You can't determine ground resistance with a single measurement on a
single VOM. Here's why:
"Ground" is the potential of the earth in bulk. When you want to
connect to "ground", you use a grounding electrode (e.g. a ground
rod), but since you aren't connecting to all of the earth, you have a
resistance between your grounding electrode and "ground". This is the
resistance of the earth in the near vicinity of your grounding
[For the case of a ground rod, think of the earth around the rod as
divided into concentric cylindrical shells of fixed thickness and
increasing diameter. Then the resistance of the individual shells
goes to zero as the diameter gets large. When you hit zero (or close
enough), you've hit "ground". The sum of the resistances of the
shells is the resistance of your grounding electrode.]
So how do you measure the ground restistance of a grounding electrode?
You can hook one lead of an ohmeter to the grounding electrode, but
where does the other one go? The only place you could possibly put it
is in the ground, but in doing so, you've created a second grounding
electrode, and you are measuring the resistance of the two in series.
[If the second electrode is just a little voltmeter lead, it will be a
very poor grounding electrode, and the resistance will be quite high.]
That is why measuring ground resistance is tricky.
[Don't quote me on this, but I think with three grounding electrodes
sufficiently far apart and a digital voltmeter you could determine the
ground resistance of each one as follows. Measure the resistance of
each pair of electrodes to get the sum of their two ground
resistances. Then solve the simple system of three linear equations
in three unknowns for the individual resistances.]
Unless you have disconnected the grounding electrode conductor
connecting your ground rods to the main panel, you aren't checking the
resistance of the ground path. It would be very unusual for the
ground path to have low enough resistance to trip a breaker at 120V.
For end use voltages (120V-480V), the ground path is never relied on
to trip breakers.
Yes, right at the ground rods I disconnected the ground wire going to
the main panel. I didn't separate the 2 rods. The hot to the two rods
did indeed trip the 30 amp breaker. To be fair though, it was after a
few days of heavy rain.
There has to be 20 feet or more of 1/2" rebar. The pieces can be
fastened with the regular rebar ties. I think one is done once the 20
minimum is reached. No need to fasten all the rebar. Rules are tougher
for livestock buildings. I would think the concrete crew could handle
The bad thing would be waiting for an inspector. We don't have
inspection for what I do in Nebraska so I can't speak from personal
experience. I've been told they do want to actually see the rebar
before the pour.
This is from the 2008 U.S. Code:
2008 NEC250.52 Grounding Electrodes.
(A) Electrodes Permitted for Grounding.
(3) Concrete-Encased Electrode. An electrode encased by at least 50 mm
(2 in.) of concrete, located horizontally near the bottom or vertically,
and within that portion of a concrete foundation or footing that is in
direct contact with the earth, consisting of at least 6.0 m (20 f.t) of
one or more bare or zinc galvanized or other electrically conductive
coated steel reinforcing bars or rods of not less than 13 mm (½ in.) in
diameter, or consisting of at least 6. 0 m (20 ft.) of bare copper
conductor not smaller than 4 AWG. Reinforcing bars shall be permitted to
be bonded together by the usual steel tie wires or other effective
means. Where multiple concrete-encased electrodes are present at a
building or structure, it shall be permissible to bond only one into the
grounding electrode system.
That is the national code. Localities can change it as they please.
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