"Dry" Megger Tests are Practically Worthless (for building wire)

[Jon456]

(This is the eight installment in my "Adventures in Wire Pulling" series.)


Many of you will recall the problems we were having with the 480VAC branch circuit conductors to one of our 250 kW solar inverters. The electrical contractor who installed our 1MW solar system, improperly pulled three 500 MCM XHHW wires through an LB, damaging the wires. His final repair solution was to apply Liquid Tape and scraps of LFMC jacket to prevent the wires from shorting. For those new to this story and wishing to learn more of the backstory, please refer to this thread: Liquid Tape: An acceptable repair for damaged 480V wire? Since then, the contractor (through his lawyer) has refused to perform any additional repairs to these wires. So I proceeded to perform testing and inspection to determine the extent of the damage and what repairs were needed to be made.

I started with "dry" insulation resistance (IR) tests of the entire run for this branch: ~700 linear feet from the inverter back to our PV distribution panel. (Note: This run consists of ~300 feet in elevated EMT conduit strapped to the back walls of buildings, transitioning to ~400 feet in buried PVC conduit. The buried sections are assumed to be wet and were not the focus of these tests. Our concern was the condition of the wires inside the elevated EMT.) The wires were disconnected at both the inverter and at the distribution panel's 400A OCPD. The wire ends at the distribution panel were taped and suspended in free air; the IR tests were performed at the inverter end. I performed the insulation resistance tests with a Megger model MIT410, set for 1,000 VDC test voltage and 60 seconds duration. The MIT410 has a maximum resolution of 100G Ohms. Each of the three phase conductors (orange, brown, yellow) was individually tested, while the remaining two phase conductors were bonded to ground with jumpers. The dry test results were as follows:

Orange-to-Gnd:	12,500 M Ohms
Brown-to-Gnd:	4,800 M Ohms
Yellow-to-Gnd:	17,500 M Ohms

These results would seem to confirm the electrical contractor's assertion that all the wires are good for service. But I was not convinced.


The next step was to perform "wet" IR tests on the wires, starting with the ~30 section between the inverter and the LB. In preparation for the test, I placed a section of garden hose (with the metal male end cut off) in the bottom of the conduit outlet (where the conduit enters under the inverter), and then sealed the conduit outlet using American Polywater FST expanding foam duct sealant. By placing the hose in the bottom outlet of the conduit, I was able to fill the conduit with water from the bottom. It also allowed me to drain all the water out of the conduit at the conclusion of testing.

At the LB condulet, I created a dam inside, using standard duct seal putty, to ensure the water was able to fully fill the horizontal section of conduit being tested. I also used duct seal putty plug the outlet of the LB to prevent water from flowing through the LB into the other section of conduit. This was so the wet test would be isolated to only the first section of wire between the inverter and the LB.

I filled the conduit with water until it spilled out over the dam in the LB, then repeated the tests performed when the wire was dry. The first-section wet test results were as follows:

Orange-to-Gnd:	0.01 M Ohms
Brown-to-Gnd:	7,000 M Ohms
Yellow-to-Gnd:	0.03 M Mhms

Houston, we have a problem.

Since 0.01 Megohms was the minimum resolution of the MIT410, I decided to perform a standard resistance test to ground on each of the two failed wires using only the Ohmmeter function of the meter (which uses its un-boosted battery pack voltage of just 4.5 VDC):

Orange-to-Gnd:	6.2 K Ohms
Brown-to-Gnd:	not tested
Yellow-to-Gnd:	24.3 K Ohms

In other words, insulation faults that a 1,000V megger test could not diagnose while the wires were dry, were able to be easily detected with a 4.5V ohmmeter when the wires were wet!

 

[Jon456]

I then began disassembling the raceway, starting at the LB. Using a sawzall, I cut through the conduit just past the outlet of the LB (grid-side of the LB), also cutting the three phase conductors and the EGC within. Removing the ~1 foot ends of the wires that had been severed, I found prior damage to the insulation on two of the wires (yellow and orange). This is where the electricity had arced phase-to-phase, and some of the copper wire strands had been melted, as well as the surrounding XHHW insulation. There was also black soot residue on the wires in the vicinity of the damage. Undoubtedly, this was the fault that caused our PV distribution panel's 400A breaker to trip 3 times last year. As I had surmised, the copper wire burned away to the point that the fault cleared itself.

Note that on the orange phase wire, the electrical contractor had applied liquid tape directly over the spot where the insulation was burned through by arcing; there was very little liquid tape elsewhere on this wire. (See the last photo above. The grey stuff to the left of the hole are remnants of the duct seal putty I used to dam the conduit for the wet tests.) That the contractor had brushed the liquid tape onto the wire exclusively at the location of the damage is strong evidence that he saw that damage to the wire when he was making his improvised repairs. In other words, he had to have known how serious that damage had been: he wasn't just covering a nick in the insulation as a precautionary measure; he was covering a fault in the wire with obvious signs of arcing and burning.


Next, I released the compression fitting at the inlet of the LB (inverter-side) and slid the LB off the end of the conduit and wires that traveled down to the inverter. At that point, it was possible to examine those wire ends and look into the open conduit. There was a deep cut in the insulation of the yellow phase wire.

But there was extreme damage to the orange phase wire from where it had been pulled in past the metal edge of the LB.

Because of the extensive damage to this orange phase wire, I removed the entire horizontal section of elevated EMT conduit (by sliding it off the wire) back to the 90 deg sweep, exposing ~12.5 feet of wire. I used nylon webbing straps to suspend the wire and prevent any new damage.

This revealed the extent of the damage to the orange phase wire:

Remember that all this damage successfully passed dry megger tests!


I did redo wet tests of all the wires remaining in inside conduit and all passed.

 

[Cow]

What was his response to these photos?

 

[GoldDigger]

What was his response to these photos?

You mean his lawyer's response?

 

[Jon456]

What was his response to these photos?

There was a lot more testing and work involved that described above. I omitted that information from the write-up for the sake of brevity.

Initial megger tests were performed on August 8th and 9th (on the 9th, the wires were cut in the LB to perferm the second set of wet tests). I wrote two reports, the first on the 8th and the second on the 11th. Our facility General Manager sent a copy of each report (with photos) to the installation contractor on the days I submitted them. We offered the installation contractor the opportunity to personally witness and confirm our tests. Instead, he had his lawyer send a letter to our General Manager. I have not seen this letter, but I was told that he objected to me cutting the wires and performing any of this work.

On August 14, we hired an independent electrical contractor to replace the last of our damaged solar feeders (one which the installation contractor had refused to correct under warranty). While this independent electrical contractor and his crew were on site, we had him witness and verify my megger tests at inverter 4. After the tests were duplicated and confirmed, I cut the conduit at the LB outlet and removed the one foot severed ends of the wires (the pieces containing the phase-to-phase fault). I showed this damage to the independent electrical contractor so he could also witness for us the damage and evidence of arcing.

On August 15, I removed the horizontal section of conduit from the inverter-side of the LB, exposing the extensive damage to the orange-phase wire. On August 16, I submitted my third report, which was then forwarded to the installation contractor. Again, we wanted to afford him the opportunity to personally inspect and witness the damage. He was also given the option to complete the repair work himself. As of this time (end-of-day on the 18th), he has not responded to my last report. Therefore, we have scheduled the independent contractor to come back tomorrow to complete the repairs.

We are losing $250 per day in electricity generation revenues, for each day this inverter is offline. We cannot afford to wait any longer to get it repaired.

 

[cadpoint]

I think I'd of gone back five more feet around the corner from where you stopped!

What a mess!

 

[ptonsparky]

I would have thought things were screwy when the first tests showed such a wide disparity. The Brown to Grnd was 1/4 to 1/3 what the others were.
Agreed, real mess. Nice job. Let us know how the legal actions turn out. If you can.

 

[kwired]

I would have thought things were screwy when the first tests showed such a wide disparity. The Brown to Grnd was 1/4 to 1/3 what the others were.
Agreed, real mess. Nice job. Let us know how the legal actions turn out. If you can.

I kind of suspected something because of that one reading as well, but at same time that one low reading was still high enough to be considered acceptable - at least for now. But he did prove there were problems just waiting to show up someday with this install.

 

[Jon456]

I think I'd of gone back five more feet around the corner from where you stopped!

Don't worry, I didn't stop. I just didn't elaborate on that part because no additional damage was discovered in that direction.

I removed a 6.5 foot section of conduit from "around the corner." In fact, I exposed & inspected this section on the back of the building before doing the 12.5 foot section on the side.

All the exposed wire looked good, including as far as I could see into the conduit end. Then I plugged the end of the conduit with duct seal, refilled it with water and repeated the wet megger tests. The wire tested good. I also did new wet megger tests with the short vertical section of conduit from the inverter up to the 90 degree sweep.

 

[Jon456]

I kind of suspected something because of that one reading as well, but at same time that one low reading was still high enough to be considered acceptable - at least for now. But he did prove there were problems just waiting to show up someday with this install.

After the installation contractor made his improvised repairs with liquid tape and LFMC jacket, he dry meggered the wire. He was using a Fluke 1587 which has a maximum rated resolution of 2000 M Ohms, so he wasn't even able to see any difference between the three phases: each wire maxed out the meter's scale, showing >2.2 G. (Not that it would have made much difference to him: in his mind, any test result greater than 100 M Ohms was acceptable.)

Of the three wires, the brown phase had the lowest dry test reading. But it was the only wire without any damage. The orange phase (with massive insulation damage and bare copper strands exposed inside the EMT) tested dry as having 2.6 times the insulation resistance of the undamaged brown phase, while the yellow phase (with a burned hole in the side and vaporized copper/carbon residue on the surface of the insulation from the arcing) tested dry as having over 3.6 times the insulation resistance as the undamaged brown phase.

I still haven't been able to come up with a logical explanation for these anomalies.

 

[Sierrasparky]

So has your company filed a complaint with the contractors license board. The'll cook his goose for not making the corrections.

I really don't know why people don't file against these type of hacks.

 

[Jon456]

So has your company filed a complaint with the contractors license board. The'll cook his goose for not making the corrections.

I really don't know why people don't file against these type of hacks.

It's not my place to comment on that.

I can, however, imagine several reasons why people don't file complaints.

 

[ActionDave]

That looks like xhh insulation. To ding it up that bad you either have make it your mission to do that much damage or be a some kind of deranged idiot that has no business pulling wire.

 

[Jon456]

That looks like xhh insulation.

You're right. XHHW-2, to be exact. The original plans called for THWN. At the commencement of construction (when I was brought on to this project) I recommended to the owner that we upgrade to XHHW because it's a superior insulation. So we paid a hefty premium to the contractor for that wire. Now we have splices all over the place.

To ding it up that bad you either have make it your mission to do that much damage or be a some kind of deranged idiot that has no business pulling wire.

The correct answer is (B).

 

[Jon456]

Correction...

I had previously stated that the arcing fault was phase-to-phase. That was an error. Yesterday morning I re-examined the LB that I had removed last Thursday. I cleaned off the remnants of liquid tape and duct seal putty, and unscrewed the compression connectors from the LB body. That's when I discovered two places on the inside corner of the LB outlet where arcing had eroded the aluminum:

It's remarkable that we had two separate phase-to-ground faults that took ~6 months for both to manifest themselves, and then both cleared themselves (by arcing and burning away metal) within a few weeks time!

 

[GoldDigger]

Correction...

I had previously stated that the arcing fault was phase-to-phase. That was an error. Yesterday morning I re-examined the LB that I had removed last Thursday. I cleaned off the remnants of liquid tape and duct seal putty, and unscrewed the compression connectors from the LB body. That's when I discovered two places on the inside corner of the LB outlet where arcing had eroded the aluminum:


It's remarkable that we had two separate phase-to-ground faults that took ~6 months for both to manifest themselves, and then both cleared themselves (by arcing and burning away metal) within a few weeks time!

Definitely an unapproved modification to the LB!

 

[Jon456]

Definitely an unapproved modification to the LB!

So was the liquid tape! :rant:

 

[GoldDigger]

So was the liquid tape! :rant:

Actually, painting the inside of the LB with liquid tape probably would have been OK if he had not damaged the LB first.
Similarly the liquid tape and plastic on the wires would have been OK if the insulation under them had not been damaged first. :angel:

 

[starbolin]

Thank you for your excellent detective work, for writing it up, posting here, and for demonstration of the value ( cough, cough ) of dry megger tests. The actions you took as far as to dissassemble first then press your case were really the only options left to you. I am sorry you were put into this situation. I am glad that you were able to confirm your suspicions.

 

[gadfly56]

I suppose that it would be totally out of line for anyone to suggest that "someone" ought to throw a blanket party for the contractor and his lawyer. :angel: