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Strike Rails:... power line ground wires
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From: Jim Lux [SMTP:jimlux-at-earthlink-dot-net]
Sent: Sunday, March 15, 1998 1:08 PM
To: Tesla List
Subject: Re: Strike Rails:... power line ground wires
> From: Bert Hickman [SMTP:bert.hickman-at-aquila-dot-com]
> Sent: Saturday, March 14, 1998 8:23 PM
> To: Tesla List
> Subject: Re: Strike Rails: The Big Question
>
> Jeff and all,
>
> Guys in the power industry do get a kick out of Tesla Coils! However,
> while that was an interesting story from the guy at the power company,
> your original supposition was indeed correct - the REAL reason the wires
> are there IS soley to provide lightning protection. Putting a grounded
> wire above the lines has virtually no impact on the E-field between the
> high tension lines and the ground immediately below.
Well... sort of.
The grounded wire also reduces induced charges resulting from a charged
body above the transmission line (e.g. a charged cloud). Under a
thunderstorm, you may get a field of several kilovolts per meter, which
works out to quite a few kV when you are 30 or so meters off the ground.
Without the ground wire, which DOES perturb the E field under it with
respect to the relatively low current cloud source, the charge would cause
a current flow or induced voltage on the power line. Power lines are
physically large (i.e. long) so they can collect a lot of power.
The "Amateur Scientist" once had an article talking about an electrostatic
motor that ran off the E field of the earth from a long wire.
Of course, the ground wire also serves as a "target" for the lightning,
providing a reasonably low impedance to ground.
There are also some issues with lightning and switching induced surges
which are influenced by the parallel C and series L of the transmission
lines (and ground wires).
Typical impedance of a transmission line (aerial) is 0.5 Ohms/km (Series L)
and 300 Kohms/km (Parallel C) for 3phase at 60 Hz. Obviously, the parallel
impedance is inversely proportional to the length (twice the length has
twice the capacitance has half the impedance). The series L is proportional
to the length. A 1000 km line (the length of California) would have a
series L of 500 ohms and a parallel C of 300 ohms.