# Confused About Megger Readings



## A Little Short (Nov 11, 2010)

I had a service call for flickering/dim lights and some receptacles not working. It was in a barn about 200' from the house main.

First I thought he had a loose or open neutral, but the voltage measurements didn't show that. I only showed around 190V to the sub panel at the barn but there wasn't any high voltage on one of the legs.
I had around 72V on one leg-neutral and around 119V -neutral on the other.

So I decided to meg the conductors. This is where I'm confused.

Readings from: Line A to neutral - 11G
Line A to ground - 11G
Same with Line B to neutral/ground

From line to line the reading was 0.0 M

I did determine from the voltage readings that the A leg conductor was bad. Probably underground somewhere since this was an underground feed. I just can't figure out why the reading was the same on both legs to neutral and ground.
I would have thought the bad conductor would have had a different reading.

I'm guessing the low reading on line to line is due to insulation damage along with damage to the wire its self. But why wouldn't it show on line to neutral and ground testing?


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## Spunk#7 (Nov 30, 2012)

Time to thump the leg with low voltage.


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## Celtic (Nov 19, 2007)

Spunk#7 said:


> Time to thump the leg with low voltage.


How?


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## Cletis (Aug 20, 2010)

Celtic said:


> How?


For 1 thing 11g is low for a meg reading


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## A Little Short (Nov 11, 2010)

Cletis said:


> For 1 thing 11g is low for a meg reading


:blink::no:


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## dronai (Apr 11, 2011)

What make and model megger do you have ?


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## Cletis (Aug 20, 2010)

Anything under 100 MOhms I am suspect, 50 M Ohms more suspect, 11 M Ohms very suspect


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## bobelectric (Feb 24, 2007)

We are talking meg ohms,not gigabits,right ?


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## bobelectric (Feb 24, 2007)

Cletis said:


> Anything under 100 MOhms I am suspect, 50 M Ohms more suspect, 11 M Ohms very suspect


 20M I would say OK.


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## Cletis (Aug 20, 2010)

bobelectric said:


> 20M I would say OK.


I've generally noticed micro-shorts or intermittent shorts happen between say 5000 ohms and 1 M Ohms on the 500 V pulse. Seems like dead shorts read from 0-5000 ohms


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## varmit (Apr 19, 2009)

Questions and comments:

A. Did you have the "main" breaker off in the barn - to avoid continuity trough any loads?

B. Is this wire aluminum? Aluminum wire is prone to dissolving if the insulation gets even a small pin hole in the insulation and the soil ph is acidic. Copper is susceptible to this also, it only takes a little longer for copper to break down. 

The cause of this conductor breakdown is the inductive heating between the conductors caused by the relatively low resistance between conductors and the inductive current caused by the downstream load.

C. 11 meg would be below the normal threshold for use. I would usually like to see around 50 meg ohm.

D. If the ground is rocky, it is almost certain to cause problems for direct burial conductors. Of course, even if the conductors are installed in conduit, which will almost always fill with water, and the conductor insulation is damaged during installation, there will be problems.


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## RobRoy (Aug 6, 2009)

Cletis said:


> For 1 thing 11g is low for a meg reading


Well Cletis, 1 giga ohm, is 1000 mega ohms. So, 11gohm is 11000mohm. Seems like a misunderstood interpretation on your part.:whistling2:


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## brian john (Mar 11, 2007)

11 gigohms =11,000 megohms

Direct burial?

It would be very unusal to have 11G Line to N and have dead short phase to phase

Did you isolate both ends of the circuit?

You have a load on the circuit? MY BET for now


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## Cow (Jan 16, 2008)

Is it aluminum wire? Direct buried? What are the voltage readings when the barn main is shut off? Is the wire properly sized for voltage drop and the load at the barn?


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## gold (Feb 15, 2008)

Did you open the ground and neutral conductors and meg them with the breaker open with respect to each other and each hot?

What kind of loads are on it? Wire size etc


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## A Little Short (Nov 11, 2010)

dronai said:


> What make and model megger do you have ?


Fluke 1507



Cletis said:


> Anything under 100 MOhms I am suspect, 50 M Ohms more suspect, 11 M Ohms very suspect


G= gigaohms = a sack full of megaohms:laughing:



varmit said:


> Questions and comments:
> 
> A. Did you have the "main" breaker off in the barn - to avoid continuity trough any loads?
> 
> ...





brian john said:


> 11 gigohms =11,000 megohms
> 
> Direct burial?
> 
> ...





Cow said:


> Is it aluminum wire? Direct buried? What are the voltage readings when the barn main is shut off? Is the wire properly sized for voltage drop and the load at the barn?





Goldagain said:


> Did you open the ground and neutral conductors and meg them with the breaker open with respect to each other and each hot?
> 
> What kind of loads are on it? Wire size etc


***Some answers in red above***

It is direct burial XLPE 2-2-4-6 AL

The readings (Meg) were taken with both ends of all 4 conductors disconnected.

As far as the voltage readings they were both with the main on and off.
There was a slight (2 volt) drop when the few loads that were on that were on the good leg.

I don't know if the ground was rocky or not. The feed goes through the edge of the horse pasture and then under a paved drive.

As far as loads, mainly just lights. There is a small water tank heater 120V that is used to keep the water trough from freezing.
There is also a 240V water heater, but obviously it wasn't working with one leg bad.

I do know there is some voltage drop because of the distance, but everything has been working ok until they lost one of the legs.

I'm not even going to try and locate the break in the conductor. I'm just going to refeed the barn.

I just don't understand why I didn't get a low reading on the bad leg to ground and or neutral, only line-line.


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## frenchelectrican (Mar 15, 2007)

I was reading this and the megaohms level you have is too low to be ok for be used as it is and you allready have a direct short or blowen conductor one of the two.

The biggest killer will be near the building where the conductours come out of the ground and paved driveway if buried less than .6 meter deep and paved driveway is good place for failure.

If this conductor been in the ground for more than 15 to 20 years then it will be a good time to replace it anyway due over the time you will get a pinhole leak on the conductour and it will eat away the alum conductour pretty good.

Merci.


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## nrp3 (Jan 24, 2009)

I have pulled out a section of 500 AL that was gone four inches of aluminum just from one pinhole.


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## A Little Short (Nov 11, 2010)

frenchelectrican said:


> I was reading this and the megaohms level you have is too low to be ok for be used as it is and you allready have a direct short or blowen conductor one of the two.
> 
> The biggest killer will be near the building where the conductours come out of the ground and paved driveway if buried less than .6 meter deep and paved driveway is good place for failure.
> 
> ...


I don't know how long the paved driveway has been there but it was definitely put in after the conductors. But everything has been working with the driveway in place for a while.
So I suspect the driveway and ground may have settled and has just now affected the conductors.

That is if that is where the break is. But is for sure a leading suspect!


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## brian john (Mar 11, 2007)

nrp3 said:


> I have pulled out a section of 500 AL that was gone four inches of aluminum just from one pinhole.


YEP aaaaaaaa


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## GEORGE D (Apr 2, 2009)

Yeah, I'm thinking you have loads connected and not all wires separated completely when you performed your test. With all loads off, even your voltage readings seem off.


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## A Little Short (Nov 11, 2010)

GEORGE D said:


> Yeah, I'm thinking you have loads connected and not all wires separated completely when you performed your test. With all loads off, even your voltage readings seem off.


There could be no loads connected if the feeder wires were disconnected at both ends! All the wires were completely separated when tested.


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## piperunner (Aug 22, 2009)

Well I just read each post when the poster asked the question he said what do you think is wrong with the test results .

Then someone asked what type of megger your joking right .

Not to jump in after the facts and the results but its great you megged it that checks the insulation . 

If you disconnected each end and tested the alu wire itself with a ohm meter that would have answered your question .
The megg reading is just for insulation. If its not grounded out it will read fine so that's what you had . 

Alu wire underground florida if you have a tiny cut or hole you cant see it with the your eyes and if you don't meg test it before you make it hot over time that tiny hole will fail . 

That's why we megg every conductor we pull it takes months to years before it goes but when it goes bad it will burn itself until the last twisted strand is holding on which would be a high resistance which would drop your voltage down by increasing resistance on that leg . Or you might get voltage across the water inside the conduit 
or the powered metal left over from the burnt out conductor.
I hear a lot of guys that say we don't pull in a ground wire we use the conduit for ground . To me every conduit should have a ground wire yet another code rule I see dropping the ball .

When we meg wire which is on every job the test is this you can get very high readings 250 meg ohms to infinity .There is no set reading standard its length location insulation were your at temp time rain cold hot that changes it .Your battery inside the megger can change your results good to bad low to high . Switch gear from 25 meg ohms up is fine some say 100 megg ohms is fine .

Ive had wire test at low readings say 75 meg to one leg to ground or another phase all the rest tested at above 150 meg turned it on under load came back an retested after one week and it was at 580 meg ohms all conductors . Damp wire wet wire new wire after time changes happen when we pull in new wire the water in the conduit gets into the insulation under load it will dry out by the nature of the heat .


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## Spunk#7 (Nov 30, 2012)

Did you thump it yet?


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## Big John (May 23, 2010)

It's not that it's impossible to have a L-L short and an 11kMΩ to ground measurement, but I agree with Brian, the odds are against it, especially underground. I think there's a connected load.

Did you measure 72V with a load on the conductor?


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## A Little Short (Nov 11, 2010)

Big John said:


> It's not that it's impossible to have a L-L short and an 11kMΩ to ground measurement, but I agree with Brian, the odds are against it, especially underground. I think there's a connected load.
> 
> Did you measure 72V with a load on the conductor?


Why does some keep saying I had an 11M reading when I clearly said 11G?
G=gigaohm

The Meg readings were taken with ALL conductors disconnected on both the main panel and sub. They were also separated from each other and anything else that they could touch.

The 72V reading was without load as everything on that leg was turned off because nothing worked. I did turn on the switch for lights that were on the bad leg but it didn't change much at all. But it wasn't phantom voltage because it didn't drop out when loads were turned on, although nothing worked.


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## jefft110 (Jul 7, 2010)

11kMΩ is 11G.


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## jefft110 (Jul 7, 2010)

Any chance that there's a tapped splice in the middle of the run feeding something else?


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## A Little Short (Nov 11, 2010)

jefft110 said:


> 11kMΩ is 11G.


I didn't see the "k" in Big John's post, but there were others that said "M" instead of "G"



jefft110 said:


> Any chance that there's a tapped splice in the middle of the run feeding something else?


0% chance


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## Spannerz (Aug 13, 2011)

Needs a thumper. 2 Ph with a 0.0M megger reading could mean a hole with water. Can you run off 1ph and disconnect the other? 

Is this cable, 3 individually insualted cores, with a screen? Or 4 Core no screen?


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## hardworkingstiff (Jan 22, 2007)

Man you guys are way more smarter than me. Based on the info in the OP, I would tell the customer he had a defective feeder to the barn and it needs to be replaced.


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## hardworkingstiff (Jan 22, 2007)

Little-Lectric said:


> The 72V reading was without load as everything on that leg was turned off because nothing worked. I did turn on the switch for lights that were on the bad leg but it didn't change much at all. But it wasn't phantom voltage because it didn't drop out when loads were turned on, although nothing worked.


Did you check for voltage drop across the OCP for the feeders? How about across the main for the barn? If you had a cable going bad over the years, I would think there would be a possibility of damage to the OCP device.


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## Big John (May 23, 2010)

hardworkingstiff said:


> Man you guys are way more smarter than me. Based on the info in the OP, I would tell the customer he had a defective feeder to the barn and it needs to be replaced.


 Stop trying to make this direct-and-to-the point! :laughing:

I agree, that's probably just the best bet, but you gotta admit those are weird test results:

Not for the least of which because if he has no resistance between legs, that means that _both_ legs would be faulted to each other, and yet they both have fantastic resistance to ground. Something just doesn't add up.

Stupid question: Did you put a DMM between them and see what resistance values it read on low scale?


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## hardworkingstiff (Jan 22, 2007)

Big John said:


> .... you gotta admit those are weird test results:
> 
> Not for the least of which because if he has no resistance between legs, that means that _both_ legs would be faulted to each other, and yet they both have fantastic resistance to ground. Something just doesn't add up.


The test results are not as weird to me as my understanding that the OP had the power turned on to the barn and the OCP did not open.

I agree, something is not adding up.


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## varmit (Apr 19, 2009)

After rereading your original post, i would believe that It would be possible that the two "hot" (current carrying) conductors are melted together (shorted) and that the neutral is burnt open completely. If the load was small, the lights would probably work from the continuity through the earth, but would be dim and or flicker if the load or earth moisture varied.

I have seen aluminum melt for several feet underground. The neutral wire could decompose for enough distance to not read on a megger between the hots and neutral. Since, I assume, the breaker has not tripped, this wire may have more than one bad spot in the run. In this scenario one of the hot wires is broken between the supply and the short preventing the supply breaker from tripping. At the hot to hot short there would be a high resistance connection causing the difference in voltage read at he barn panel. 

No matter what the actual cause may be, it appears that it is time for a new circuit to the barn.


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## Big John (May 23, 2010)

hardworkingstiff said:


> The test results are not as weird to me as my understanding that the OP had the power turned on to the barn and the OCP did not open....


 Not for the megger reading. He says he had both ends of each conductor in free-air.

OP, when you did your L-N and L-G megs, are you certain the bonding jumper was in place and this service was correctly earthed? My only thinking is that the fault does exist, but your ground-return path for your test was open, giving you abnormally high readings.

What voltage did you meg at?


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## gold (Feb 15, 2008)

If I got voltage readings like that with no load I'd switch testers then check the breaker and gec/egc before I meggered. Assuming it wasn't coming from the transformer that way. Did you check the voltage in the main panel? Under load and with no load?


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## A Little Short (Nov 11, 2010)

hardworkingstiff said:


> Man you guys are way more smarter than me. Based on the info in the OP, I would tell the customer he had a defective feeder to the barn and it needs to be replaced.


I already stated that I'm going to re-feed the barn.



Big John said:


> Stop trying to make this direct-and-to-the point! :laughing:
> 
> I agree, that's probably just the best bet, but you gotta admit those are weird test results:
> 
> ...


I did check with a DMM and it didn't show any low resistance.



Big John said:


> Not for the megger reading. He says he had both ends of each conductor in free-air.
> 
> OP, when you did your L-N and L-G megs, are you certain the bonding jumper was in place and this service was correctly earthed? My only thinking is that the fault does exist, but your ground-return path for your test was open, giving you abnormally high readings.
> 
> What voltage did you meg at?


What would the bonding jumper have to do with anything with the reading when I megged *all* the conductors disconnected and separated from each other?

I tried all the voltage ranges on the megger, but I forgot what the lower voltage readings were. They also showed low resistance but the 0.0 M was on the 1000V range.



Goldagain said:


> If I got voltage readings like that with no load I'd switch testers then check the breaker and gec/egc before I meggered. Assuming it wasn't coming from the transformer that way. Did you check the voltage in the main panel? Under load and with no load?


Voltage at the main panel was good.

*Update: After talking to the HO again he said it tripped the 100A feeder breaker in the main panel once before he called me. He turned everything in the barn off and then reset the breaker. He said he turned everything back on except the breaker for the 240V water heater. Nothing worked on the leg with the low voltage.*


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## gold (Feb 15, 2008)

Did you meg the breaker?


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## A Little Short (Nov 11, 2010)

Goldagain said:


> Did you meg the breaker?


No, only disconnected conductors.


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## piperunner (Aug 22, 2009)

Little-Lectric said:


> No, only disconnected conductors.


Well little lectric the pros here will recapture your post with how they do it. Just tell them your battery was low on your multi meter that you rented from radio shack and you contracting a sub contractor out of state with ground penetrating radar to locate the break . LOL This way they will all agree and you will get poster of the week . But take some pictures of your tool bag or the rented meter they like photos of that stuff . 
Sorry I just laugh at the comments this is typical on the forum .:laughing:
Next the engineers from MH will chime in and give us a code rule or formulas to solve this problem .


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## brian john (Mar 11, 2007)

piperunner said:


> Well little lectric the pros here will recapture your post with how they do it. Just tell them your battery was low on your multi meter that you rented from radio shack and you contracting a sub contractor out of state with ground penetrating radar to locate the break . LOL This way they will all agree and you will get poster of the week . But take some pictures of your tool bag or the rented meter they like photos of that stuff .
> Sorry I just laugh at the comments this is typical on the forum .:laughing:
> Next the engineers from MH will chime in and give us a code rule or formulas to solve this problem .


For once we agree, I really feel ET should close the electrical sections of this forum, screw discussing issues that may affect electricians, lets just keep the open topic section and open a new section the PR bl*ws himself section.:laughing::laughing::laughing::laughing:


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## brian john (Mar 11, 2007)

When we have any conductors, you would be remiss not to megger the cables.
Then if you have unusual readings you suspect, your testing process, then test set and what you overlooked. Unusual readings are generally from something the electrician overlooked. 

Second if we determine the readings are correct, we would get an underground fault locator. Locate the fault and make repairs.

I still feel something was overlooked.


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## varmit (Apr 19, 2009)

This "oh by the way" from the owner would seem to validate my post #35.


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## hardworkingstiff (Jan 22, 2007)

Little-Lectric said:


> What would the bonding jumper have to do with anything with the reading when I megged *all* the conductors disconnected and separated from each other?


I would like to know the answer to this question also.


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## brian john (Mar 11, 2007)

hardworkingstiff said:


> I would like to know the answer to this question also.


All conductors isolated on both ends?

THEN NOTHING unless it is the electron bank stored in the Earth messing up the readings?:001_huh::blink::no:


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## piperunner (Aug 22, 2009)

brian john said:


> For once we agree, I really feel ET should close the electrical sections of this forum, screw discussing issues that may affect electricians, lets just keep the open topic section and open a new section the PR bl*ws himself section.:laughing::laughing::laughing::laughing:


 
Well will leave that bl*ws stuff to the high tech qualified testing guys we don't do stuff like that. Is that part of your essential training Brian


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## Cletis (Aug 20, 2010)

Hey, I didn't go back and read anything but first post but if I got this call 

"I had a service call for flickering/dim lights and some receptacles not working.


i'd just be looking for a high resistance connection. probably use my extech cs70 at both ends and start dividing ?


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## hardworkingstiff (Jan 22, 2007)

brian john said:


> All conductors isolated on both ends?
> 
> THEN NOTHING


I think most of us agree with you, I was trying to cryptically call out the poster that made that statement.


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## denny3992 (Jul 12, 2010)

Cletis said:


> Anything under 100 MOhms I am suspect, 50 M Ohms more suspect, 11 M Ohms very suspect


Balderdash... 10-15 meg is ok on a motor...


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## pete87 (Oct 22, 2012)

There is a Dead Rat with Braces in the Ground .









Pete


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## Spunk#7 (Nov 30, 2012)

What happened when you thumped it?


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## A Little Short (Nov 11, 2010)

*Update*

I think I mentioned earlier that the HO told me after the fact that the 100A breaker that fed the sub at the barn had tripped a couple of times before he called me. That told me the reason for the low megger readings on the line-line.

I disconnected the leg that had the low voltage reading so that there was no line fault, and so he could have 120V to use the barn lights until a new line could be run.

I get a call yesterday that he has no power to the barn but the breaker hadn't tripped. I figure the other leg finally burnt through. I explained why the breaker didn't trip because I had already disconnected the other leg and there was nothing for the connected leg to short to.

It's funny how info always comes up after the fact! I noticed the indentation where the trench was filled coming from under his deck heading out through the pasture. It went about 15', then straight towards a fence post. I asked him when the fence was installed and he said only a few months!

He hadn't thought to tell me about the fence:no:, only about the paved driveway from a year or so ago. I had already planned on running a new feed, but I'm fairly certain the line was hit when installing the fence.

I don't have a fault locator but thought I would just try something. I connected my toner to the conductors at the panel, then followed the path from the house to the fence. I could just barely hear the tone until I got to the fence post. Then it got loud/strong and nothing a foot or so past the post. Pretty sure the break is there! So I think digging there and splicing (if I find the break) will be better than digging up the pasture since it's so wet out now.


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## Big John (May 23, 2010)

Interesting. Sounds like you found it, but I still can't explain the readings you got. Post pictures if you can.


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## papaotis (Jun 8, 2013)

always comes back to something they didnt tell you! thats why i try to ask all the right questions first! the problem is, you dont always know the right questions


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## brian john (Mar 11, 2007)

papaotis said:


> always comes back to something they didnt tell you! thats why i try to ask all the right questions first! the problem is, you dont always know the right questions


I seldom believe what they tell me.


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## A Little Short (Nov 11, 2010)

Big John said:


> Interesting. Sounds like you found it, but I still can't explain the readings you got. Post pictures if you can.


Careful what you ask for.......here's your pics!


Found the break right at the fence post as I thought.





Polaris blues to the rescue!





And the winning number is>>>:thumbup: with all loads on!


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## Big John (May 23, 2010)

I appreciate the pictures. I still don't see how it's can be that a direct ground fault in obviously damp earth that was severe enough to show dead short between conductors when megged, but would show up as 11G when measured line to ground. Are you certain you didn't inadvertently go to an isolated neutral bar or something?


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## A Little Short (Nov 11, 2010)

Big John said:


> I appreciate the pictures. I still don't see how it's can be that a direct ground fault in obviously damp earth that was severe enough to show dead short between conductors when megged, but would show up as 11G when measured line to ground. Are you certain you didn't inadvertently go to an isolated neutral bar or something?


As I said before, all the conductors were completely isolated from the panel(s) and from each other when I megged them.
When the readings were taken, I still had one leg, well didn't know then how bad it was, that was good enough to get close to 120V to the barn.
The other leg finally burnt/corroded through, then I had nothing to the barn.
The neutral and ground had a good reading and also didn't show any damage when this was dug up.

I can't explain the readings I got, but I am 100% sure the conductors were in free air when the test was done.


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## Wirenuting (Sep 12, 2010)

Big John said:


> I appreciate the pictures. I still don't see how it's can be that a direct ground fault in obviously damp earth that was severe enough to show dead short between conductors when megged, but would show up as 11G when measured line to ground. Are you certain you didn't inadvertently go to an isolated neutral bar or something?


I've seen that below a concrete galley floor. The conduit was rotted thru and the lines had melted thru. The short was between phases and yet clear to ground. If it wasn't for good prints we had at the hospital I would have been searching for a buried jobs. These prints were great, if the line drawing was drawn with a sweeping curve, you could be that the conduit also had a sweep. Old time prints are golden and we also had all the working as-builts with notes and curses and beer can locations..
It turned out that many of the runs with in 10' of a broken drain line were just rotting after 40 years. 
Each drain line we dug up we would expand the hole to include any nearby conduit..


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## Big John (May 23, 2010)

Wirenuting said:


> I've seen that below a concrete galley floor. The conduit was rotted thru and the lines had melted thru. The short was between phases and yet clear to ground....


 I've seen it in conduit, especially PVC, but it can be explained because you know there are air voids in there.

I can't explain it when everything is surrounded by wet dirt. I'll file it in the memory banks, maybe I'll run into it one of these days.


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## Wirenuting (Sep 12, 2010)

Big John said:


> I've seen it in conduit, especially PVC, but it can be explained because you know there are air voids in there. I can't explain it when everything is surrounded by wet dirt. I'll file it in the memory banks, maybe I'll run into it one of these days.


That was the only time I remember seeing it.. The ground below was clay above packed gravel atop clay. Digging below the gravel would allow water to puddle.. This was in the 15 story hospital and was only a quarter mile or so from the bluff over looking Lake Michigan.


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