Thank you for your question regarding GPR and GPD acceptable limits. It is our pleasure to help.
There are actually no specified limits to either of these electrical phenomena’s, as they are simply indicative factors for other specifications. Let us explain:
The Ground Potential Rise (GPR) is a factor that describes the maximum voltage that will be seen during an electrical fault on a give grounding system. You 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. If your building rises to 500 volts or 50,000 volts, it can be either good or bad, depending on the current flow at critical points. Here is some more information on GPRs:
The Ground Potential Difference (GPD) tells us that during a GPR event, the grounding system will have voltages that vary by a certain amount. For example, if your building has a 50,000 volt GPR, ranging from 49,900 volts to 50,000 volts, then your GPD is 100 volts. If your building has a 500 volt GPR, ranging from 100 volts to 500 volts, than you have a 400 volt GPD. This means that from one end of your grounding grid to the other you will see a 400 volt difference in potential. Which is worse? The 100 volt GPD or the 400 volt GPD? Actually, we still don’t know.
If the building with the 400 volt GPD will only push a few amps across any given point, it may be perfectly fine. On the other hand, the building with the 100 volt GPD might push hundreds of amps across the grid, and that could be very bad indeed. When dealing with human safety, we calculate what is called “Step & Touch Voltages” to determine if the amount of current that can enter a person will be enough to cause the human heart to fibrillate or not. When protecting equipment, we must look at manufacturers recommendations for maximum voltage and current levels. But in almost all cases, if it is safe for people (Step & Touch) then it is safe for the equipment too.
Now how do you provide proper protection? It is a combination of removing electrical energy from the grid (electrodes), balancing electrical energy (grids), bringing electrical energy closer to people and equipment in certain cases (bonding), and taking it away in other cases (insulation). Frankly, it can get very complex. Much more than we could ever really cover in a simple blog.
Please feel free to give us a call 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/rutlo/3107300716/sizes/l/in/photostream/