Subject: Re earth cables (fwd)
Date: Fri, 28 Aug 1998 083338 -0500 (CDT)
From: "Roy L. Beavers" <rbeavers@mail.llion.org>
To: emfguru@hotmail.com
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---------- Forwarded message ----------
Date: Fri, 28 Aug 1998 15:07:30 +0200
From: Clas Tegenfeldt
To: "Roy L. Beavers"
Cc: Ingrid.High@dnv.com, aphilips@gn.apc.org, tegen@bemi.se
Subject: Re: earth cables (fwd)
About underground cables instead of overhead wires.
Be very careful about sum currents when discussing overhead lines versus
cables! Sum currents gives off magnetic fields both on overhead power wires
as well as buried cables!!!
What happens is that the wires are brought closer toghether, instead of meters
apart the wires will lie centimeters apart. It is the /distance/ between the
wires that reduces the fields dramatically, it doesn´t matter if the wires
are in air, in rubber, or buried into the ground.
It doesn´t really matter if the cable is hung between poles or
buried into the ground! It is the change to cable that makes the difference.
Everything in this email assumes a three phase system!
The resulting magnetic field depends on:
1. sum current over the whole set of wires
2. the distance between the wires
3. balance between phases
The sum current gives off a magnetic field that falls of
proportionally against distance from the power line.
The distance between the wires in combination with the
phase current balance gives a magnetic field that falls
of more or less rapidly with distance, often about square law.
The resulting electric field will be low for the cable since one almost
always have a metallic protective shield around the live wires. A
secondary effect is the shielding of wet soil, but this is only valid
when resistivity is low, for dry sand the electric field around an
unshielded cable will be greater. The electric field is also reduced by
the close distance between the wires.
VERY IMPORTANT THING TO NOTICE: IF the power line has
a net sum current, the cable will also have that sum current and thus a
magnetic field directly proportional to the sum current (as for a single
wire) thus NOT providing the drastic decrease in magnetic field as wanted.
In fact the situation may not improve significantly at all...
On the other hand, if one makes sure that an overhead power line has no
net sum current (say less than one ampere) and a good balance between
phases the magnetic field can be lowered for a very low cost. The next step
is the cable, but only after the first step is concluded, and only to a
high cost.
At 06:28 1998-08-28 -0500, you wrote:
>---------- Forwarded message ----------
>Date: Fri, 28 Aug 1998 11:16:58
>From: Alasdair Philips
>To: "Roy L. Beavers"
>Cc: "High, Ingrid"
>Subject: Re: earth cables (fwd)
>
>Ingrid
>The magnetic field from a simple single-circuit unbalanced overhead line
>(as in your friends case) falls off approximately linearly with distance as
>it would from a single wire (i.e. Flux = const*current/distance to wire).
>From a reasonable balanced double-circuit reverse phased overhead line it
>falls off approx. inversely with dist.squared.
...snipp...
Clas Tegenfeldt ,,,
(o o)
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Reprinted with permission of Roy Beavers, http://www.feb.se/EMF-L/EMF-L.html