Measuring Ohms Resistance

I have to meg a ground rod to see what the Ohms resistance is.
How do I go about it? Can I use a fluke and measure the Ohms, and if so,
how would I do it?
Or do I need a megger? And if so, how do I do it?
Code states that I can have only a maximum of 25, Ohms and I need to see
what the Ohms reading is on the ground rod.
Please delete RemoveThis in my email addy to reply by email.
Thanks!
~Skip
Reply to
Skip
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Your going to need a ground tester. Fluke makes them as well as AVO and Biddle. I doubt that you have one.
The NEC allows for an acceptation, just drive another ground rod 10 feet away and connect accordingly to the service. Here in the Phoenix area we have been ignoring the testing issue for a long time buy using the second ground rod method. Ground test meters cost 2k up.
There are 3 methods of testing a ground rod that I know about, could be more. I have used a 4 point and 3 point method and then there is the Amec ground tester that looks like an amprobe that tests the ground rod directly. (Might be made by others now) Check WW Grainger and bring about 2k for purchase.
The last issue with most AHJ's (authority having jurisdiction) is the certification of the meter and the skill of the person doing the test. At least here there are issues.
25 ohms is the minimum to trip a circuit breaker. Lower is always better. I have installed ground systems with less than 5 ohms to ground even here in the desert. 10 ohms used to be considered performance grounding and then it moved to 5. I have see 2 ohms in specs, and actually had a customer ask for zero. Ya like that will happen.
Reply to
Zathera
You'll simply need an ohm meter to measure resistances in this range. A Fluke should work nicely.
Meggers are use to measure leakage resistance at high voltages, hence are not applicable to your need.
Harry C.
Reply to
Harry Conover
Sorry, I missed the "how do I do it" part.
You need a reference ground to measure the resistance of your ground against.
If you have a metal water service to your home, that should work nicely as the reference. If not, you're likely going to have to drive a second ground rod at some specified distance from the first.
The idea is to use an ohm meter to measure the resistance between the reference ground and the ground rod you are testing.
Harry C.
Reply to
Harry Conover
Skip,
A standard Megger wont accurately test the resistance to ground of your Grounding Electrode System. An Ohm Meter wont work at all. A Ground Resistance, or Earth Resistance, tester would be required.
They are costly, and require an individual trained in their operation to effectively conduct a test.
Do a search for and you will get an idea of how the testing is conducted, and the type of equipment used.
Having a low resistance to earth ground is very important to personnel safety, and proper operation of equipment. I would recommend that you secure the services of a properly trained, and equipped, electrician, or engineer, to conduct your test.
Louis
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Reply to
Louis Bybee
This won't give accurate results. The OP will have to get ahold of a proper "Earth Resistivity Tester" if they want an accurate result - and also to folow directions carefully - like to avoid influence on the reslts from large electrical systems nearby.
HR.
Reply to
Rowbotth
You can't use a regular ohmmeter to measure ground rod resistance. He needs a three-wire ground resistance tester. Megger (the AVO brand) makes them, as well as AEMC, Amprobe, and others.
Ben Miller
Reply to
Ben Miller
On Tue, 7 Oct 2003 08:07:04 -0700, "Zathera" Gave us:
I wonder what the Statue of Liberty would read.
Reply to
DarkMatter
Why do you say that? 25 Ohms is 25 Ohms on any resistance measuring instrument.
If you feel compelled to use a special, one function measuring device for this purpose, somebody has been doing a sales job on you! :-)
I've installed ground systems for radio stations, airports, industrial sites, and residential properties, and for me a simple ohm meter has always proved adequate to do the job.
Some government work will required you to have your meter calibrated to a standard traceable to the National Bureau of Standards, but any calibration lab can provide this servoce for a nominal fee (typically $50-100).
If you respond, please be specific in citing any local, state, or other requirement/regulation that precludes the use of a simple ohm meter for this purpose.
Harry C.
Reply to
Harry Conover
On 7 Oct 2003 19:47:19 -0700, snipped-for-privacy@yahoo.com (Harry Conover) Gave us:
Not when the instrument is susceptible to stray fields it isn't.
If you insist that simple DVM readings are accurate, you have some magic on the rest of the industry. NOT!
No. The ohmmeter and your method were cursory observations, which YOU deemed adequate.
Not. The METER TYPE IS ALSO SPECIFIED.
It is not accurate. Admit it. Move on.
It would be more accurate to pass a known voltage through the two rod circuit, read volts on a shunt resistor, and calculate system resistance. That would be a voltage HIGHER than that of the test voltage of a simple ohm meter or DVM.
Try again.
Reply to
DarkMatter
I only "feel compelled" to use a speciality piece of equipment to effect Ground Resistance Testing because that is the only method that will produce an accurite, and meaningful result.
An even greater reason is due to the fact that where the topic comes up I have yet to experience any AHJ that would accept the use of an Ohm Meter. Where the testing is required in engineering specs (every government job of any significance, and most high tech facilities), or specificed by the AHJ because of questionable conditions, the test results including, identifying the equipment used, the test proceedure used, and current calibration documentation, was also required. In most areas I've worked an additional grounding electrode is systematically installed, and accepted in lieu of testing, to accomodate 250.56.
If it is of importance to you I will inquire as to the specific governing bodies requirements, and ask for the individual sources where they can be parused. The bottom line is that if they wont accept the test it doesn't matter what I think. So I continue to use the speciality equipment.
We have all meters used to document quantifying test proceedures, calibrated at least once a year, and it has come in very handy haveing the certification on more than one occasion where figures were questioned in litigation.
Where you indicate the use of an Ohm Meter for Ground Resistance Testing, if the leads are connected between a ground rod (with a grounding electrode conductor connected to it), and a water pipe, I suspect the test is really quantifying the bond between each through the service panel.
Do a web search for "Ground Resistance Testing", and you will find an enormous quantity of information from multiple sources. If you find any reliable sources indicating an Ohm Meter is acceptable, please advise me. It could save a great deal of money if I can only convince the AHJ.
Louis-- ********************************************* Remove the fish in address to respond
Reply to
Louis Bybee
Neither a standard megger nor a standard ohmmeter are adequate. See NEC 2002 Handbook Exhibit 250.25, IEEE Std 81 "IEEE Guide for Measureing Earth Resistivity, Ground Impedance, and Earth Surface Potentials of a Ground System (Part 1)" and IEEE Std 142 "IEEE Recommended Practice fo Grounding of Industrial and Commercial Power Systems". In particular, in the last reference, Section 4.4.2 - "Also, ordinary low-resistance ohm meters lack sufficient voltage for separating out the grounding resistance of the auxiliary electrodes needed to make the test."
Reply to
Jim Ghrist
Depends on how they did it originally. I do not have much experience with salt water installations. What little I have done has been in San Diego, LA, and Pearl Harbor. If the equipment is exposed to the salt air then with in a year there is corrosion. Corrosion makes for a high impedance connection. I used to go to a plant each year in August in Diego worked on equipment 50 feet from the ocean. Always the same location and always the same result. Rags and dirt gentle cleaning and back to service in 72 hours. They had been doing that for ever
Pearl's installations were all inside, sealed when possible and the main gear a/c'd. We were installing power supplies for subs. Ya, 4000 amps front and rear for each boat. The 24 hour load banking was really interesting to watch. Load banks were so hot that 30 feet away was as close as I wanted to get.
Reply to
Zathera
I believe it has to do with the resistivity of the path between the two (or 4) probes. Most ohmeters use a square 9 volt battery (like the ones you see in electronic devices like alarm clocks and battery-powered smoke detectors) as power to drive current which the ohmmeter measures; the proper device uses aone of the big square 6 volt lantern-type batteries to drive the current. This is important because as well as the resistivity of the earth, there is typically a cople of hundred feet of copper conductor between the current probes and the voltage probes, and the meter. Many points must be measured and the results plotted before the true "resistance" can be measured, to ensure that you are getting an actual value.
The battery which most ohmmeters use just doesn't have the capacity to drive the current through the circuit to be measured.
HR.
Reply to
Rowbotth
Here is a perfect example on how untrained persons try and act like a trained qualified IBEW electrician. An IBEW electrician would never ask such a basic question. Anyone who does, has no business working with electrical current.
I wonder if anyone here would even know how too increase the conductivity of a ground field? And for those who don't know, conductivity is the opposite of resistance. Remove RATSandSCABS to reply Impeach George Bush.
Reply to
Mr. Sandman
Where I am, local rules require a 2m long 16mm diameter stake for the customer earth. No maximim resistance specified.
There was an 'outbreak' of lines staff swapping active and neutral open wire services when replacing poles etc, and in one case ordinarily cold water ran hot! (copper pipes inside house and non metallic street pipes).
My boss reckoned earthing should be adequate to rupture the 63 amp HRC pole fuse in such cases.
This is a 230 volt system and the neutral and earth are 'linked' at customer's main switchboard.
Go and figure this.
Reply to
Peter
My dog knows how to do that and he doesn't have his journeyman's card. Once he increases the conductivity a little every other dog in the neighborhood will help him out.
Reply to
Gfretwell
But let him try it just once with some current running through it. He won't be back, and neither will the others.
Reply to
indago
As in MHOs? You must have worked with tubes before. ;-)
michael
Reply to
Anthony Fremont
At least with a BEM you can just say "Carry my tools boy." They'll do it everytime. They are fun to humiliate. Watch 'em teach a new BEM how to roll up an extension cord...really funnny!
BEM=electrician
Reply to
Johan Lexington

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