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#129970 - 12/05/05 03:18 PM less than 25 ohms  
PEdoubleNIZZLE  Offline
Joined: Jun 2004
Posts: 176
McKeesport, PA, USA
As I have read in previous posts, ground rods must be installed so that there is less than 25 ohms resistance. What I want to know is less than 25 ohms between what two points? I know I'm missing something here, and this may have already been covered, but where do you measure the resistance?

Disclaimer: I'm not an EC, handy man, etc. I'm not even an EE... yet. It's not homework, and I'm not attempting to use this information to install anything. I'm ust a very curious person who has trouble googling things, and figured this is the best place to ask. :-)

Tools for Electricians:

#129971 - 12/05/05 04:48 PM Re: less than 25 ohms  
winnie  Offline
Joined: Sep 2003
Posts: 649
boston, ma
You are looking for 25 ohms between the electrode and the 'earth'.

Soil is _not_ a particularly good conductor. But there is quite a lot of it. So even though the conductivity in ohms per meter is poor, you can consider the earth a very good conductor because the cross section is so large.

It is a reasonable approximation to say that the earth itself has zero resistance, but that the grounding electrode has a certain amount of resistance connecting to the earth. We are approximating the finite resistance of the electrode and the soil as though the earth were a superconducting sphere but the electrode were a resistor connecting to that superconducting sphere.

If you have two ground electrodes, and they have a resistance of X and Y to 'earth', then the resistance measured between these two electrodes would be approximately X+Y.

This approximation breaks down in many ways, for example if the electrodes are very close together.

This is a trick that you will see used all over the place in physics. You take some measurement that must made between two points, and you make one of those points something extremely large or far away, and call that your 'reference'. We do this with voltage: voltage is always defined between two points, but if you select the 'earth' as your reference zero, then you can identify any other point by its voltage relative to this reference zero. Now if I happen to have two points, and know their individual 'voltage relative to earth', then I can calculate the voltage between those two points.


#129972 - 12/05/05 05:13 PM Re: less than 25 ohms  
rather_large_ben  Offline
Joined: Oct 2004
Posts: 18
They make 'ground rod resistance testers'. Ours has two stakes you put in the ground different distances apart and the meter measures from them to the rod.

#129973 - 12/05/05 07:59 PM Re: less than 25 ohms  
Roger  Offline
Joined: May 2002
Posts: 1,716
The following link is very informative.


#129974 - 12/06/05 12:32 AM Re: less than 25 ohms  
PEdoubleNIZZLE  Offline
Joined: Jun 2004
Posts: 176
McKeesport, PA, USA
Thanks everyone, you have satisfied my thirst for knowledge.

(evil grin)Also, I was thinking... When we terraform Mars a few millenia from now, we should start fresh and bury a copper mesh around the planet to ground to. Although the resistance would be great over a long distance, it would be better than stabbing the soil. (/evil grin)

#129975 - 12/06/05 04:59 PM Re: less than 25 ohms  
Dave T  Offline
Joined: Jan 2005
Posts: 155
Waukesha, WI, USA
I've tried to figure out just how one confirms that there is less thank 25ohms of resistance anyway and the time that it takes to do so other than having a ground resistance tester. It is of my opinion that going to plan 'B,' the cost of a a second ground rod and few feet of cable and drivng it and calling it a day may make more sense or cents.

#129976 - 12/11/05 10:28 AM Re: less than 25 ohms  
trekkie76  Offline
Joined: Jun 2004
Posts: 220
baileyville, maine, usa
This is what the army has to say on the subject. couldn't get the pic to come up, but it shows a megger with the one lead on the ground rod, the other on the water pipe.

-3. Resistance. The resistance between a grounding rod and the earth should be less than 25 ohms. Check the ground resistance with a megohm meter (Figure 1-12). This check should be made with the grounding jumper between the water pipe or a good ground and the electrical system. A metallic cold-water system makes a good point for testing the resistance of an electrical grounding system. If 25 ohms to ground cannot be achieved, take the following actions:

following actions:

Figure 1-12. Megohm meter

a. Drive additional 8-foot grounding rods into the earth and bond them together. The rods must be at least 6 feet apart (Figure 1-13).

[This message has been edited by trekkie76 (edited 12-11-2005).]

#129977 - 12/11/05 11:05 AM Re: less than 25 ohms  
trekkie76  Offline
Joined: Jun 2004
Posts: 220
baileyville, maine, usa

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