you have any further question on how to interconnect the various grounding systems, how to provide noise-free grounding for your computer server rooms, how to tie in the lightning protection system, or how to get a basic grounding study and computer model conducted, please do not hesitate to contact one of our Engineers directly, and they will be glad to discuss your project with you free of charge.
The Engineering Team at E&S Grounding Solutions