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I've always read about step potential in terms of substations and high voltage powerlines, where we are talking in the kilovolt+ range.

What if there is a manhole cover and a mains(240V/400V) wire touching it and making it live? Given that the manhole is actually partially in the ground, would there be a danger if you are walking and step with one foot on the ground, other foot on the manhole cover? My assumption is since the manhole is actually lying on the ground, it would lower the potential that would pass through you. Am I thinking in the wrong way?

alex.b.bg
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Where is two different things. Electrical ground and earth surface aka ground is not the same things. But if manhole is grounded electrically, ground fault protection should work. Otherwise electrical potential spreads over surface depends on soil conditions. And step voltage is potential difference, so the value depends of length of step and potential spreading. But if manhole cover lying on some concrete structure, not electrically grounded and live wire touch it you may get full voltage between cover and soil.

user263983
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  • So the way I understand it, in case there is no earthing or grounding of the cover, it's the same as "touch potential" and it doesn't matter that the cover is sitting on the ground. Now the way I look at it, it's a rather stupid question to ask, but thanks for the answer :) – alex.b.bg Feb 16 '21 at 02:33
  • Yes. Keyword is "ground". Different meaning in different cases. – user263983 Feb 16 '21 at 11:58
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There is actually a technical term for what you are describing. It's called "step voltage".

Ground has actually a finite impedance so if you put some kV on the ground (say, a fallen pole) even without a manhole cover, there is a potential distribution around the dispersion point that starts at full voltage and goes toward zero as you are going away from it. Think of many many resistors in series, each of one is, like, a grain of sand.

This can actually radiate for tens of meters. If you walk on it you are shunting some of these with your body (two feet at something like a meter of distance) and get a nasty shock. Historical trivia: with the first distribution systems horses fell dead on the road (an horse is a lot longer so it shunts more potential).

To counteract this now we have 1) safety relays and 2) huge grounded steelworks beneath high voltage structures to ensure that fault current can do damage.

Lorenzo Marcantonio
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  • I indeed have read about step potential/voltage, but the examples have always been in the many-KV range, so I was wondering if such a danger would exist with a low, line-level voltage. But as user263983 mentioned, in this case, it doesn't look like a step potential scenario - rather, it's just a metal object that has become live and you touch it with one leg, and your other leg is stepping on the ground, so it should be the same as touching a live wire... – alex.b.bg Feb 17 '21 at 11:54
  • Well, with a 400VAC feed step voltage is rarely an issue. It is difficult to estimate a manhole impedance to ground but if it's too high you could get a dangerous touch voltage too (50VAC is considered dangerous by standards, but you need to be very unlucky to get hurt from that) – Lorenzo Marcantonio Feb 17 '21 at 11:58