(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:
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:
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):
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!
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!
