The 25 Ohm Rule and the right meter...

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Bob Anchorite

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I am a student taking code classes...

The guys who sell the ground resistance meters say that your regular ohm meter will not read the ohms to ground accurately and that you need a real ground resistance meter. Is this true, if so please explain... at $1200 to $2500 for this type of meter I would like to see why etc...

Bob
 
Google Ground Testing 3 point. you'll get all the information you need, besides the meter you need the expierence, to obtain results, this is not a simple matter of just taking a reading.

Also go to avo megger website, look for the publication Down to Earth.
 
on pounding a 2nd rod...

on pounding a 2nd rod...

Of course pounding a 2nd rod gets you past the code requirements, but getting past the code is not equal to true understanding. I think I-wire would support that - and there are times when ground resistance can be say 500 ohms and two rods leave you with a solid 250 ohms of resistance...

As a student I am trying to focus on the understanding side.

Bob

Ps: Thanks to Brian John for his suggestions.
 
While its great that you want to know how keep in mind that most ahj will still want 2 rods.They will not just take your word that you measured it and it would be far to costly to perform the test with them watching.
 
Bob, go to this page and use the drop down window at the top right for some pretty good info on ground testing.

Roger
 
A good ground tester will evaluate the soil around the electrode for existing stray voltages or currents. If the voltage level is too high, the tester may be unable to give accurate results. If the voltage is within reasonable limits - or non existent - the tester will send a signal that is incompatible with any signal that may be present, so that the resulting current flow will be independent of existing influences. A multi-meter can not do this.

In addition, the three point method conforms to IEEE 81 "Recommended Guide for Measuring Ground Resistance and Potential Gradients in the Earth"
 
I have an IDEAL #61-920 ground clamp tester and to be honest, it doesn't seem to work very well. That or I am not using it correctly... I thought it would be useful for some research I plan on doing, but the readings don't seem to be reliable...
 
Bob Anchorite said:
I think I-wire would support that - and there are times when ground resistance can be say 500 ohms and two rods leave you with a solid 250 ohms of resistance...
Click to expand...

What makes you think that putting in two rods will make the ground resistance half what it is with one rod?
 
Ohm's Recommendation?!?

Ohm's Recommendation?!?

petersonra said:
What makes you think that putting in two rods will make the ground resistance half what it is with one rod?
Click to expand...

Ohm's Law?

1 / Rtotal = 1 / R1 + 1 / R2 + ... + 1 / Rn
 
Why is it important to have less than "25" ohms to ground?
We spend a lot of time fussing around with 2 ground rods, install a ufer ground and be done with it. For the typical dwelling unit, thats better than good enough.
What we really need to spend our time on is bonding to make sure the OCD will trip quickly.
 
tallgirl said:
Ohm's Law?

1 / Rtotal = 1 / R1 + 1 / R2 + ... + 1 / Rn
Click to expand...

If one has two resistors in parallel and they are of equal resistance, it is indeed true that the net resistance of the circuit is half either resistor.

There is no guarantee whatsoever that two rods pounded into the ground 6 feet apart are going to have equal resistance to earth.
 
Typically on projects where we do ground electrode testing it is not a residence, but a data center, hospital, telecom, radio/TV ECT. The multiple rods are usually a specification. Sometimes arrived at by a SWAG, other times by research that included numerous four point tests, to determine the necessary design and number of driven electrodes to obtain the specified resistance. After installation several three point test are performed to determine if the installation meets design spec.


As for the Clamp on Ground Testers, I own several and in the right instance they are adequate for the job, but I have these test devices misused more that they are correctly used. By others of course??
 
tom baker said:
Why is it important to have less than "25" ohms to ground?
We spend a lot of time fussing around with 2 ground rods, install a ufer ground and be done with it. For the typical dwelling unit, thats better than good enough.
What we really need to spend our time on is bonding to make sure the OCD will trip quickly.
Click to expand...


Grounding Electrodes are for Lightning, and prevent Surface Arcing... You will probably only get 50 ohms....
 
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I completely agree with Mr. Baker. Way too much wasted time and energy is wasted on the grounding electrode system on this forum. For most installations, all evidence tends to indicate the most basic and minimalist-grounding system is sufficient for its intended purpose.
 
bphgravity said:
I completely agree with Mr. Baker. Way too much wasted time and energy is wasted on the grounding electrode system on this forum. For most installations, all evidence tends to indicate the most basic and minimalist-grounding system is sufficient for its intended purpose.
Click to expand...

Maybe, but a lot of people still think a ground rod is for clearing faults... or better yet actually providing a ground...
 
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