# Making some sense of megger readings



## nrp3 (Jan 24, 2009)

Over the last couple of weeks I have been resurrecting some pole light circuits that have been out for a couple of years. These are all run with UF. I trace out the runs and use the Megger to find which segments are bad, then get out the fault locator to find the damaged areas. I then cut out the bad sections and use the Megger to see if there are more faults to be found in either direction on the run. I also double check the reading after I use the UF splice kits. What was strange in couple of instances was you would cut out a bad section and Megger in both directions and get seemingly good readings but splice it back together and the readings would drop to what would seem to be really bad. Thinking I made a bad splice, I would cut out the splice I made and redo it only to get the same results. Wondering if this is similar to what has been mentioned about long extension cords and gfcis. These cables are close to thirty years old and are buried no more than 6 to 12 inches. I dug out many tape and wirenut splices. Also wonder if in some cases my Megger technique might not be right, I am doing this as a fairly quick go, no go deal. Like to see a quick rise into the gig ohm range or assuming that's a good cable. These ones that are acting funky, I'm sure are compromised somewhat still, but are holding, no more than half a dozen cfls at most.


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## nrp3 (Jan 24, 2009)

I was thinking about it this morning and would guess this, the old parallel resistor deal we all learned in apprentice school, the total resistance of the circuit will be less than the smallest resistor in parallel.


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## mrmike (Dec 10, 2010)

Somehow, IMO, it has something to do with the splice changing the direction of the magnetic field, and showing leakage to the meggar.
Years ago I found a similar experience with a Tic Tac voltage tracer following a cables & came across a splice. The tracer would show voltage on one side of the splice & nothing on the other side. I found this many times after & informed all my fellow electricians to be aware of it......................


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## Big John (May 23, 2010)

Can you give some approximate numbers on what the "good" and "bad" megger readings were?

I'm wondering if for some reason after they are first installed, the readings on those splice kits are poor. Don't they have adhesive heat-shrink? When that adhesive is still warm maybe it has a tendency to track more.

-John


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## nrp3 (Jan 24, 2009)

My totally unscientific assumption that if the readings rose quickly into the gigohm range that the cable was ok. Bad would be down in the low megohms, or at least compromised. Unsteady readings as well. I tried a few different types, Ideal brand, ilsco, and some cully epoxy type. I only used a couple of the epoxy ones. Same problem occurred with the other two brands.

What I saw in the first case was 50 megohms H-N. Joining the two together caused a drop to 2.45 meg with a steady rise up to around 16 meg. H or N to G would go right to the gig range. The second case was N to G with even lower numbers. H-N would go to 500 meg or so after joining. That one I could see water had gotten in. You strip the wire in different spots and it was green.


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## Sparky J (May 17, 2011)

A megger expert I am not and I do find this interesting as I am trying to utilize a megger more for troubleshooting and verification. But could the funky readings have something to do with the water infiltration (or crappy installation and age). I mean when you cut the wire and tested it was fine but after splicing it back together there were bad readings.

This is one problem sometimes with UF especially poorly installed (depending on the soil type) you fix it at one break and you find a few more.

Depending on how many breaks your finding it may be more cost effective to scrap it and run new conduit and wire. IMHO


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## nrp3 (Jan 24, 2009)

The one I was working on most recently got hit when they replaced the sidewalk. Since it was only down maybe 8/10 inches, it got hit by the bucket of the excavator and pulled right out of the pole. It was all scuffed up and when I cut out as much as I could, you could see the moisture around the neutral and ground. I suspect that if it failed again, I could dig on the other side of the walkway and pound a piece of emt through to the other side. Cut out that section under the walk and most likely get the rest of the holes. You could see where the cable takes a nose dive heading under the walkway. Probably where it is compromised.


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## varmit (Apr 19, 2009)

A few ideas and statements:

If the cable is damaged even a small amount or spiced poorly, you will get low megger readings. 

If there is any moisture present, this will cause lower readings. 

If you are using a small, low current megger, it may not have the ability to put a charge on the entire run. Yes, total impedance comes into play.

Some soil types can dissolve conductors if there is any insulation damage.

It could be faster/cheaper to reinstall the cable than attempting a bunch of splicing and never being sure if everything is fixed 100%.

If the runs are several hundred feet long, you may need to leave the megger connected for several minutes to get an accurate reading.


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## nrp3 (Jan 24, 2009)

Using a Megger MIT 420. Not sure whether this has a high output or not, assuming not. I have seen an aluminum secondary totally dissolved through a small opening. Mostly short runs between poles, fifty feet or so. Digging ranges from soft sand, to bony, rocky hell. Moisture was definitely in the last one I worked on.


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