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Posted by Don Foreman on May 2, 2008, 12:53 pm
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On Fri, 02 May 2008 16:56:08 +0700, Bruce in Bangkok
>>
>>I think Martin is generalizing incorrectly about the skin effect, which
>>applies only to high frequency AC. It's a phenomenon that becomes important
>>at radio frequencies. At DC, or at low frequencies, conduction is uniform,
>>or nearly so, across the whole section of a conductor.
>>
>>Of course, copper has something like 10 times the conductivity of steel, so
>>you have to take the thickness of the copper cladding into account, too.
>
>
>Most of the copper coated ground rods I see are electroplated and the
>plating is maybe 0.0005 thick. I don't believe it carries much
>current.
Lightning events do have high-frequency content so skin effect does
come into play. But you're right, .0005" of copper won't make much
difference.
Consider that even steel has far higher conductivity than the earth
into which it's driven. Making a ground rod more conductive than steel
would serve no useful purpose. A better course for low-Z ground is to
use more ground rods, because surface area is what determines
effectiveness.
The diameter can be smaller and resistance can be higher than the
cables coming out of a welder because it only handles high current
during very short transient events, and even tens of volts of drop are
acceptable during lightning transients.
Residential utility ground current should be very low, well under an
amp and more like milliamps. There is a substantial neutral wire
running back to the pole transformer. The reason for a groud
reference is to prevent the system from attaining a common-mode
voltage significantly above ground potential --e.g., 120 volts line
to neutral but neutral is 400 volts above ground due to static buildup
somewhere so the terminal voltages w.r.t. gnd are 400 and 520. Zap!
Copper is a better choice for corrosion protection because zinc is
very vulnerable to even weak acids. Slightly acidic soil would eat
the zinc off a galvanized steel rod rather quickly.
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Posted by Leo Lichtman on May 2, 2008, 3:10 pm
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You could get a better conduction path by stringing a bare copper wire (say
8 ga.) next to the steel rod, than by plating the rod. 'course, that
doesn't provide any corrosion protection, but my point is that doesn't do
anything for you electrically. As far as a lightning strike is concerned,
the current is enormous, producing a voltage drop so high that lightning may
even jump LATERALLY from a tree trunk to a person standing under the tree.
In order to get to the ground rod, the current will have traveled from the
roof somewhere, probably along several paths.
I believe that the ground rod is mainly there to establish that the house
neutral is not floating, as Don Foreman said (more eloquently.)
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Posted by dcaster@krl.org on May 2, 2008, 2:47 pm
Please log in for more thread options > I think Martin is generalizing incorrectly about the skin effect, which
> applies only to high frequency AC. It's a phenomenon that becomes importan=
t
> at radio frequencies. At DC, or at low frequencies, conduction is uniform,=
> or nearly so, across the whole section of a conductor.
>
>
> --
> Ed Huntress- Hide quoted text -
>
Skin effect is important at 60 hz. too. I forget the exact figure but
as I remember at 60 hz it is not worth making conductors over 4 inch
in diameter.
Recently saw a house with lightning rods where the wire to ground was
Litz wire.
Dan
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Posted by Martin H. Eastburn on May 2, 2008, 9:38 pm
Please log in for more thread options Actually Ed is not far off but missed himself.
Current density for all points on a cross section of A is j=i/A
However since the resistivity of material varies greatly, the
current flows in the copper.
The skin is the thin plating on the steel rod that every house
has pounded into the ground near the power meter.
What messes up the even density in a solid copper rod (we don't have)
is that of the electromagnetic field that drives the current outward.
So the real density is loaded at the outer edges and this process really
takes place in massive form in RF.
TV's are fed by an antenna. The antenna is fed by a air core coax. The
central wire is a tube. No reason for using copper that electrons won't
go. This makes the coax a little more flexible.
28 cycles that was intended to be used on Navy ships - with large iron
transformers acting as ballast actually tunnels to the center of the
conductor. An arm placed across two power posts will have the marrow
burnt out. The Navy aborted the experiments.
Martin
Martin H. Eastburn
@ home at Lions' Lair with our computer lionslair at consolidated dot net
TSRA, Endowed; NRA LOH & Patron Member, Golden Eagle, Patriot's Medal.
NRA Second Amendment Task Force Charter Founder
IHMSA and NRA Metallic Silhouette maker & member.
http://lufkinced.com/
Ed Huntress wrote:
>> On Thu, 01 May 2008 20:42:06 -0500, "Martin H. Eastburn"
>>
>>> The copper clad can carry hundreds of amps when asked upon. The steel
>>> can't.
>>> Electrons flow on the skin. A clamp on the outside puts or gets electrons
>>> to or from the ground.
>> Just a quick question here. If the extremely thin electroplated
>> coating on a steel ground rod will carry hundreds of amps, as you say,
>> hows come my 250 amp welder has them big thick cables coming out the
>> front, there?
>>
>>
>> Bruce-in-Bangkok
>> (correct Address is bpaige125atgmaildotcom)
>
> I think Martin is generalizing incorrectly about the skin effect, which
> applies only to high frequency AC. It's a phenomenon that becomes important
> at radio frequencies. At DC, or at low frequencies, conduction is uniform,
> or nearly so, across the whole section of a conductor.
>
> Of course, copper has something like 10 times the conductivity of steel, so
> you have to take the thickness of the copper cladding into account, too.
>
> --
> Ed Huntress
>
>
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Posted by Randy on May 2, 2008, 10:13 am
Please log in for more thread options On Fri, 02 May 2008 13:03:02 +0700, Bruce in Bangkok
>On Thu, 01 May 2008 20:42:06 -0500, "Martin H. Eastburn"
>
>>The copper clad can carry hundreds of amps when asked upon. The steel can't.
>>Electrons flow on the skin. A clamp on the outside puts or gets electrons
>>to or from the ground.
>
>Just a quick question here. If the extremely thin electroplated
>coating on a steel ground rod will carry hundreds of amps, as you say,
>hows come my 250 amp welder has them big thick cables coming out the
>front, there?
>
>
That ground rod is only needed to carry that hundreds of amps for a
split second*, until the breaker trips or the lighting strike is over.
*Or a few seconds as a worst case senerio.
Thank You,
Randy
Remove 333 from email address to reply.
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>>I think Martin is generalizing incorrectly about the skin effect, which
>>applies only to high frequency AC. It's a phenomenon that becomes important
>>at radio frequencies. At DC, or at low frequencies, conduction is uniform,
>>or nearly so, across the whole section of a conductor.
>>
>>Of course, copper has something like 10 times the conductivity of steel, so
>>you have to take the thickness of the copper cladding into account, too.