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From: firstname.lastname@example.org (N. Thornton)
Subject: Re: Ground Resistance monitor
Date: 1 Nov 2002 08:41:30 -0800
References: <3DBA8DBA.6205CB0A@execpc.com> <3DBB2BEF.40C7ED0B@hotmail.com> <3DBD5CEC.86AFAFE3@boeing.com> <3DC01EB4.A18EC28F@boeing.com>
NNTP-Posting-Date: 1 Nov 2002 16:41:30 GMT
Paul Hovnanian wrote in message news:<3DC01EB4.A18EC28F@boeing.com>...
> "N. Thornton" wrote:
> > Paul Hovnanian wrote in message news:<3DBD5CEC.86AFAFE3@boeing.com>...
> > > w_tom wrote:
> > > > To test for earth ground, the measuring system must complete
> > > > a circuit that includes the earth ground rod. If a meter
> > > > connects to an outlet, then where is the other side of a
> > > > complete circuit that passes through earth ground rod? No
> > > > circuit through the earth ground connection means no valid
> > > > test of earth ground.
> > >
> > > This is correct. At a minimum, the ground test will require
> > > two additional ground electrodes to be installed for a test.
> > > A current is run between the ground rod and circuit under
> > > test and a high current test probe. The resulting voltage
> > > drop will be measured between the rod under test and a voltage
> > > probe. The physical layout of the test probes is important and
> > > will require seperations of many feet.
> > If you think laterally you'll notice that the live and earth lines are
> > close to earth potential from an rf point of view: hence one can test
> > without introducng an extra rod.
> That's because there isn't a significant RF source on the circuit.
> Just being at the same potential doesn't mean anything about the
> impedance at that frequency.
I thought it was because of the capacitance to ground in the wiring,
connected appliances, and supply system. I dont know just what that
impedance is, and whether its low enough to be useful.
> > > This sort of test is usually run prior to placing the grounding
> > > system into service. I think that any attempt to build this
> > > capability into the proposed equipment would violate NEC rules
> > > since it would intentionally energize the grounding electrode
> > > and system.
> > I dont see anything wrong with doing that at low voltage, and with non
> > hazardous currents. Now sultanas - they would be unacceptable :)
> > Do you think the rf test might work?
> Possibly, but this could be tricky. At higher frequencies (maybe even
> 10s or 100s of kHz), the reactance may be much larger than the
> resistance of the grounding circuit.
Yup. There might be the question of differentiating between high
ground R and high supply R too.
> Measureing the resistance
> will become much more involved than a simple voltage drop due to
> a given current type problem. Its not impossible, just difficult.
> And RF doesn't eliminate safety concerns that code enforcement
> officials may have with regard to injecting test currents into
> a grounding circuit.
One is allowed to connect the cooker to the earth rod, as well as the
radio antenna circuit: one could perhaps come up with something that
used such small currents?
I have an idea: measure 3 resistances at rf, L-N, N-E, L-E. That
should make it poss to get a much more accurate E resistance reading.
What contribution would inductance make at 100kHz? Would it be large
enough and variable enough to prevent a pass or fail reading?
I'm not saying I'm convinced, but it sounds like a maybe. If it did
work it would be primarily a chip and a small mains dropping resistor
in a box you could plug into the socket outlet. Is there enough call
for these things to warrant developing one?
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