amdx wrote:


I was in my workshed which is about 60 ft from the house drilling some
aluminum. I got a faint tingle in my hand, I touched the case of my
drill a couple times and didn't feel any thing, so I started to drill
again. I noticed the tingle again, I went through this a few times until
I decided there's a problem. The tingle was at a tiny red spot on my
finger, I later removed a small metalic sliver. If not for the sliver I
probably would not have noticed the missing ground, for I don't know how
long.

Depending on the grounding arrangements, this can sometimes still happen
even if there isn't a complete disconnection or any serious leakage
current.

The sensation of electrical shock is entirely dependent on the current
density in the susceptible tissues, rather than the voltage applied.
Human tissues are very conductive, but dry skin usually acts as a
partial insulator so low voltages don't create enough current to cause
any sensation.

With a sliver of metal puncturing the skin and leading directly into the
underlying tissue, probably quite close to nerve endings, a very low
voltage would develop enough current to be easily detectable. Tests
have suggested that A.C. voltages below the 2v level can be detected by
the tongue when the feet are in a wet, conductive environment.

This is a particular problem where the neutral and earth conductors are
bonded ("P.M.E." in the UK) because the small voltage dropped along the
neutral will apear on any bonded "earthed" metalwork. In farms and
other wet environments, this can lead to animals receiving shocks from
licking handrails, feeding troughs - and from milking machines through
the milk lines directly to their teats.


--
~ Adrian Tuddenham ~
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