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building on a slope needing lightning protection

Would it be best to place the earth protection (the copper spike) for a building on a slope downhill (A) or uphill (B) from the structure? Assume that lightning is most likely to strike at (C), the hill which is also much higher than the lightning rod on top of the building. Assume the soil is rocky and there is no way to drive the stake in at B far enough to get BELOW the foundation.

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closed as off-topic by John Rennie, stafusa, sammy gerbil, Kyle Kanos, JMac Aug 9 '18 at 13:48

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  • $\begingroup$ A, perhaps, might ensure protection of the lower floors which might not be protected if you were to place it at B. Just as the lightning rod is always placed at the highest point, the earth rod should also be placed at the lowest point. Since the lightning is most likely to strike the hill, the ground rod being the furthest from the strike would actually best protect the building, as any charges in the ground are pulling more charge from further down, and carrying them up, so they are likely to route through the protection system, which is the point. $\endgroup$ – Brian Russell Aug 7 '18 at 20:36
  • $\begingroup$ Reposted to Electrical Engineering. $\endgroup$ – rob Aug 10 '18 at 19:21
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I got this response from Bud at lightning.org:

I can give you my opinion by answering the question “Does it matter?” with a qualified NO. The nature of lightning is that it wants to reach earth ground to neutralize the charge imbalance created in the clouds. Once the lightning reaches earth ground at any point, it spreads out over the surface depending on the electrolytic content of the soil strata until neutralized. I’m not sure it makes a lot of difference, all things being equal, where you place it into the earth, the effect will be the same.

On the other hand the soil itself makes a difference. Lightning will normally seek the lower resistance ground path. If you have a ground in good clay/loam soil with moisture versus a ground driven into rock or sand with little to no moisture content, the lightning will seek the better path to ground. In system design, we normally try to split the lightning along multiple paths to minimize the charge on any one lead and avoid side-flashing to interior systems. I’m aware of a project I worked on many years ago, similar to what you have drawn, with a square building and all four corners grounded – 2 along line B and 2 along line A. Along line B they had blasted out of a cliff for the project making very rocky conditions on this side. Line A was good dirt backfill. Lightning struck this building nearer the B roofline and the majority charge started down those leads, but at basement level it came into close proximity with the electrical grounding inside – flashed through the basement wall and onto the electrical grounding which had been interconnected with the lightning protection grounds on the A building side only. This caused much damage to equipment connected electrically in the basement. The problem was resolved by installing a ground loop, interconnecting all the grounds to maintain equal potential on all downleads and system elements. This is why I think the grounding medium makes more difference than the uphill/downhill side.

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