Cable Heating From Using Steel Conduit Elbows? 5

[tefaber]

We had an interesting situation come up here recently...wondering if anyone had seen anything like this:
The city was seeing intermittent trips on a 15kV breaker which was tied to an underground feeder. Within a week, one of the phases (they run each phase in a separate conduit), faulted. They pulled the faulted cable out and replaced it. Within a week, another phase faulted. Both of the faults occurred at a 90 degree turn in the conduit. The conduits were installed as pvc with steel elbows at the turns. The cable manufacturer said that by using the steel elbows, the magnetics would create heat causing the breakdown. The city only was using about 60 percent of that cable's capacity so I can't see how the magnetic heating effect could create enough heat to push the cable's temperature high enough to create that fault. Any thoughts/comments?

 

[Skogsgurra]

Interesting.

You have 535x0.60 = 321 amps in the 13 kV circuit. And 430x.80 = 344 amps in your 4 kV circuit.

And you have what is supposed to be magnetic heating in the 13 kV circuit and not in the 4 kV circuit (where you actually seem to have a higher current). Are conditions comparable otherwise? I do not understand US cable numbers - is the 4 kV cable a single phase cable like the 13 kV cable is?

Gunnar Englund

http://www.gke.org

 

[MakeItSpin]

I had an experience with this phenomenon years ago at the beginning of my career.

We were sent out to wire a large submersible well pump, something along the lines of 200hp. When we got to the site there were three individual conductors stubbed out of 3-1" threaded hubs on the top of the wellhead. Re-pulling the cable was not an option, the well was several hundred feet deep with the motor at the bottom. My boss at the time decides to use 3 1 x 12 rigid nipples to support a junction box, running each wire of the 3 phase through it own nipple. When we started it up it was a matter of seconds before the 12" nipples were too hot to touch.

We bought aluminum nipples, sawed between the knockouts in the junction box like NEC requires, end of heating problem. The top of the well head with the three hubs was cast iron, I realize there were eddy currents there, but the mass of the casting and the cold water moving past kept it cool.