Grounding Resistance Measurement
- Thread starter royboy
- Start date
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ryan_618
Senior Member
- Location
- Salt Lake City, Utah
Re: Grounding Resistance Measurement
I hope you didn't bet on the resistance measurement. 25 Ohms is the magic number, not 75
I hope you didn't bet on the resistance measurement. 25 Ohms is the magic number, not 75
- Location
- Illinois
- Occupation
- retired electrician
Re: Grounding Resistance Measurement
You can't make the test with an ohm meter. You need a ground resistance tester.
Don
You can't make the test with an ohm meter. You need a ground resistance tester.
Don
Re: Grounding Resistance Measurement
When and if you do access a Ground Test Set, you may be surprised at the high readings you will get. This is especially true in loamy or sandy soils. Clay, or salted (fertilized) soils can improve the readings.
Roger
[ February 08, 2004, 06:21 PM: Message edited by: roger ]
When and if you do access a Ground Test Set, you may be surprised at the high readings you will get. This is especially true in loamy or sandy soils. Clay, or salted (fertilized) soils can improve the readings.
Roger
[ February 08, 2004, 06:21 PM: Message edited by: roger ]
speedypetey
Senior Member
- Location
- Hudson Valley, NY
Re: Grounding Resistance Measurement
While we are on this subject, how many of you have actually checked the resistance of a ground rod?
In 15 years I have never nor have I ever seen or heard of an inspector doing it.
How common is this check?
While we are on this subject, how many of you have actually checked the resistance of a ground rod?
In 15 years I have never nor have I ever seen or heard of an inspector doing it.
How common is this check?
Re: Grounding Resistance Measurement
Speedy, we do at times, but not directly for the NEC requirement. Our test equipment is manufactured by James G Biddell which is now AEMC I think.
As far as taking the equipment out and testing for the NEC requirement, it's just easier to drive two rods and be done with it,
What we do test for is "Performance Grounding", this may be in the contract documents for commercial, institutional, industrial, healthcare, telcom, etc... specifications.
Roger
Speedy, we do at times, but not directly for the NEC requirement. Our test equipment is manufactured by James G Biddell which is now AEMC I think.
As far as taking the equipment out and testing for the NEC requirement, it's just easier to drive two rods and be done with it,
What we do test for is "Performance Grounding", this may be in the contract documents for commercial, institutional, industrial, healthcare, telcom, etc... specifications.
Roger
Re: Grounding Resistance Measurement
In order to be accurate with the 3-point system or "Fall-Of-Potential" method the "C" lead spacing needs to be at least 6-to-12 times ( the further the more accurate, 70' standard)) the rod length of a single rod system. If you are talking about a multiple rod system or rings, it is still 6-to-12 times the perimeter or straight line distances of the system. The measurement must also be taken at several different radials to be sure no underground pipping interferes with the readings. So how many electricians, or inspectors for that matter, are going to bother?
Drive two rods and go home!
[ February 08, 2004, 08:03 PM: Message edited by: dereckbc ]
The only correct place to take the test is at the UUT (Unit Under Test), or at the rod with it disconnected from the AC system or anything else. It's not 75, but 25 or less.
In order to be accurate with the 3-point system or "Fall-Of-Potential" method the "C" lead spacing needs to be at least 6-to-12 times ( the further the more accurate, 70' standard)) the rod length of a single rod system. If you are talking about a multiple rod system or rings, it is still 6-to-12 times the perimeter or straight line distances of the system. The measurement must also be taken at several different radials to be sure no underground pipping interferes with the readings. So how many electricians, or inspectors for that matter, are going to bother?
Drive two rods and go home!
[ February 08, 2004, 08:03 PM: Message edited by: dereckbc ]
ryan_618
Senior Member
- Location
- Salt Lake City, Utah
Re: Grounding Resistance Measurement
One of the gentlemen on the Utah IAEI board of directors performs the test on a regular basis, for reasons that Roger pointed out. He tells me that due to the fact that we are on a lakebed, he gets less than 25 about 75 percent of the time, and even gets 10 Ohms on occasion. From what he was telling me, the farther out of the vally he gets, the worst the tests score.
I realize that is of little value to most people here, but I found it quite interesting.
One of the gentlemen on the Utah IAEI board of directors performs the test on a regular basis, for reasons that Roger pointed out. He tells me that due to the fact that we are on a lakebed, he gets less than 25 about 75 percent of the time, and even gets 10 Ohms on occasion. From what he was telling me, the farther out of the vally he gets, the worst the tests score.
I realize that is of little value to most people here, but I found it quite interesting.
Re: Grounding Resistance Measurement
For residential and other non critical applications, "Drive Two Rods" and call it quits.
Probable when an impedance is specified in contract obligations, otherwise inspector's judgement. In all my applications, which is telecom, 5 ohm's or less as measured with "Fall-Of-Potential" method.
For residential and other non critical applications, "Drive Two Rods" and call it quits.
pierre
Senior Member
- Location
- Westchester County, New York
Re: Grounding Resistance Measurement
I purchased the ground tester from AEMC (a lot of $$$) and use it for my self indulgence. The grounding system does not have to be disconnected, it works very similar to a clamp on amp meter. The readings in lower NY are pretty high - ranging from 70 ohms to much higher. I have yet to measure, even with two rods, 25 ohms to ground.
I agree with Dereck, when purchasing the material for a service, just pick up two rods.
Pierre
I purchased the ground tester from AEMC (a lot of $$$) and use it for my self indulgence. The grounding system does not have to be disconnected, it works very similar to a clamp on amp meter. The readings in lower NY are pretty high - ranging from 70 ohms to much higher. I have yet to measure, even with two rods, 25 ohms to ground.
I agree with Dereck, when purchasing the material for a service, just pick up two rods.
Pierre
caosesvida
Senior Member
Re: Grounding Resistance Measurement
my father told me they used to hook the ground rod to a fuse and put a small amp fuse in and see if it would fail. If not they would supplement the ground rod.
my father told me they used to hook the ground rod to a fuse and put a small amp fuse in and see if it would fail. If not they would supplement the ground rod.
Re: Grounding Resistance Measurement
I have the AEMC meter, and only use it to for inspection purposes. It cannot be used to measure ground resistance, as it is not accurate. It will always reads high due too series resistance of the UUT plus the ground resistance at the transformer loop. For example, if I were too specify a 5-ohm ground and used the AEMC and it measured 4-ohms, I would accept the system (this has never happened in my experience). I know the actual ground resistance is something lower than 4-ohms. On the other hand if the AEMC were to read 8-ohms, I would have no idea what the UUT actually is. I would have to follow up with a 3-point "Fall-of-Potential test to find out.
Pierre, I was referring too a 3-point "Fall-of-Potential" test method.
I have the AEMC meter, and only use it to for inspection purposes. It cannot be used to measure ground resistance, as it is not accurate. It will always reads high due too series resistance of the UUT plus the ground resistance at the transformer loop. For example, if I were too specify a 5-ohm ground and used the AEMC and it measured 4-ohms, I would accept the system (this has never happened in my experience). I know the actual ground resistance is something lower than 4-ohms. On the other hand if the AEMC were to read 8-ohms, I would have no idea what the UUT actually is. I would have to follow up with a 3-point "Fall-of-Potential test to find out.
Re: Grounding Resistance Measurement
Unless the fuse was very small, I doubt any fuse would operate, ground resistance is too high to carry enough fault current to operate a OCPD. However I have seen and heard of this method used as a "poor man's" method of measuring ground resistance.
jimwalker
Senior Member
- Location
- TAMPA FLORIDA
Re: Grounding Resistance Measurement
So for those of us without fancy testers ,what if we measured the amps and used a 15 amp breaker.If we read 5 or more amps we pass ? Would OSHA approve ?
So for those of us without fancy testers ,what if we measured the amps and used a 15 amp breaker.If we read 5 or more amps we pass ? Would OSHA approve ?
charlie
Senior Member
- Location
- Indianapolis
Re: Grounding Resistance Measurement
If ground resistance were that simple, the fall of potential or clamp on type of testers would not be necessary. After learning Ohm's law, you would think if you got 40 ohms on one rod, driving a second one would give you 20 ohms . . . but it doesn't. Sorry, this isn't linear or simple. The best way is to just count on driving two rods and walk (OSHA Approved).
If ground resistance were that simple, the fall of potential or clamp on type of testers would not be necessary. After learning Ohm's law, you would think if you got 40 ohms on one rod, driving a second one would give you 20 ohms . . . but it doesn't. Sorry, this isn't linear or simple. The best way is to just count on driving two rods and walk (OSHA Approved).
Re: Grounding Resistance Measurement
I have recently been called out by the engineer as far as having tested the system. He specificaly specified 25 ohms or less dry season and 10 ohms or less wet season(florida), also he made it clear 2 rods wouldn't be the answer if i didn't meet the requirements AND it was required to be done by an independant contractor, NOT CHEAP, even though with the testing equipment mentioned earlier it takes literally about 2 seconds. food for thought, READ THE SPECS! Justin
I have recently been called out by the engineer as far as having tested the system. He specificaly specified 25 ohms or less dry season and 10 ohms or less wet season(florida), also he made it clear 2 rods wouldn't be the answer if i didn't meet the requirements AND it was required to be done by an independant contractor, NOT CHEAP, even though with the testing equipment mentioned earlier it takes literally about 2 seconds. food for thought, READ THE SPECS! Justin
brian john
Senior Member
- Location
- Leesburg, VA
Re: Grounding Resistance Measurement
I own several three/four point testers and also utilize the clamp on testers . We utilize all testers monthly at a minimum, sometimes weekly. Depends on what projects are on the books.
To obtain an accurate (?) reading you must take time to follow the instructions, so many of our competition IMO utilize the testers improperly. The customers prefer their method, as it is faster and cheaper. Oh Well……….
I own several three/four point testers and also utilize the clamp on testers . We utilize all testers monthly at a minimum, sometimes weekly. Depends on what projects are on the books.
To obtain an accurate (?) reading you must take time to follow the instructions, so many of our competition IMO utilize the testers improperly. The customers prefer their method, as it is faster and cheaper. Oh Well……….
caosesvida
Senior Member
Re: Grounding Resistance Measurement
I think my dad did say it was a 5 amp fuse. Primitive, true but in the 30's and in ww2 I guess you made due with what you had.
I think my dad did say it was a 5 amp fuse. Primitive, true but in the 30's and in ww2 I guess you made due with what you had.
charlie tuna
Senior Member
- Location
- Florida
Re: Grounding Resistance Measurement
osha is gonna get you for working on exposed energized equipment!!!!
osha is gonna get you for working on exposed energized equipment!!!!
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