Wires Failing in PVC Conduit

[cm1031]

Plant has a 277/480 volt service and it is having issues with wires going bad on the smaller circuits where the #12 THHN will be burnt off in the pvc conduit. One circuit the wires have been replaced twice and any ideas what would be causing this?

 

[charlie b]

Much more information is needed. Are these 480 volt circuits? What is the load on the #12 wires? What is the overcurrent protection? What is the ambient temperature? How long after a wire is replaced will it burn up again? Are there multiple circuits sharing the same conduit?

 

[cm1031]

More Info

More Info

20 amp three pole breaker, 480 volt, heat is not an issue, no other circuit wires in the conduit, and checking for the load on the wires. No time pattern between the failures.

 

[charlie b]

My prime suspect would be a load in excess of 20 amps, and a breaker not tripping on that overload due to internal failure. If you measure the actual load, and if it turns out to be less than 20 amps, then I have no more suggestions to offer.

But I do have two more questions.

1. Does the insulation fail at only one location (e.g., near a termination point), or all along the length of the wire?
2. How is this failure discovered (e.g., someone looks at the terminations, or smoke alarm goes off, or thermographic test, or a short circuit takes place)?

 

[mcclary's electrical]

20 amp three pole breaker, 480 volt, heat is not an issue, no other circuit wires in the conduit, and checking for the load on the wires. No time pattern between the failures.

Two things would have to happen at the same time then.

1) something overloading the circuit
2) OCPD is stuck in the closed position

 

[dpeter]

Perhap the wires in question have been subject to a voltage spike that has damaged the insulation and have arced and burned themselves clear or insulation was damaged as it was pulled.

 

[cadpoint]

Scope the run of pvc as a last result with all equipment checking out!

 

[LarryFine]

My guess is insulation damage coupled with water in the pipe. This would involve at least two wires.

 

[Ken9876]

Rodents are the cause of many shorts and burnoffs, if the pipe is big enough. Or it could be like others said, damaged pipe damaged wire, maybe use thicker insulated wire.

 

[Cow]

You could run run a rag or some sort of conduit piston through the conduit first to make sure it's clear. Then pull the new wire in and immediately meg it. Follow up by megging at routine intervals to see when the insulation starts degrading. You could also try XHHW rather than THWN.

 

[cadpoint]

Sudo cheap check, if the Client will pay for the servicing...

I think I'd run a fish through to the other end then attached a 4' large knotted rage and pull is back through just 3/4 the size of the hole, then go have the rag chemically checked.
Attack the rag to the side of wire or meter tape. Just be ready for the evidence,

Then, I'd pull a plug cleaner through, on meter tape. (also doubled ended) So if you get stuck you'll know where to dig!

I don't know what a session on a mass spectrometer is; but that'd be the fastest test around !

If your in an industrial situation, You might go back againest the weather records and note rain fall amounts... just a thought.

Good Luck!

 

[USMC1302]

What kind of plant or process is this serving? I think as some others may have alluded, could something have gotten into the conduit and is degrading the conductors? What do they look like when you pull them out. I have two wastewater treatment plants and the Hydrogen Sulfide tears everything up(none of the conduits were sealed). The enclosure components look like they're 50 years old.

 

[Pierre C Belarge]

My guess is insulation damage coupled with water in the pipe. This would involve at least two wires.

How long has the original conductors been in the PVC before this started to occur?


Once the fault occurred, there may have been damage to the PVC due to the fault. Each time the new conductors are pulled in, they get damaged.



Does the fault occur in the same location each time?

 

[mxslick]

Wish the OP would come back with some info on this..I doubt it since he's been here since 2004 with only 4 posts...:roll:

What he said sounds like he has had more than one circuit (pipe?) with this problem...if so my votes are for either voltage spikes/lightning damage or as USMC1302 said some corrosive/degrading gas/liquid.