Thank you for your question regarding substation grounding on the top floor of a high-rise building. It is our pleasure to help.
Grounding is always a difficult subject, but having a substation on the 15th floor, definitely presents some extra problems. Obviously, all 29 CFR 1910.269 and IEEE-80 standards must be applied to the 15th floor to insure proper Human Safety in regards to Hazardous Step & Touch Voltages. We are more than happy to help you with these critical issues, but it sounds from your email that you are more interested in how to get a good earth connection.
Here is some information regarding Step & Touch Voltage Hazards:
To get a good earth connection, the best method will be to run dedicated down conductors from the 15th floor to grade-level. The number of down conductors should be based on the shape of your building and the short-circuit line-to-ground fault your site will experience. A good guideline would be to have the down conductors meet lightning protection standards such as the NFPA 780. Once the down conductors reach grade-level, a buried ring surrounding the building and tying all the down conductors together is best. Of course standard 10-ft ground rods at 20-ft intervals bonded to the perimeter ring is ideal. Based on the local soil conditions you may want to consider adding some deep ground wells to lower the overall resistance-to-ground of the system, which will reduce the Ground Potential Rise (GPR) and lower the Hazardous Step & Touch Voltages you will experience during a fault.
Remember that proper bonding of the conduits routing the down conductors to the earth will be critical. Improperly bonded metallic conduit can actually prevent current flow, which can result in equipment failure, fire, serious injury or even death.
You will also need to bond this system into building steel. Ideally, the grounding system will help to protect the actual structure from foundational damage caused by electrical faults. Special attention should be taken to ensure that these important bonds are not only securely made, but made in such a manner as to route electrical faults down the grounding system, and not into the building structure itself.
Lightning is an additional concern for these structures and you will need to consider these effects as well. The large mass of metal on top of the building will make it an exceptional target for lightning strikes.
Because your site is on top of a building, it will be vital that you model this system using a computer simulation. The simulation will ensure that the fault currents will not exceed the capacity of the down conductors to handle the current, preventing accidental burn-outs of the conductor. Additionally, the Step & Touch hazards will need to be carefully analyzed. Only a computer can accurately demonstrate what will be needed at your site. We use and recommend the CDEGS computer simulation software by Safe Engineering Services ltd.
We are of course more than happy to help you out with this difficult project, but if not us, please use someone who has the tools and experience to make your site safe and effective. Please feel free to contact us at anytime.
The Engineering Team at E&S Grounding Solutions