Melissa tells us: I have two transformers next to an electrical building. A 13.8-4160 and a 4160-480. MV transformer is resistance grounded to 100A. LV is solidly grounded. My engineer is telling me I need a beefy grid around both and it should be modeled as a substation due to the fact a transformer winding may become shorted to the transformer enclosure creating a SLG fault. He says if that happens, 34.4kA will go into the ground and this will create harmful step and touch potentials in about a 100-150ft diameter around the transformer. I don’t know much about this sort of thing, but I know typically, a very simple ground system is put into place, and not one with many many rods and runs of conductor. Can you tell me, not knowing more details, if his argument holds water and I should consider his design? I am told the soil is moderately conductive. 3350 ohm-cm to 6250 ohm-cm.
Thank you for your question regarding step and touch voltages around your transformers. It is our pleasure to help.
Yes, your engineer is correct. A Single Line to Ground (SLG) fault at your transformer will create a significant Ground Potential Rise (GPR) which could in fact result in both step and touch voltage hazards to the public. 29 CFR 1910.269 mandates that these hazards be mitigated to protect personnel working near the transformers, and/or the public.
Your conductive soil can be beneficial, but it can also be harmful, given certain conditions. If the conductivity gets worse with depth, the electrical energy is going to want to stay near the surface of the earth, creating a larger step voltage concern.
You will need Soil resistivity data, and electrical fault data to analyze these hazards properly. We recommend a series of Wenner 4-point soil resistivity tests with spacing’s at least as large as your transformer area. For electrical data, you will need the SLG fault current, the X/R ratio (or zero sequence impedance), and the fault clearing time.
We are of course more than happy to help you with this important project. But if not us, please get someone with the proper simulation software and expertise to help you design a safe and effective grid that is in compliance with 29CFR1910.269 requirements.
Please feel free to call us at 310-318-7151 if you should have any further questions.
The Engineering Team at E&S Grounding Solutions
Photo credit: E&S Grounding Solutions