Finding earth ground impedence (ohms) at a residential electrical service panel???

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jango

jango

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What is the most effective way to measure the ohms or earth ground impedence at a residential electrical service that has a ground rod. I believe the NEC calls for 25 ohms. What's the most effective way to measure this, and what can be done to reduce the ohms/resistence down to perhaps 5 to 10 ohms? Any feedback on this topic would be greatly appreciated.
 
Why worry about the actual impedance of a single rod when you can simply add a second and call it good?

250.56 Resistance of Rod, Pipe, and Plate Electrodes.
A single electrode consisting of a rod, pipe, or plate that does not have a resistance to ground of 25 ohms or less shall be augmented by one additional electrode of any of the types specified by 250.52(A)(4) through (A)(8).
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I'm being requested by the property manager at the site to test the ohms of the existing earth ground and try to achieve 10-15 ohms or less. If I were to drive two or more 8' ground rods, what would be the required/recommended distance between the rods? and I'm assuming they would be linked with a solid copper conductor?
 
The best way is a 3 point fall of potential tester. You could go to any meter company web sight and find all kinds of testers, for a price.
Or you could figure out how to make an amp meter work for you, keeping in mind, voltage divided by amps equals OHMS. It can be done with just an amp meter but it is not the safest way to go, so I will not give a step by step instructions on how to do it. Sorry.
Why do they want such a low result? It is usually an in-educated person asking for it.
 
If you can test the rods, but don't get the required result try pouring salt water around the rod.
 
jango said:
I'm being requested by the property manager at the site to test the ohms of the existing earth ground and try to achieve 10-15 ohms or less. If I were to drive two or more 8' ground rods, what would be the required/recommended distance between the rods? and I'm assuming they would be linked with a solid copper conductor?
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You realize you could install a 100 rods and still not reach 25 ohms... best of luck with this quest. :)
 
I am wondering why at a "residence". Sounds like a home owner read or heard it somewhere, Telco sights always seem to want 5 OHMS or less.
The lower the resistance on the ground, the higher the current flowing on it instead of the neutral, back to the transformer. The ground rod is only one part of a good grounding system, the metal water supply should be far superior to a dozen ground rods. Think surface area in contact with the earth.
 
jango said:
... If I were to drive two or more 8' ground rods, what would be the required/recommended distance between the rods? ...
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The code says they have to be at least 6' apart. They are most effective in lowing the resistance when they are at least twice the length of the rod apart.
 
zbang said:
Why worry about the actual impedance of a single rod when you can simply add a second and call it good?
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I Ditto that, I kind of found it dumb to think that in ordere to measure the impedance that you would first need to drice a first rod and then a second so that you could measure the impedance. Now you have (2) rods driven anyway.
 
jango said:
What is the most effective way to measure the ohms or earth ground impedence at a residential electrical service that has a ground rod. I believe the NEC calls for 25 ohms. What's the most effective way to measure this, and what can be done to reduce the ohms/resistence down to perhaps 5 to 10 ohms? Any feedback on this topic would be greatly appreciated.
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Personally, I would sub it out. The cost of the meter alone is steep and if you do not do this regularly, you never get the ROI back.

I have never had this requirement at at residential place, although some crazy audiophiles will ask about it.
 
jango said:
I'm being requested by the property manager at the site to test the ohms of the existing earth ground and try to achieve 10-15 ohms or less. If I were to drive two or more 8' ground rods, what would be the required/recommended distance between the rods? and I'm assuming they would be linked with a solid copper conductor?
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Does the property manager have a reason for the request?

Roger
 
cowboyjwc said:
You could go with a chemical ground, if you don't get the readings you want, but that only starts to work when money is not an object.
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And you know on a house, money will not be the issue.
I have only done one chemical rod and they work great, the older they get the better they work.
 
acrwc10 said:
And you know on a house, money will not be the issue.
I have only done one chemical rod and they work great, the older they get the better they work.
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How do you know it 'works great'?

I just finished bonding some gas pipe due to the presence of CSST supplying gas to a dryer. There was no bond from the well point to the water piping as the pump had not been jumpered, and of course no bond to the metal gas pipe feeding the CSST.

It had been that way for years with no problems. Does that mean it was 'working great' before I bonded everything? I would have to do testing to even notice the difference unless there was a fault, and a rather large one at that.

Just curious, why do you feel the need for a chem ground at your house?
 
120V/10ohms=12A. Take out the bond between the neutral and the ground(make sure the ground is COMPLETELY isolated from the neutral), put an inline fuse of 12A between a 20A breaker and the ground and turn on the breaker, if the fuse blows, you got less than 10 ohms.
HOWEVER, keep in mind that soil conditions change, and so will your ground rods resistance to earth. Where you might get less than 10ohms during rainy season, you might not get that low during a dry season. From my experience is very hard to achieve 10 ohms to cover ALL conditions. I did a job one time where the customer INSISTED on less than 10 ohms and we ended up driving 2 sets of 30' ground rods. and this was in South Florida where, if you dig 2-3', you got a well.

jango said:
I'm being requested by the property manager at the site to test the ohms of the existing earth ground and try to achieve 10-15 ohms or less. If I were to drive two or more 8' ground rods, what would be the required/recommended distance between the rods? and I'm assuming they would be linked with a solid copper conductor?
Click to expand...
 
Strife said:
120V/10ohms=12A. Take out the bond between the neutral and the ground(make sure the ground is COMPLETELY isolated from the neutral), put an inline fuse of 12A between a 20A breaker and the ground and turn on the breaker, if the fuse blows, you got less than 10 ohms.
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That test will not tell you the impedance of the electrodes to the earth. It will tell you the impedance of the electrodes to the neutral.
 
HUH?
Didn't I mention(I'm sure I did):"make sure the ground is COMPLETELY ISOLATED from the neutral"?
There it is(from my previous post):"(make sure the ground is COMPLETELY isolated from the neutral),"

K8MHZ said:
That test will not tell you the impedance of the electrodes to the earth. It will tell you the impedance of the electrodes to the neutral.
Click to expand...
 
Strife said:
HUH?
Didn't I mention(I'm sure I did):"make sure the ground is COMPLETELY ISOLATED from the neutral"?
There it is(from my previous post):"(make sure the ground is COMPLETELY isolated from the neutral),"
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Yes, you did.

Remember, electricity is seeking any and all paths back to it's source. The current flow you are measuring with your test is the flow from the one terminal of a transformer back to it's neutral, with a small portion of the earth being only one part of that path.

Read the pdf I linked to. Better yet, spend the 10 bucks and get the book. I got the book in school. It's a great book and an must have for anyone testing electrode resistance. That, and a megohmeter. I have three of them. Two battery powered (one is a real Megger) and a crank type.

The amount of current flowing through a breaker back to it's source is no indication of the amount of resistance between an electrode and the earth.

Like I said, read the book. It's far better at explaining electrode resistance than I'll ever hope to be.
 
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