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Donuts > Fried Eggs
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Discussion Starter · #1 ·
I realize that you need to disconnect the electrode system from the utility before doing a resistance test on it, and I understand why.

The problem I have is we often run into a lot of pushback from customers/ECs/etc, especially on commissioning, when they realize we need to "modify" a system they just finished installing.

I'm trying to figure out if there's a possible work-around that allows us to leave the electrode connection in place.

Because the test-set pushes out pulsed DC it's easy enough to monitor where that current is flowing. What do y'all think of the idea of making an off-the-cuff assessment of the amount of current (if any) flowing along the GEC back towards the utility connection, to try and determine how much the utility electrode system is impacting the test?

Obviously the test wouldn't be up to NETA standards, but as it stands now we have clients who simply say "Test it as-is." and that definitely isn't up to NETA standards.
 

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I know it too late but test the ground grid before commercial power is hooked up.

If not possable the next best thing is to shut off the mains and pull the neutral. Get a good base line, hook everything back up and repeat the test and see if your calculations are valid for future testing.
 

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Donuts > Fried Eggs
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Discussion Starter · #3 ·
...If not possable the next best thing is to shut off the mains and pull the neutral...
It's amazing how many jobs we get to commission sites that are already running. "They turned it on? So why the hell are we going, they commissioned it themselves when they threw the breaker!" :laughing: But that's the unfortunate reality.

I don't even have to lift the neutral, often I can separate the GEC from the system, and if there's no current flow, I'm comfortable doing that. But often we're told not to even do that.
 

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I realize that you need to disconnect the electrode system from the utility before doing a resistance test on it, and I understand why.

The problem I have is we often run into a lot of pushback from customers/ECs/etc, especially on commissioning, when they realize we need to "modify" a system they just finished installing.

I'm trying to figure out if there's a possible work-around that allows us to leave the electrode connection in place.

Because the test-set pushes out pulsed DC it's easy enough to monitor where that current is flowing. What do y'all think of the idea of making an off-the-cuff assessment of the amount of current (if any) flowing along the GEC back towards the utility connection, to try and determine how much the utility electrode system is impacting the test?

Obviously the test wouldn't be up to NETA standards, but as it stands now we have clients who simply say "Test it as-is." and that definitely isn't up to NETA standards.
Under this circumstance test with a clamp on ground resistance tester (AEMC 3711) You won't need to lift the N-G bond.
 

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The number of jobs we get called to test

The day before they occupy the site
Downtown with not exposed ground or no way to go out even 100'
To test a grid 100'x100' on a 100'x100' plot of land and no trespassing signs all over.
The ufer, driven electrode, building steel, cold water and system bond are all bare coming out of poured concrete and no one knows which is which, continuity is "0" between them
And the engineer and EC are crapping their pants
 

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Under this circumstance test with a clamp on ground resistance tester (AEMC 3711) You won't need to lift the N-G bond.
Not a valid test, once electrical is connected you put every ground rod on the electrical system in parallel.
 

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Not a valid test, once electrical is connected you put every ground rod on the electrical system in parallel.
I agree but under the circumstances what choice do they have if they can't lift any bonds or grounds? Testing "as is" would be invalid as well.

Kinda weird this is a commissioning test as they have already lit this thing up.
 

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I agree but under the circumstances what choice do they have if they can't lift any bonds or grounds? Testing "as is" would be invalid as well.

Kinda weird this is a commissioning test as they have already lit this thing up.
90% of the test we are called to perform are on line connected and we spell this out in our quotes. That we must test the electrode prior to final connection to the system.

Heck one job I went early to explain this to the electrician and he stilled screwed the pooch.
 

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90% of the test we are called to perform are on line connected and we spell this out in our quotes. That we must test the electrode prior to final connection to the system.

Heck one job I went early to explain this to the electrician and he stilled screwed the pooch.
Preaching to the choir....
 

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Donuts > Fried Eggs
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Discussion Starter · #11 ·
Under this circumstance test with a clamp on ground resistance tester (AEMC 3711) You won't need to lift the N-G bond.
When doing a grid or even large building with lots of interconnected metal, in my experience the accuracy with those is much less than a fall-of-potential. While I agree the high frequency tends to rule out the utility MGN, it can also exclude distant parts of a very large grid, while concurrently reading all the adjacent parallel paths of the grid.

I've done compliant tests with a fall-of-potential and gotten ideal results, and couldn't duplicate any of them with the clamp-on tester.
The number of jobs we get called to test

The day before they occupy the site...and the engineer and EC are crapping their pants
Someone who feels my pain. I've had a couple of cases where I've shown up and reminded the guys that it says in their specs that we need to disconnect and megger every single feeder that they just spent the last two weeks terminating.... I instantly make a lot of friends.

So when you guys run into a similar situation do you just put a footnote "Test not valid to NETA specs"? That's basically what we do now.

Also I thought you guys might get a kick out of this, I was looking through IEEE-81 and found this:
...This influence is determined and allowed for during the test on ground grids or deep-driven ground rods of 1 Ω ορ λεσσ. Ιν τηε χασε οφ σµαλλ-....
Great editing, boys. I hope that second part doesn't say something important. :laughing:
 

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There isn't much else to say, if they want an accurate test accepted by industry standards you have to isolate the grid to test it.

Tell them it is like wanting an oil change but telling them to not turn the engine off. :blink:
 

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Donuts > Fried Eggs
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Discussion Starter · #14 ·
There isn't much else to say, if they want an accurate test accepted by industry standards you have to isolate the grid to test it.

Tell them it is like wanting an oil change but telling them to not turn the engine off. :blink:
Nuts.

What about using the star-delta method to deliberately stay within the zone of influence of the test grid? It would make it much easier to exclude utility influence.

I didn't see that method specifically described in IEEE, but it seems to be a variation of the "Ratio Method."
 

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It's a mess if they're up and running. Even if they're all hooked up but the power is off we still need to undo some of their work to test properly.

The very people who call us in to test are the ones who allowed the power to be turned on and the EGC to be connected.

Really all they want is a certificate from us sharing the liability.
 

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There isn't much else to say, if they want an accurate test accepted by industry standards you have to isolate the grid to test it.

Tell them it is like wanting an oil change but telling them to not turn the engine off. :blink:

My grandmother use to say You might as well take a bath and not change your socks and underwear.
 

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we did some work for the navy and they spec'd that the ground grid and downcomers from the aerial grid had accessible handholes with grid connections that had to be removeable (we found some erico or robinson two-piece connections that were approved). if it's too late, then it's time for the jackhammer and some cad-welding I guess (at the contractor's expense, if that was spec'd)
 

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Donuts > Fried Eggs
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Discussion Starter · #18 ·
Incidentally, maybe I'm really behind the curve on ground-grid testing, but a lot of manufacturers are offering models with what's called "selective mode" testing, which appears to be exactly what I was talking about above: It uses CTs to look at other current paths and automatically guards out any current flow that isn't going through the earth on the electrode under test.

We don't have any ground resistance testers that do that, but I'm betting I can use DC ammeters and get a pretty accurate reading and just do the math myself.

It wouldn't be a measurement I could stand behind for commissioning, but I might be pretty darn comfortable using that method for just general testing (with obvious footnotes).

I'll run some tests and keep y'all posted if it appears to work.
 
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