ground rod - 0 ohms
- Thread starter gmv5158
- Start date
- Status
- Location
- Illinois
- Occupation
- retired electrician
Re: ground rod - 0 ohms
First, you can never have any type of circuit with 0 ohms. What is your application, and why do you think that you need a very low resistance grounding electrode system?
Don
First, you can never have any type of circuit with 0 ohms. What is your application, and why do you think that you need a very low resistance grounding electrode system?
Don
Re: ground rod - 0 ohms
thanks for your interest don, my application is a lightning protection job i am bidding on that has ten down leads on a single story 20 foot high building. There are no engineered notes requiring 0 ohms, but thru conversations with the electrical contractor they want as low as possible with in financial reason.
I gave them a bid price for ten 10ft copper clad rods placed 50 ft apart as lpi calls for and stated in note that cost plus would apply to achieve the lowest ohms possible.if they want to pay we'll drivum to china.I've never had to check for low ohms and want to know how you physically hook the meter to read what you got at the grd. rod?
thanks for your interest don, my application is a lightning protection job i am bidding on that has ten down leads on a single story 20 foot high building. There are no engineered notes requiring 0 ohms, but thru conversations with the electrical contractor they want as low as possible with in financial reason.
I gave them a bid price for ten 10ft copper clad rods placed 50 ft apart as lpi calls for and stated in note that cost plus would apply to achieve the lowest ohms possible.if they want to pay we'll drivum to china.I've never had to check for low ohms and want to know how you physically hook the meter to read what you got at the grd. rod?
bphgravity
Senior Member
- Location
- Florida
Re: ground rod - 0 ohms
I would simply follow the suggested installation standard of the NFPA 780 and not even worry what the total resistance outcome is. It will vary so greatly from location to location and even from week to week in some areas. So you may be able to acheive 5-ohms today but a month from now it may not be as soil conditions change.
If you have a normal soil resistivity of something around 4,000 to 50,000 ohm-centimeters, a ground resistance of anything around 15 to 200 ohms will be sufficient for basic lightning protection. Deep rods are of almost no use as lightning typical drives no more than 2 feet into the earth before extending out laterally from its strike point.
I would focus on providing adequate TVSS and other surge protection equipment at the service and any other sensitive equipment. This is your best option.
I would simply follow the suggested installation standard of the NFPA 780 and not even worry what the total resistance outcome is. It will vary so greatly from location to location and even from week to week in some areas. So you may be able to acheive 5-ohms today but a month from now it may not be as soil conditions change.
If you have a normal soil resistivity of something around 4,000 to 50,000 ohm-centimeters, a ground resistance of anything around 15 to 200 ohms will be sufficient for basic lightning protection. Deep rods are of almost no use as lightning typical drives no more than 2 feet into the earth before extending out laterally from its strike point.
I would focus on providing adequate TVSS and other surge protection equipment at the service and any other sensitive equipment. This is your best option.
Re: ground rod - 0 ohms
I will try to help you out as I have done a lot of GES and lightning protection for telephone companies requiring 5 ohms or less.
Seriously doubt anyone is going to be willing to pay to get down to 5-ohms or less unless you just happen to be lucky and have good soil. Achieving the 5 ohms or less take engineering, planning and soil test, it just does not happen on it's own.
OK now to answer your question. There are two methods of measuring ground resistance, one semi accurate, the other is ball park.
The semi accurate method is known as "Dead Fall Of Potential" method using either a 3-point or 4-point system and an instrument called a Meggar. The down side is this. For the test to be accurate, the 3 or 4 points has to be at least 6 times as large as the area of the GES, preferable 10 times. So just for a two rod system spaced say 20-feet apart, the two furthest test points would need to be 120 to 200 -feet apart from one another. To complicate things, the test must be performed several times moving the probes on a different plane. After all the measurements are completed you then average the reading to get the mean average. I think you can see the problem real quick. Lets say your building is 100 x 100 feet, or diagonally 140 feet. 140 times 10 is 1400 feet. That means you need cable at 1400, and another at 600 feet to perform the test. Couple that with multiple test. Where are you going to find that much space to string those cable across neighborhoods multiple times.
The other method is easy. You use a clamp on ground tester. You clamp one of the rods, read it and walk away your done. Problem is it is not accurate because of the way the meter works. It takes the resistance of the rod under test and adds it to the series of the rest of the rods in parallel. Not much of a problem though, because whatever it reads you actual impedance is really lower than it reads.
I will try to help you out as I have done a lot of GES and lightning protection for telephone companies requiring 5 ohms or less.
Seriously doubt anyone is going to be willing to pay to get down to 5-ohms or less unless you just happen to be lucky and have good soil. Achieving the 5 ohms or less take engineering, planning and soil test, it just does not happen on it's own.
OK now to answer your question. There are two methods of measuring ground resistance, one semi accurate, the other is ball park.
The semi accurate method is known as "Dead Fall Of Potential" method using either a 3-point or 4-point system and an instrument called a Meggar. The down side is this. For the test to be accurate, the 3 or 4 points has to be at least 6 times as large as the area of the GES, preferable 10 times. So just for a two rod system spaced say 20-feet apart, the two furthest test points would need to be 120 to 200 -feet apart from one another. To complicate things, the test must be performed several times moving the probes on a different plane. After all the measurements are completed you then average the reading to get the mean average. I think you can see the problem real quick. Lets say your building is 100 x 100 feet, or diagonally 140 feet. 140 times 10 is 1400 feet. That means you need cable at 1400, and another at 600 feet to perform the test. Couple that with multiple test. Where are you going to find that much space to string those cable across neighborhoods multiple times.
The other method is easy. You use a clamp on ground tester. You clamp one of the rods, read it and walk away your done. Problem is it is not accurate because of the way the meter works. It takes the resistance of the rod under test and adds it to the series of the rest of the rods in parallel. Not much of a problem though, because whatever it reads you actual impedance is really lower than it reads.
Re: ground rod - 0 ohms
thanks for the info BPHGRAVITY, soil resistivity and its direct result was part of the discussion at the meeting. I think the electrical contractor was phishing me to see if I knew what I was talking about since this would be my first timeworking with this outfit if I get awarded the job. thanks again, just lining up my ducks.
happy new year
thanks for the info BPHGRAVITY, soil resistivity and its direct result was part of the discussion at the meeting. I think the electrical contractor was phishing me to see if I knew what I was talking about since this would be my first timeworking with this outfit if I get awarded the job. thanks again, just lining up my ducks.
happy new year
Re: ground rod - 0 ohms
thanks DERECKBC for the 411.
I also doubt if they are willing to go cost plus on an unknown either. I have never personally used the clamp on ground tester but have seen it used and knew about the meggar instrament but had no idea how it worked. thanks for sharing the knowledge and all the help.
thanks DERECKBC for the 411.
I also doubt if they are willing to go cost plus on an unknown either. I have never personally used the clamp on ground tester but have seen it used and knew about the meggar instrament but had no idea how it worked. thanks for sharing the knowledge and all the help.
al hildenbrand
Senior Member
- Location
- Minnesota
- Occupation
- Electrical Contractor, Electrical Consultant, Electrical Engineer
Re: ground rod - 0 ohms
Check out this Adobe Acrobat Reader document: Understand Ground Resistance Testing.
Check out this Adobe Acrobat Reader document: Understand Ground Resistance Testing.
Re: ground rod - 0 ohms
hey thanks AL, fantastic sight, I searched for this exact info last night but I guess I didn't have the right sequence of key words, alas, had I not been looking I wouldn't have found this sight either. two finds are better than one.
thanks for sharing and have a happy new year.
hey thanks AL, fantastic sight, I searched for this exact info last night but I guess I didn't have the right sequence of key words, alas, had I not been looking I wouldn't have found this sight either. two finds are better than one.
thanks for sharing and have a happy new year.
Re: ground rod - 0 ohms
To be honest I would do things slightly different to make the system more effective. As Bryan pointed out the depth of the rods is not important, nor is the actual impedance after completion. What I would design is where each down conductor went to earth, sink a standard 8' ground rod, then run a radial outward away from the building, in either a single radil or crows foot pattern. Use thermal welds for all connections at or below grade.
I do not have a problem with that, so yes.
To be honest I would do things slightly different to make the system more effective. As Bryan pointed out the depth of the rods is not important, nor is the actual impedance after completion. What I would design is where each down conductor went to earth, sink a standard 8' ground rod, then run a radial outward away from the building, in either a single radil or crows foot pattern. Use thermal welds for all connections at or below grade.
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