Grounding testing

[TerryBarnhill]

Howdy,

Anyone testing for 25 ohms?

I have been tasked to check several buildings scattered all over several miles around my area for grounding issues. The company has had several insurance claims due to what I was told, "lighting damages from poor service grounding". As it stands, I have not laid eyes on but one of these buildings up to this point. I don't know the age of the buildings, if they are made of steel, sticks, or mud, but would like to have at my disposal the tool for the job when I go.

More than likely, there could be broken conduits, bad circuit grounding, or any number of issues sight unseen. However, I wasn't tasked with doing circuit troubleshooting, but rather to check the services. I would like to show proper grounding at the service to facilitate further investigation as needed. I just don't know if any of the testers can save that kind of data.

I found http://www.extech.com/instruments/product.asp?catid=34&prodid=591 as a three point tester, but have no idea if I have bare earth I can tap into at all the buildings. According to this page, http://ecmweb.com/content/who-cares-about-25-ohms-or-less , clamp on meters aren't always accurate.

Just curious if anyone has been down this road, before.

 

[infinity]

I'm not sure where this testing will take you. Only a single ground rod requires 25 ohms or less. Any other electrodes that are used can have a 1000 ohms of resitance and are still code compliant.

 

[TerryBarnhill]

I was mainly looking to show that the service is correct, (if indeed it is), then proceed to the circuit troubleshooting. If I go look at the grounding, find nothing amiss, then they have more issues with computer equipment it could make me look like I failed my inspection. Clients only see black and white, we all know that. I know the code says 25 ohms or two ground rods, but if I were to show what they actually had, that would further my cause. I really doubt there is much of an issue at the service connections, but I'm certainly willing to go look.

 

[infinity]

You said correct so that implies that it's code compliant which you could probably check by a visual inspection. The question then is what do you do next, simply report that it's code compliant and move on or try to find other solutions/problems.

 

[petersonra]

Howdy,

Anyone testing for 25 ohms?

I have been tasked to check several buildings scattered all over several miles around my area for grounding issues. The company has had several insurance claims due to what I was told, "lighting damages from poor service grounding". As it stands, I have not laid eyes on but one of these buildings up to this point. I don't know the age of the buildings, if they are made of steel, sticks, or mud, but would like to have at my disposal the tool for the job when I go.

More than likely, there could be broken conduits, bad circuit grounding, or any number of issues sight unseen. However, I wasn't tasked with doing circuit troubleshooting, but rather to check the services. I would like to show proper grounding at the service to facilitate further investigation as needed. I just don't know if any of the testers can save that kind of data.

I found http://www.extech.com/instruments/product.asp?catid=34&prodid=591 as a three point tester, but have no idea if I have bare earth I can tap into at all the buildings. According to this page, http://ecmweb.com/content/who-cares-about-25-ohms-or-less , clamp on meters aren't always accurate.

Just curious if anyone has been down this road, before.

I do not believe "poor service grounding" had anything to do with their problems.

You could have 1 Ohm ground resistance on the GES and it would make no difference.

Best bet is to put in some kind of SPD at the service point. That may actually help.

All the ground rods in the world won't help them much, if at all. More likely not at all.

Service grounding is either code compliant or not. You can tell that by a visual inspection.

Lightning protection has almost nothing to do with service grounding.

 

[infinity]

I do not believe "poor service grounding" had anything to do with their problems.

You could have 1 Ohm ground resistance on the GES and it would make no difference.

Best bet is to put in some kind of SPD at the service point. That may actually help.

All the ground rods in the world won't help them much, if at all. More likely not at all.

Since were discussing lightning protection wouldn't 1 ohm be pretty good?

 

[petersonra]

Since were discussing lightning protection wouldn't 1 ohm be pretty good?

would not change anything at all as far as protection from lightning goes.

it could be 1000 ohms and it probably would not matter either.

 

[dereckbc]

The earth impedance could be 1 ohm, but that would not help any lightning issues. The actual earth impedance is irrelevant, it is the deign and workmanship that make it work. If I had to guess like 99% of the installations I have investigated with lightning damage nothing was done to mitigate lightning damage. Yet it still met code compliance. Lightning protection has to be designed. It just does not happen by following code.

 

[ATSman]

Howdy,

Anyone testing for 25 ohms?

I have been tasked to check several buildings scattered all over several miles around my area for grounding issues. The company has had several insurance claims due to what I was told, "lighting damages from poor service grounding". As it stands, I have not laid eyes on but one of these buildings up to this point. I don't know the age of the buildings, if they are made of steel, sticks, or mud, but would like to have at my disposal the tool for the job when I go.

More than likely, there could be broken conduits, bad circuit grounding, or any number of issues sight unseen. However, I wasn't tasked with doing circuit troubleshooting, but rather to check the services. I would like to show proper grounding at the service to facilitate further investigation as needed. I just don't know if any of the testers can save that kind of data.

I found http://www.extech.com/instruments/product.asp?catid=34&prodid=591 as a three point tester, but have no idea if I have bare earth I can tap into at all the buildings. According to this page, http://ecmweb.com/content/who-cares-about-25-ohms-or-less , clamp on meters aren't always accurate.

Just curious if anyone has been down this road, before.

I have performed many ground resistance tests in my day whether it be a simple ground rod, multiple rods, Ufer grounding systems on large buildings, etc. All of tests were measuring the resistance (bond) between the grounding system and the earth which it is connected to and considered to be the lowest reference point. The type of earth, sandy, clay, how much moisture , depth of the rods, cadweld integrity, etc, during the test were all factors in the results of the tests. The 3-point test is the most accurate test since the test rods are out of the sphere of influence of the ground system (150 ft.) That is, if you have the open space to drive the 2 test rods. Otherwise the 2-point method has to do.
As far as the other posts stating that proper grounding has nothing to do with lightning damage, I beg to differ. Why then are lightning rods solidly grounded to metal water pipes or ground rods? These strikes tend to raise the voltage of the whole electrical system and will cause damage if it exceeds the insulating rating of the wiring or input filters of the devices. Sure SPD will help to mitigate the strikes but experience shows that they should only be considered a second defense to a properly grounded system. Only thru proper testing (not just visual code compliant inspections) can these systems be verified to be wired correctly and provide reliable service during all conditions.
I suggest you read the following which is considered to be the industry standard in ground resistance testing:
http://www.biddlemegger.com/biddle/A Stitch In Time.pdf
Also I would google Lightning Strike Protection for more info.

 

[AdrianWint]

From reading this thread I'm guessing that earth-loop impedance is not a standard tests that you guys have to do?

As part of our (UK) testing on any new install or service work we must perform an earth loop impedance test on the earth electrode with all bonding disconnected. We must then compare the result with previous results & do our calculations to ensure that the protective device will open in 0.4s with the earth fault current that would flow in that installation.

Do you guys not do similar?

Adrian

 

[GoldDigger]

From reading this thread I'm guessing that earth-loop impedance is not a standard tests that you guys have to do?

As part of our (UK) testing on any new install or service work we must perform an earth loop impedance test on the earth electrode with all bonding disconnected. We must then compare the result with previous results & do our calculations to ensure that the protective device will open in 0.4s with the earth fault current that would flow in that installation.

Do you guys not do similar?

Adrian

No, we do not.
One reason is that at 120V rather than 240V there is slim to no chance that current through the earth ground electrode would trip an overcurrent device (unless maybe with a Ufer ground (Concrete Encased Electrode) instead of a rod electrode.)
All of the US installation techniques rely on the Equipment Grounding Conductor (equivalent of PE) being bonded to the service neutral and that will be the return path for any fault current.

A fault to actual earth rather than to bonded metal will NOT trip a breaker. It will just energize the earth in the zone of influence of the fault connection.
At higher voltages like 480 maybe. And at medium voltages as used in distribution, the earth finally becomes a viable fault current path.

 

[AdrianWint]

No, we do not.
One reason is that at 120V rather than 240V there is slim to no chance that current through the earth ground electrode would trip an overcurrent device (unless maybe with a Ufer ground (Concrete Encased Electrode) instead of a rod electrode.)
All of the US installation techniques rely on the Equipment Grounding Conductor (equivalent of PE) being bonded to the service neutral and that will be the return path for any fault current.

A fault to actual earth rather than to bonded metal will NOT trip a breaker. It will just energize the earth in the zone of influence of the fault connection.
At higher voltages like 480 maybe. And at medium voltages as used in distribution, the earth finally becomes a viable fault current path.

Interesting.... we too use TNCS whereby the earth & neutral are bonded at the cutout (disconnect?) but we must still prove the earth loop impedance. In the case of TNCS it must be less than 0.8 Ohms

So does a TT install (earth & neutral not bonded, earth only via an electrode) not exist in the US? They can exist in the UK but the whole installation must be covered by a 30mA RCD (GFCI)

Adrian

Adrian

 

[dereckbc]

Do you guys not do similar?

In the USA using earth as a conductor below 600 volts is forbidden and worthless as a conductor. Only electric utilities can use earth as a conductor, but they operate at very high voltages.

In the USA earth impedance is of no real importance, nor capable of operating a over current protection device at either 120 or even your countries 240 volt system. Stop and think about it at what impedance is required to operate a even a simple small 20 amp circuit @ 120 or 240 volts in .4 seconds?

It is a bit of a trick but to solve the problem it requires the fault path to have an impedance low enough to produce at least 6 x rated current of 20 amps so we get 120 amps of fault current. Well that means it takes a 1 ohm fault path @120 volts or 2 ohms at your 240 volts. You could bury a copper mine of copper into the earth and not even come close to those impedance's. Now what do you do when you have a 30, 40, or 50 amp circuit for that HVAC or Hot Water Heater?. There is not enough copper in the UK or a deep enough hole to bury that much copper to operate a single breaker.

So us Yanks we just run an EGC with each circuit referenced to earth to provide a fault path to operate a OCPD. Cheap and effective. Does not even need to be referenced to earth to operate. So who cares what the earth impedance actually is as it does not matter. It could be 1 ohm or a 1000 ohms, makes no difference. Even at an impossible 1 ohm is of no use.

You can even make a case that the lower the electrode impedance becomes, the more dangerous it becomes.

 

[GoldDigger]

Interesting.... we too use TNCS whereby the earth & neutral are bonded at the cutout (disconnect?) but we must still prove the earth loop impedance. In the case of TNCS it must be less than 0.8 Ohms

So does a TT install (earth & neutral not bonded, earth only via an electrode) not exist in the US? They can exist in the UK but the whole installation must be covered by a 30mA RCD (GFCI)

Adrian

Adrian

TT does not really exist in the US on the customer side.
The closest thing is the floating (not earthed) secondary of a separately derived service (almost always three phase) or much less common a floating (not earthed) mains service from the power company (POCO).
These are used infrequently these days and POCOs are not offering new floating services. This setup is primarily for industrial processes where an unplanned power outage would be very expensive and/or could cause damage to production equipment. In those situations NEC requires a detector for a fault connection to ground (since no fault current will flow). That detection allows for a planned shutdown and repair of the equipment or wiring before a second fault can occur which would cause fault current to flow, including potentially a high current on the bonding/grounding connections inside the plant.

Since the EGC with bond to neutral is the designated fault clearing path, the resistance of the ground electrode is not really a concern.
Although specific industrial and communications environments may supply their own specifications under contract, the NEC only requires an earth electrode resistance to be less than 25 ohms.
And if a single rod electrode measures more than 25 ohms, all the NEC requires is that a second electrode be added, with no upper limit at all on the combined resistance of the two electrode system.

As you can see, the underlying design philosophy with respect to earthing and bonding is completely different.
But taken as a whole, each set of standards provides for a safe system, just by a different set of means.

The danger, if any, occurs when you indiscriminately try to mix the two systems.

BTW, we would find it extremely difficult to get an earth electrode resistance as low as .8 ohms even with the addition of Ufer grounds and connecting the water distribution system as an electrode.

 

[dereckbc]

Anyone testing for 25 ohms?

I found http://www.extech.com/instruments/product.asp?catid=34&prodid=591 as a three point tester, but have no idea if I have bare earth I can tap into at all the buildings. According to this page, http://ecmweb.com/content/who-cares-about-25-ohms-or-less , clamp on meters aren't always accurate.

OK first problem with a 3-point tester is it can take a lot of cable to make an accurate test, and it has to be made from several angles. It can take a full day to run the test. That Extech kit you show can only test a single rod system. Bet you money the longest lead is only 15 to 20 feet. A two rod system assuming the rods are spaced 8 feet will take a 64 feet test cable. Spacing or length of test cable has to be 8X larger then the ground field. So for example if you had a 3 rod system with 16 feet spacing between each rod is a total field of 32 feet. Your 100% test rod has to be a minimum of 256 feet away and not cross any underground metallic object and taken from multiple angles. Not realistic.

OK now let's talk about a LOOP TESTER like an AEMC 2117 Ground Loop Tester. This is the type of instruments utility companies, telephone companies, CATV companies all use to determine if a spec is being met. Are they accurate? Nope but who cares because they always read higher than true impedance. They read the impedance of the ground under test, plus the loop resistance.

So let's say I ordered a spec 10 Ohm Electrode. My contractor drove 10 rods spaced 16 feet apart and called it good. I go out with my loop tester and clamp the first rod at either end and read 9 Ohms. I sign off on the job as a job well done and well below spec of a less than 1 Ohm Electrode. How do I know that you ask? Because I know how the meter works. Remember I said the loop tester reads the UUT plus the impedance of the loop. When I clamped the single rod I got 9 ohms. That means the single rod impedance was just under 9 Ohms, and the series resistance of the remaining 9 rods is less than 1 Ohm. So I now know each rod is approximately 8 to 9 ohms, and with 10 of them in parallel gives me .8 to .9 Ohm Ground Electrode Impedance. Far exceeds my spec. Do I care if it is . 8 or .9 Ohms or the exact impedance. Nope I could careless because it exceeds my spec and .1 Ohm dos not make any difference. to me. I was looking for anything 100 ohms or less on the single rod. 9 blew it away.

So if your objective is to test for 25 ohms or less, get a Loop Tester. It will take you roughly 1 to 5 minutes to complete and can say with certainty if it passes or not. If the loop tester says 25 or less call it a day and collect a check because you are certain it is below 25 ohms. Done. You will not know exactly how low it is, but you will know if it is 25 or less.

 

[TerryBarnhill]

Thanks for all of the replies. I thoroughly enjoy reading everyone's opinions on the different subjects brought up here in this forum!

My initial thoughts on three point was I probably couldn't do one, based on parking lots being paved around buildings. I was leaning towards a loop tester, but just didn't know if anyone had used one. Although, $1600 out of pocket is quite a hole for something that I wouldn't realistically use all that often after this testing.

All good things to ponder!

 

[dereckbc]

I was leaning towards a loop tester, but just didn't know if anyone had used one. Although, $1600 out of pocket is quite a hole for something that I wouldn't realistically use all that often after this testing.

Yep they are pricey I have a used AEMC model 3730 in a case with all the factory documentation for sale if interested. $750 shipping included. PM me if you want it or have questions. It is in mint condition. I am not that far from you as I live just north of McKinney TX now up around Sherman.

Another option for you is to lease one. They go for $250/month with 1 month minimum. Do not know how many building you have to test or what you are charging, but either option is a legitimate business expense and can be written off on taxes. Even if not should leave you with a nice profit. Not sure what city or if all are in one city but with a Clamp On Loop Tester you should be able to do 8 to 10 sites a day in a town the size of Tyler. Using a 3 point deaf fall potential could take weeks. Time is money.

 

[brian john]

I have performed many ground resistance tests in my day whether it be a simple ground rod, multiple rods, Ufer grounding systems on large buildings, etc. All of tests were measuring the resistance (bond) between the grounding system and the earth which it is connected to and considered to be the lowest reference point. The type of earth, sandy, clay, how much moisture , depth of the rods, cadweld integrity, etc, during the test were all factors in the results of the tests. The 3-point test is the most accurate test since the test rods are out of the sphere of influence of the ground system (150 ft.) That is, if you have the open space to drive the 2 test rods. Otherwise the 2-point method has to do.
fo.

Where did you come up with 150' being outside the sphere of influence?

Typically we go 10 times the diagonal