Efficiency of an electrical ground

Stan in Oly, WA

Well-known Member
I've read that different grounding methods provide different degrees of grounding capacity. For instance, I've seen figures showing that grounding rods in dry soil make poor grounds, but I can't remember the measurement that was used. How can I determine the efficiency of a ground?

Stan
 
(quoted from post at 12:00:06 07/28/17) I've read that different grounding methods provide different degrees of grounding capacity. For instance, I've seen figures showing that grounding rods in dry soil make poor grounds, but I can't remember the measurement that was used. How can I determine the efficiency of a ground?

Stan
EC wants the ground rod resistance to be 25 Ohms or less. The way I have determined this ground rod resistance is by connecting a known load (I used a 60W incandescent bulb, which on 120 volts is a 1/2 ampere current) from line through bulb to the ground rod.
First, measure the open circuit (zero current) line to rod voltage (example=123 volts). Connect bulb & re-measure voltage (example=114 volts). (123-114)= 9 volts. Now calculate rod resistance using V=IR. R=V/R=9/0.5= 18 Ohm ground rod resistance.
 
I'm not sure I understand. If I connect a 12 gauge wire from a breaker to the rod, won't that trip the breaker if the ground is good?

Stan
 
(quoted from post at 13:47:20 07/28/17) I'm not sure I understand. If I connect a 12 gauge wire from a breaker to the rod, won't that trip the breaker if the ground is good?

Stan
e don't know how good the "ground" is, but if as poor as 25 Ohms, then, you will measure about 5 amperes & no trip. But if really excellent and 1 Ohm, the result will be 120 amperes and a fast breaker trip!
 
If the soil is dry it will take a second or two and give time to get a reading. {note a ground rod is only supplemental grounding according to code]
 
(quoted from post at 10:33:25 07/28/17) 25 ohms - or just run a 12 gauge wire from a breaker to the rod and measure with amprobe.

It does sound like a good way to get electrocuted if its not done right!?
 
Stan, very good question: Indeed in some locations and some earth subject to moisture and mineral contents, a rod driven into mother earth IS A POOR (yet may still satisfy codes) GROUNDING ELECTRODE. Buried braided copper cables and buried ground plates and building structural steel or foundation grids or some metallic buried utility pipes can be far superior to a rod driven into dry earth.

When I practiced electrical power distribution engineering at our facility and jurisdiction we were required (in addition to other grounding electrode availability) to drive one earth ground rod and if it didn't pass we had to drive a second rod, but if it still didn't pass we didn't have to drive anymore and it was accepted HOWEVER that was ONLY in our jurisdiction and the NEC at the time, other jurisdictions and later codes may be far different . Our practice then was to bond the incoming utility Neutral to "All readily available grounding electrodes" such as building structural steel, buried perimeter ground grids, metallic buried utility pipes and if none of those were available then we could use a "made grounding electrode" like driven into the earth copper ground rod or rods. We earth grounded the Neutral at EITHER the service drop,,,,,the meter base,,,,,,,,or inside the main panel, and we then created our Neutral Ground Bond inside the main dist panel.


NOTE FYI earth grounding of the Neutral is mostly for safety, lightning and surge protection, and to keep the utility and grid at a common low voltage reference IE MOTHER EARTH. Earth grounding of the Neutral is NOT the same as Neutral Ground Bonding.

NOTE its the purpose of the Equipment Grounding Conductor to provide a dedicated low resistance return current path for FAULT CURRENT and that can still work regardless of the quality of the Neutrals earth ground be it via a rod or grid or pipes etc

Many non sparkies don't understand the difference in Neutral and Ground, and Earth Grounding of the Neutral versus Neutral Ground Bonding etc etc.

Typically electrical questions get the most responses and opinions. If in doubt consult local authority (if any exists) and trained professional electricians and engineers or do as you wish or Billy Bob and Bubba say is fine by me, its your money and your risk.

John T Longggggggggggg retired from EE practice so no warranty, consult trained current professional electricians and engineers and local authority if any exists, they make the rules NOT any of us here lol
 
John,
Why did I have a major power surge that fired many things in house, when neighbor's tree fell on triplex, broke the bare wire. I still had a grounding rod on a new 200 amp service.

No essay required, keep it simple, I'm simple.
geo
 
New local code requirement here is a "ufer" ground. Look it up. It was developed by a guy named Ufer in NM, IIRC. It involves tying a conductor to the rebar in your foundation and then attaching that wire after it comes out of the concrete to the panel. It's been around a long time (WWII) in dry environments but I don't understand why it's now required in my township in Michigan where the water table is about 4'.
 
Cant answer in depth with short n simple so it wont be all correct, but there was still enough surge current that DID NOT get shunted to ground that trashed some of your homes sensitive circuits. I just cant cover in a sentence or two what could fill entire libraries lol

John T
 
Sure a ufer ground is one good grounding electrode, but it seems strict for a township.

John T
 
It was like the last resort if there weren't other better grounding electrodes when I practiced.

John T
 
At my company we have to check our grounding every time we move tbe plants and we have a mter called a "megger" spelling? The the maintanance guys use to chelk ohms to ground. Sometimes it is very hard to get in spec when you set up on the floor of a quarry, we drive a lot of rods, what is called a "mat" what does all of this have to with your question??LOL not sure how common those ground check meters are but of you have access to one it sure is comforting to know that my home grounding meets mine specs.
 



if it was a triplex and you lost the neutral...

each hot leg had to feed back to and through the other hot leg to get a return to the power company... and if you had a 1000 watt appliance on one leg feeding a 100 watt appliance on the other leg.. the non balance or non equal resistance meant that one appliance got about 30 volts and the other appliance got 190 volts through it. Loosing the neutral puts 220 on everything based on its balance/resistance of each leg, against the other leg.. So the small current appliances took the over voltage.. the high current appliance saw low voltage...

Had a tree rubbing the power line and it took out the non insulated conductor on the triplex feed to the house, and it did the same thing.

had a neighbor on the same transformer loose a neutral and made my voltage all wonky,, one side high and one side low. Drove me crazy till I traced it back to the transformer with my house cut off... Then realized it was his house doing the dance. Tv would not work at his house unless your turn on two lights that were on the other side. He was glad I came along and helped him get a temp neutral cut in, till the buried lead could be replaced. I was glad to keep my server farm up and running.
 
> At my company we have to check our grounding every time we move tbe plants and we have a mter called a "megger"

A megger (megohmmeter) is used to check insulation resistance at high voltage. You wouldn't use one to check ground continuity. Your boys may call their meter a "megger", but that's not what it is. They would be using a ground continuity tester, which checks continuity at high current. Exactly the opposite of a megohmmeter.

FWIW, I used to work with wiring harness testers, which do both: check insulation resistance at high voltage and continuity at high current.
 
Here is a link to our Megger grounding test set up, a very spendy piece of equipment. It would appear that our discrepancy here is Megger is an equipment brand name to me and to you it is a generic term? Anyway we call it the megger and the term is used throughout the mine industry.
Megger
 
>Megger is an equipment brand name to me and to you it is a generic term?

Yeah, for most EEs, the term "megger" is a generic term, like "kleenex". I didn't realize that the Megger company makes products other than megohmmeters. Thanks for the info.
 
It was enough surge to fry the biggest surge suppressor sold for a computer. Computer was saved.

After that I paid big bucks, bought a surge suppressor from power company. It goes behind meter. Suppose to protect the entire house.

I always thought the bare wire was connected to ground, mother earth, and all current returns there. But apparently mother earth couldn't handle it when the bare wire snapped. I still had 220 and pump ran just fine on 220. It was the 110 items that got fried.
 
Butch, at out huge naval industrial facility our electricians called it a "megger" that they used when they checked for grounding electrode quality and integrity. If there were no other "readily available" grounding electrodes and they were left with using "made grounding electrodes" such as a rod driven into mother earth, they drove one, used what they called a "megger" to test and if it failed they drove an additional ground rod. I'm NOT saying what they called it is right or wrong I'm ONLY saying thats FWIW what they all called it............

John T Retired electrical engineer
 
Mark,
It's been a life time ago that I used a megger to measure insulation on motors. If my failing memory is correct, didn't they have a hand crank that turned a generator, like used on old telephones, to produce a high voltage to shock the motor's insulation?
 
Indeed George, good old mother earth isn't any great conductor of electricity and it depends on the soil and moisture and mineral content as to how good or bad it actually is...........

I know you know all this but FYI for non sparkies, the energy source is the utility transformer which has 240 VAC line to line but only half that or 120 VAC either line to the center tapped Neutral. At that point and again at the service drop the Neutral is connected to a grounding electrode (such as a driven ground rod). Loads are then either line to line 240 or line to Neutral 120 and the normal return current path is via the wires NOT mother earth. BUT LIGHTNING AND SURGES YIKES MAY RETURN VIA ALL SORTS OF PATHS AND FRY SOME ELECTRONICS

Fun chattin with you

John T
 
I TOLD YA SO LOL Legal or Electrical questions draw more responses and opinions (me included) then most any other topics !!!

John T
 
> didn't they have a hand crank that turned a generator?

The "old school" megohmmeters had a crank. I think most are now battery-powered.
 
I have done this many times when setting up ground grids for our generators and lightning grounds. We try to get down below 10 ohms resistance. Surface area of the ground rod or rods has a big effect on lowering the resistance. In practice yes the device is a Megger but a special type of megger. Tests can be done to check resistance of the rod to ground coupling and also general tests to determine the resistivity of the soil in an area. Vibroground used to make ground testing meters. AEMC makes the current model I have used. One lead goes to the ground rod and the other 3 leads go to stakes spread out at a predetermined distance from the ground under test.
 
If you ever grabbed a hold of an old hand crank phone, you would get a nice shock.

I knew a kid that made an IQ tester. If you put your fingers on two touch pads, pushed a button you would get a shock when you released the button. Then he rigged up a LCD display to spell out fool.

Do you think the old hand crank phone would make a great IQ tester?
 
Interesting, thanks. We are required to use concrete-encased grounding electrodes in commercial services here in Iowa but never knew the Ufer name behind it. Always seem to pick up some new facts on this site.
 
> Do you think the old hand crank phone would make a great IQ tester?

George, I don't know about that, but crank telephones have been used as torture devices in prisons and allegedly to interrogate prisoners in Vietnam.
Tucker Telephone
 

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