wolfgang@haddock.UUCP (06/24/87)
Here is a question that I never saw discussed anywhere before:
What is the resistance of earth ground? Let's say we have two
good connections to the earth, say two Chevettes :-), dropped into the
ocean, one into the Pacific and the other Atlantic. With an ohm-meter
connected between them, what kind of resistance can I expect to see?
(I can probably even donate 1/2 of the electrodes for the
experiment, but that is another story ... ).
-wolfgang
--
Wolfgang Rupprecht
{decvax!cca|yale|ihnp4|cbosgd|bbncca|harvard}!ima!haddock!wolfgangdrw@cullvax.UUCP (Dale Worley) (06/25/87)
wolfgang@haddock.UUCP (Wolfgang Rupprecht) writes: > What is the resistance of earth ground? Let's say we have two > good connections to the earth, say two Chevettes :-), dropped into the > ocean, one into the Pacific and the other Atlantic. With an ohm-meter > connected between them, what kind of resistance can I expect to see? One thing you run into is that with a point contact to a 3D medium, resistance of the contact is dominated by what happens near the contact: Consider a spherical shell A of finite thickness around the contact, and another B with double the radius and thickness. The contribution of A to the contact resistance is twice that of B. If the contact is a perfect point, the contact resistance is infinite! Getting back to the example, the Chevettes make pretty good contact with a lot of ocean, so the resistance would probably be damn close to zero. Dale -- Dale Worley Cullinet Software ARPA: cullvax!drw@eddie.mit.edu UUCP: ...!seismo!harvard!mit-eddie!cullvax!drw ************** REPLACE THIS LINE WITH ONE EXACTLY LIKE IT **************
g-rh@cca.CCA.COM (Richard Harter) (06/27/87)
In article <616@haddock.UUCP> wolfgang@haddock.ISC.COM.UUCP (Wolfgang Rupprecht) writes: >Here is a question that I never saw discussed anywhere before: > > What is the resistance of earth ground? Let's say we have two >good connections to the earth, say two Chevettes :-), dropped into the >ocean, one into the Pacific and the other Atlantic. With an ohm-meter >connected between them, what kind of resistance can I expect to see? > You will measure zero resistence. Where do you think the term "ground" came from? Actually you will measure some impedance -- however this is actually an electrode impedance. The physics of the situation are classical and elementary. If you put an electrode in the Earth the current spreads out spherically with the potential falling off 1/r^2 if the Earth is homogenous. With two electrodes you can calculate the potential function for each one and combine them additively. The case where the Earth is not homogenous is of considerable practical interest. Earth resistivity measurement is a standard technique in Geophysical exploration. What you do is stick a pair of electrodes in the Earth and measure potential differences on the surface in a lot if places. You then attempt to infer the underlying resistivity structure. The general idea is that an underlying low resistivity body (e.g. a metal ore deposit) will distort the observed surface potential function because the body sucks up the current. The problem of calculating surface potentials for a given model is an interesting one, albeit one of simple classical physics. I was working in this area ~15 years ago. At that time one and two dimentsional models were tractable but there were no good techniques for three dimensional model, other than simple special cases. The inversion problem (deducing the underlying structure from observed data) is mathematically ill conditioned but one can do a good deal with it. Other techniques include magneto-telluric measurements and gravity logging. The oil industry mostly uses acoustic methods. It's been quite some time and I am not current, so I expect the techniques and geophysics today are a great deal more sophisticated. -- Richard Harter, SMDS Inc. [Disclaimers not permitted by company S!E#I1#I
jl@fornax.uucp (JL) (06/28/87)
> wolfgang@haddock.UUCP (Wolfgang Rupprecht) writes: > > What is the resistance of earth ground? Let's say we have two > > good connections to the earth, say two Chevettes :-), dropped into the > > ocean, one into the Pacific and the other Atlantic. With an ohm-meter > > connected between them, what kind of resistance can I expect to see? ...explanation... > Getting back to the example, the Chevettes make pretty good contact > with a lot of ocean, so the resistance would probably be damn close to > zero. > > Dale Oh, my. 1.) No matter HOW good the local contact, you are still limited by the intervening material, which has a VERY low resistance. Even though there is a LOT of the material, the net result is a VERY VERY HIGH resistance. We are talking >> Megaohms here! 2.) If this were not true, you could well expect to electrocute a fair portion of the world any time a power-line came into good contact with the ground or water (which happens fairly often in storms that blow power lines down,...). -- Jay-El
g-rh@cca.UUCP (06/29/87)
In article <327@fornax.uucp> jl@fornax.uucp (JL) writes: >> wolfgang@haddock.UUCP (Wolfgang Rupprecht) writes: >> > What is the resistance of earth ground? Let's say we have two >> > good connections to the earth, say two Chevettes :-), dropped into the >> > ocean, one into the Pacific and the other Atlantic. With an ohm-meter >> > connected between them, what kind of resistance can I expect to see? > ...explanation... >1.) No matter HOW good the local contact, you are still limited > by the intervening material, which has a VERY low resistance. > Even though there is a LOT of the material, the net result is > a VERY VERY HIGH resistance. We are talking >> Megaohms here! Wrongo. We are talking a very low resistance, namely zero. In the old days they used a single wire for telegraph lines, relying on the Earth to supply a return path. Where do you think the term ground came from, anyway. Although there is a lot of material, current flow through it is in parallel (in effect.) The surface through which the current is spreading is a hemisphere. What you actually will measure is the effective electrode impedance which is quite another matter. >2.) If this were not true, you could well expect to electrocute > a fair portion of the world any time a power-line came into > good contact with the ground or water (which happens fairly > often in storms that blow power lines down,...). Again wrong. When a source of current in inserted into the Earth the current density drops off as 1/R^2 (in a homogenous Earth). When a power line comes in good contact with the Earth the current density (and potential difference) is negligible except when you are close to the current source. -- Richard Harter, SMDS Inc. [Disclaimers not permitted by company policy.] [I set company policy.]
jeffw@midas.TEK.COM (Jeff Winslow) (06/30/87)
Oh, my. Maybe I'm just dense, but... In article <327@fornax.uucp> jl@fornax.uucp (JL) writes: >1.) No matter HOW good the local contact, you are still limited > by the intervening material, which has a VERY low resistance. ^^^ > Even though there is a LOT of the material, the net result is > a VERY VERY HIGH resistance. We are talking >> Megaohms here! ^^^^ I'm confused. Are you saying it's high, or low? >2.) If this were not true, you could well expect to electrocute > a fair portion of the world any time a power-line came into > good contact with the ground or water (which happens fairly > often in storms that blow power lines down,...). Not true. All that's required to prevent electrocution is that the voltage drop across any two points on the ground to be low enough that it doesn't put a lethal amount of current through you if you touch them. Two of many scenarios to satisfy this are: 1. The ground resistance is so low that most of the voltage is dropped in the shorted line. 2. Most of the current flows below the surface (in the water table, for instance) so that the current density at the surface, away from the point of short, is very low. Jeff Winslow
lorraine@batcomputer.tn.cornell.edu (Peter Lorraine) (06/30/87)
In a reply to the original question about the resistivity of the
earth someone mentioned that enormous potentials exist across continental
distances. What is the mechanism that generates these potentials?
Is there a return current balancing the generation mechanism? I've
heard that your typical (whatever a high school geography teacher meant
10 or so years back) lightning flash carries several coulombs. The
electrostatic energy in a capacitor is 0.5CV**2. How much energy does
the earth churn out each day moving charge around?
As long as my mind wanders, can anyone think of a way to collect
some of this energy? How about sticking superconducting rods from the
ground up two or so miles and collecting current on a continuous basis?
This is probably not very practical for some fairly obvious reason
(other than the engineering ones :).
Thank you,
Peter Lorraine (lorraine @ tcgould.tn.cornell.edu)cmcmanis%pepper@Sun.COM (Chuck McManis) (06/30/87)
Well to put in my two cents worth, lets look at the problem again..
First, the original poster stipulated electrodes that consisted of
two chevettes, one dropped into the Pacific ocean and one dropped
into the Atlantic ocean.
Right off the bat we can determine that little or no current will
flow through the continental United states because the oceans
are a pretty decent electrolyte and connected at both the arctic
ocean and the Cape of Good Hope. This circuit can be modelled as :
__________________________
/ U.S.of A. \
Chevette 'A' o-----+----------/\/\/\------------+----o Chevette 'B'
\__________________________/
As you can see, no current flows in the US because it is 'shorted' by
the oceans.
Secondly assuming you dropped the chevettes into the dirt somewhere
and they did manage to set up a current flow, someone else claimed
'if the resistance were zero every time a powerline fell down it
would electrocute everybody.' or something to that effect. Which
I don't believe primarily because I have been within 100 yds of a
downed high voltage wire and not suffered any ill effects. However
in the interest of science I offer the following model :
| /
\ <---- You, and open /
/ circuit resistor. / <--The power line
\ /
/ /
_____________|_______________________________/______
__|__ __|__ ^ __|__
/ / / / / / | / / /
|
The Ground
As you can see, no current flows in you because you develop no potential.
(no this isn't a slur on ones abilities!)
These are of course the simplest cases and there are lots of other things
that one should consider, such as 'is the voltage varying with time, and
if so why didn't you use an impedence model?' or 'What about the skin
effect of charged bodies, and the potential for coronal discharge?'
These we leave as an exercise for the reader.
--Chuck McManis
uucp: {anywhere}!sun!cmcmanis BIX: cmcmanis ARPAnet: cmcmanis@sun.com
These opinions are my own and no one elses, but you knew that didn't you.