5 ohms Ground Resistance in Commercial Buildings
5 ohms Ground Resistance in Commercial Buildings
(OP)
Hi all,
Ground Potential Rise (GPR) = Ig * Rg
For 500 kVA, 13800-480/277V, Dyn effectively grounded at Z=5%, Ig = 12kA taking an infinite primary source.
The contractors are providing a loop grounding grid buried around the building and aiming to obtain 5 ohms maximum.
Based on 5 ohms, GPR = 12,000 * 5 = 60 kV
This is quite very high.
IEEE 80 provides formula to calculate the tolerable touch and step potentials....however, the design GPR is very high. When asked, the contractor informed us that it has been their practice to get Rg = 5 ohms and call it a day.
The Code is even specifying for a higher one ( I think 25 ohms).
Is this a valid design application? Is the GPR neglected (regardless of Ig) in the building design?
Ground Potential Rise (GPR) = Ig * Rg
For 500 kVA, 13800-480/277V, Dyn effectively grounded at Z=5%, Ig = 12kA taking an infinite primary source.
The contractors are providing a loop grounding grid buried around the building and aiming to obtain 5 ohms maximum.
Based on 5 ohms, GPR = 12,000 * 5 = 60 kV
This is quite very high.
IEEE 80 provides formula to calculate the tolerable touch and step potentials....however, the design GPR is very high. When asked, the contractor informed us that it has been their practice to get Rg = 5 ohms and call it a day.
The Code is even specifying for a higher one ( I think 25 ohms).
Is this a valid design application? Is the GPR neglected (regardless of Ig) in the building design?






RE: 5 ohms Ground Resistance in Commercial Buildings
RE: 5 ohms Ground Resistance in Commercial Buildings
From the rule of the thumb premise that the bolted line to ground fault at the transformer secondary terminals = bolted 3 phase fault, this is where I took 12 kA as a line-to-ground fault. The total ground resistance Rg = 5 ohms.
I assume that the 12 kA line-to-ground fault is injected into the ground grid loop having a total Rg = 5 ohms.
Are you implying that I should treat the ground grid resistance as an impedance that limits the line-to-ground fault current?
RE: 5 ohms Ground Resistance in Commercial Buildings
Very little fault current will flow into the ground grid. The current that does will be no more than 277V/5ohm = 55A.
RE: 5 ohms Ground Resistance in Commercial Buildings
RE: 5 ohms Ground Resistance in Commercial Buildings
The neutral of the transformer secondary is directly connected to the ground grid loop. Hence, if there is a local line to ground fault at the transformer secondary terminals (or near to it). The line-ground fault current will get injected first to the ground and up to the transformer neutral conductor.
If there is a line-to-ground fault at the transformer secondary terminals, then the only limiting impedance would be the transfomer winding impedance + neutral conductor impedance (which is relatively short). I agree that the fault current will not be as high as 12 kA owing to the positive + negative + zero sequence impedances.
But looking into IEEE 80, the allowable touch and step potentials are calculated based on 50 or 70 kg person with soil/surface resistivities included. Then the GPR is compared against them. In the GPR calculations, the line-ground fault Ig is calculated based on the three sequence impedances. Since our LV system is having a neutral which is effectivley grounded, the Rg = 5 to 25 ohms seems to be high with respect to what the GPR and the line-ground current would be. I apologize for insisting for now and hope I get clarified even further as I am following IEEE 80 at the moment and looking at the building for applicability.
Thank you for your patience.
RE: 5 ohms Ground Resistance in Commercial Buildings
RE: 5 ohms Ground Resistance in Commercial Buildings
The 12kA LV fault level isn't of huge concern for the GPR, you need to consider the available fault from the HV system. If you have an impedance-earthed system then the GPR is likely to be quite low. Determine your available fault current and apply Ohm's law to see what happens to the GPR. Remember that you can't have GPR greater than the source voltage. With a typical resistance-earthed system of, say, 400A your GPR would be limited to 2000V through a 5Ω resistance IF the whole fault current flowed to earth and none returned via the cable shield or armour.
RE: 5 ohms Ground Resistance in Commercial Buildings
ThePunisher ,
Ground Potential Rise (GPR) = Ig * Rg <== OK
Your interpretation of Ig is wrong.
See examples attached and which case is applied in your system