# Electrical code Q

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 electrode.
[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.
Cheers, Wayne
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Wayne Whitney wrote:

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.
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I'm reminded of that old popular science "worm collecor" project that involved two ground rods, a light bulb and 120vac. :-)
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Tony wrote:

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 that part. 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 NEC250.52 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.
End quote. .
From: http://tinyurl.com/2alupxw
That is the national code. Localities can change it as they please.
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On 5/28/2010 10:29 AM, Joe J wrote:

yes
--
Steve Barker
remove the "not" from my address to email
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