OT: Spiral burn on motor cable

Anybody ever see this before?

I've got a 60HP motor on a drive that has been drawing a little higher than normal currents, about 125%, something noticed during a routine inspection. Mechanically there is no explanation. Motor megs out normal. After further investigation Otis found that one of the wire legs has a spiral burn in the insulation that follows one of the strand bundles. The spiral burn mark is about two to three feet long. I've never seen a wire exhibit a burn pattern like that before, I would have guessed that if there was a poor connection that currents and heat would have distributed across the cable within a few inches, not following one of the strand bundles for a couple of feet. Just wondering if something was wrong with the cable - Otis is replacing it now. Since there probably isn't much the combined forum hasn't seen, has anyone seen that before?

Never seen it before, but if it's only one core, and only for a short length, I'd suggest that it's probably a dodgy section of cable - impurities in the copper perhaps? If it was due to external heat, all the cores would have been affected the same way. If it was due to one phase being overloaded, it would have been the whole length of the cable. If it were due to a loose connection or a hot joint, as you have said, it would surely dissipate in a few inches. For it to be only in one section and one core would seem to indicate that that piece of copper has a higher resistance than the surrounding copper and cores, and therefore generated more heat. And I guess once it started to burn, the resistance would have increased and it became a self-perpetuating cycle.

Just my own wild guesses based on what you described. If it were me, I'd save the piece of cable and dissect it later. I hate not knowing why

The only thing I can think of, besides the impurity possibility, would be a termination so bad that most of the current was trying to travel in that single strand, until at that distance, the casual contact with adjacent strands became good enough to even out the current flow among more of them. What kind of cable, what kind of terminations?

SO cord. Not really a fan of it with a drive, I prefer drive cable if a cable is going to be used. But it is what the contractor used. I had a chance to inspect the wires with the insulation peeled back. It was a typical cable where strands are twisted into three 'cords' that are twisted around each other. The strands under the burned insulation exhibited typical overheating discoloration, but the strands in the other two cords had an odd color to them. It didn't look thermal, it looked like a yellow patina, not the dark orange-brown of copper that has been hot. I've been in a wire manufacturing plant and watched cables made, and for the life of me, considering how they are made, I can't imagine two parallel cords of strands being contaminated with something and the third one not. So I have to conclude that patina it is not what it appears to be but is thermally caused. Yet at the same time only one of the copper cords was getting hot. I've not made sense of what I'm seeing here. Oh well, I don't have time to analyze it and I'm not curious enough to send it down to our met lab.

Maybe there is a problem with the drive?? If one phase has higher voltage spikes than the others (assuming this is an inverter) it might have degraded the insulation similar to the way an inverter can break down the windings in an old motor.

How is the cable routed? Maybe it was caused by an "induced" current caused by being near a high current or voltage load nearby??

At a previous job, I have seen THWN wire used on a drive produce some funny looking marks when it fails. Usually, I would find holes in the insulation near the spot where it eventually shorted, but the pinholes would be all around that area. I told the boss we need to use XLP wire but apparently it is cheaper than downtime and motors and labor and troubleshooting errors to "Do what we have always done"...

Some scientist could probably examine the wire or even a picture of it and provide a rational explanation.

Interesting. I'd love to see a picture of it once you're done replacing it.

Man ... a 60 HP motor on SO

maybe something happened to it during manufacture
one time I had a piece of (much smaller ) SO where all three conductors were connected together under the rubber sheath I assume some operator ran outta conductors ... connected them to new conductors and ran the whole shebang through whatever puts on the sheath

We have a few motors fed with SO cable. It's nice and flexible for motors that need to move. I just finished repairing an 80 HP motor with a flexible cord lead. The teck cable on the other side of the junction box got hit.

Were there any breaks in the strands of the failed conductor?
While SO Cord is very flexible, if it is subjected to extreme or sharp bends (pulling around a sharp corner for example, or stored folded rather than rolled) individual strands can easily break reducing the capacity of the conductor.