TT:
This phenonmenon, known in the electric utility industry as step
potential, is, indeed, real and potentially lethal. In designing our
electrical substations and power plants, we electrical engineers at the
power company included a ground ring (typically 4/0 galvanized steel plus
driven ground/earthing rods) around the inside and outside of all our
facilities (generating stations as well as substations.) We then bonded all
structures, including the chain link fence, posts and gates, to this ring.
The reason is that, during electrical faults including lightning
strikes, significant current could flow from the structures into the earth.
If there were no method of reducing the resulting rise in earth potential,
the voltage gradient (differential) away from the facility to distant
earth/ground would increase to very high levels. In some instances, based
on the actual earth resistivity, that potential difference might reach
lethal levels over a distance as short as a couple of feet - a typical step
length for the average person.
To protect our maintenance personnel (inside the facilities) and the
public (outside the factilties) during a ground fault event, we installed
the above-mentioned ground rings. These served to reduce the step potential
to below-lethal levels.
73 de
Gene Smar AD3F
-----Original Message-----
From: Dick Flanagan <dick@twohams.com>
To: towertalk@contesting.com <towertalk@contesting.com>
To: <towertalk@contesting.com>
Date: Saturday, March 15, 2003 12:55 PM
Subject: Re: [TowerTalk] Shack ground disconnect?
>I don't know whether the cow/lightning story is real, but the premise is
>valid and well-known. There have been documented cases where horse-mounted
>police had their horses die under them as they approached an electrical
>source that was shorted to ground. The electrical potential radiated out
>through the soil from the power source, high near the source and lowering
>as the distance increased. Since a horse's front and rear feet are
>relatively far apart, as the horses approached the electrical source, the
>potential under their front and rear feet was sufficiently different to
>cause a current flow through their body that killed them. In one incident,
>two horses died before someone realized what was happening.
>
>The whole idea of a common point ground is to remove this potential (no pun
>intended).
>
>73, Dick
>
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