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From: firstname.lastname@example.org (N. Thornton)
Subject: Re: Ground Resistance monitor
Date: 2 Nov 2002 06:51:25 -0800
References: <3DBA8DBA.6205CB0A@execpc.com> <3DBB2BEF.40C7ED0B@hotmail.com> <3DBD5CEC.86AFAFE3@boeing.com> <3DC01EB4.A18EC28F@boeing.com> <3DC2E3F3.150B675D@boeing.com>
NNTP-Posting-Date: 2 Nov 2002 14:51:25 GMT
Paul Hovnanian wrote in message news:<3DC2E3F3.150B675D@boeing.com>...
> "N. Thornton" wrote:
> > 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?
> In both cases, I'm assuming that you are referring to the equipment
> Equipment 'gounding' conductors are not current-carrying.
In reality they are, thats why I mentioned cookers. Cookers are famous
for earth leakage. Ground / earth wires are current carrying, just not
supply carrying. In every installation there is capacitance between E
and L and N, and the inevitable result is current flowing in the earth
rod. Only when the L/N current imbalance goes beyond a predetermined
threshold is the supply cut off by the ELCB/RCD/GFCI/any other name.
I doubt I've ever seen a supply installation with no earth current.
As far as code goes, small L-E and N-E capacitors are perfectly legal
in plug in equipment.
> Note: the
> 'grounding' conductor is not the 'grounded' conductor, which in the USA
> is the neutral.
> > 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.
> For the above measurements, what are you considering to be 'E'?
The mains earth wire, or ground as you call it.
> the equipment under consideration in the original post, the connection
> to 'earth potential' is only through the (unknown) resistance through
> the service entrance conductors and the grounding system to true earth.
> Without some external reference to true earth, you are essentially
> trying to measure a resistor's value with one probe from an ohmmeter.
What I proposed was:
Each of the 3 conductors, L N and E have an impedance to true earth.
That impedance will be close to identical for L - true E and N - true
E. Measuring L-E N-E and L-N will enable us to calculate the impedance
of the E wire to true earth.
> > What contribution would inductance make at 100kHz? Would it be large
> > enough and variable enough to prevent a pass or fail reading?
> It may (or may not) work. It depends on how you go about differentiating
> the reactive components of the impedance from the resistive. Its not
> impossible. But I'm not certain that using RF will get around the need
> to inject a test current into the grounding electrode (a code compliance
> and safety problem) as well as a need to create a loop through the
> electrode under test, through the earth and then through some other
> test electrode back to the test equipment.
I hope I've explained that one a bit more clearly now.
> > 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?
> I don't think that will do the trick.
Now it is explained more clearly...?
> There is currently (no pun intended) test equipment on the market
> for measuring grounding electrode (and system) resistances. At tis time,
> it doesn't appear that inspection authorities are creating the demand
> for such monitoring, either for new construction or during the life of
> most installations to encourage much further development.
> In other words, what's out there now seems to do the trick.
Yes, but at what price, and with what (expert) labour? What I propose,
if it works, would do it for peanuts, with no disruption to service
and continuous monitoring. I still dont know if there's a demand for
that, I'm not a power engineer.
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