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?

 

[cuky2000]

Elbows is one of the areas were cable are exposed to high mechanical stresses do to wall pressure during the installation phase. If additional stresses are added in this area during cable operation, a premature failure may happen sooner than anticipated. Here is a possible scenario to explain the premature failure in the cable:

Single conductor cable routed in separate ferrous metallic conduits produce a magnetic field induction creating circulating current throughout the metal conduit elbow. The heat generated by the Joule effect (RI^2) may exceed the thermal rating causing a premature failure in the insulation.

Cables in ac circuits should not be installed with each phase in separate magnetic conduits under any circumstances due to the high inductance under such conditions. Cables in a-c circuits should not be installed with each phase in separate metallic non-magnetic conduit when their size exceeds 4/0 unless the conduit is insulated to prevent circulating currents.

http://www.okonite.com/engineering/shielding.html

 

[davidbeach]

Elbows do cause mechanical stress during pulls, but that is stress to all of the cable that passed through the elbow. Since both failures are reported to have occurred at the elbows, I would not suspect pulling damage as that would show up at random locations beyond the elbow.

 

[cuky2000]

Davibeach,

Failure on any electrical component is characterized by the sum of the stresses in time.

We are suspecting the thermal stresses are the lead cause of failure and we may add that metallic elbow is not a good engineering practice with single conductor cable. However, at this moment there is not sufficient information to rule out any potential combination of causes or determine the exact reason of the failure.

 

[davidbeach]

cuky2000, I completely agree with your first statement.

On the second statement, while there may be other contributing factors the description of the failures is exactly what is to be expected given the steel elbows. With an installation problem known to cause the exact failure found, and a problem that is expected to cause such failures in the future, why look for other possible causes rather than just dealing with the one?

 

[Skogsgurra]

Davidbeach,

I appreciate the education, but I still insist that the magnetic heating is (close to) an I2R effect. It is well known that hysterisis has a slightly higher exponent. Steinmetz, I think, used 2.3 - 2.6 for hysterisis heating. Depending on steel quality, frequency and penetration.

The eddy current effect is a pure I2R effect. It is hard to know which effect is dominant as long as we do not have data from the installation.

So, with the higher exponent (2.6 instead of 2) the resulting heat would be even less at 60 percent current - about 13 or 14 percent. If we only have eddy currents, it would be 36 percent. As said before.

What I mean to say is that one should not always jump to the first conclusion that presents itself. It is sometimes wise to think twice - or thrice.



Gunnar Englund

http://www.gke.org

 

[davidbeach]

skogsgurra, I haven't tried to figure out or look up the effect of additional current on inductive heating, so you may well be correct on the impact of additional current on the amount of heating. My point is that it is the unbalanced current that is doing the heating. If all three phases had been installed in one conduit, the amount of current doing the inductive heating, even with the cables carrying their maximum current, would be very close to zero. In the case under consideration, the current causing the heating is many, many times that near zero value. If the expected net (zero-sequence) current were 5% of rated current, we are now seeing 60%, or about 12 times. Square that 12 (or raise it to the 2.6 power if you wish). Now we are looking at 15,000% {',' per US convention of thousand's separator, not European convention of radix} or more, of the heating that would occur with limited net current. Even if the heating to be expected for a balanced 3-phase were extremely small, 15,000% of that amount can be enough to cause significant damage. It is that extreme heating compared to the balanced three-phase case (or a properly constructed segregated phase installation) that makes me question the need to find other causes.

When there's a four foot diameter hole in the bottom of the boat, why look for slow leaks around the joints?

 

[Skogsgurra]

Sorry davidbeach, I know all these things. Including the "news" that currents in a three-phase cable cancel.

The OP presented the "four foot diameter hole" and said that he would like some comments and thoughts beyond that. That is what I have tried to give him. I am sorry that you do not like that.

Gunnar Englund

http://www.gke.org

 

[davidbeach]

skogsgurra, the way I read the question, he was discounting the possibility of heating caused by the steel elbows. What I took was the cable manufacturer blamed the steel elbows, but the OP wanted to find the "real reason". My guess is that the steel elbows will be quite difficult (expensive) to get to and replace and he wants a magic bullet.

I read your comments as discounting the possibility of the steel elbows generating enough heat to cause the problem, if that was not your intent, I'm sorry I misread.

Sure, there could be many other things wrong with that circuit, but as long as those steel elbows are in place, the other problems won't have enough time to develop a failure before the elbows melt the insulation again.

 

[Skogsgurra]

Agree. I do not usually "behold the mote that is in my brother's eye, without considering the beam that is in mine own eye" Or how to put it. I wouldn't have done that (using steel elbows) either. We cannot get any further than this, I guess.

Gunnar Englund

http://www.gke.org

 

[lengould]

One minor addition might be that a contributing cause to "the failure always occuring at the elbows" is that that's the (only/best) conductive path to ground or another phase, the rest of the run having the individual conductors enclosed in pvc and even if badly stripped would stand up longer than at the steel elbows. Even if so, still doesn't change the fact that the elbows need to be changed out due to magnetic heating.


Pechez les vaches.

 

[mc5w]

Also, the steel elbows are probably not solidly grounded resulting in step potentials when a ground fault current does occur.

You can get an installation that is very mechanically strong by encasing PVC elbows in concrete. Just be careful that rebars do not go in between phases which will also create an induction heating hazard. ( NB: Rebars really only need to be on the outside of the concrete encasement anyways. ) For an important feeder or circuit concrete encasement increases circuit integrity. Having 6 to 12 inches of concrete over the topmost conduit provides better protection against digins and coloring the concrete red porvides even better protection yet.

Also, concrete encasement helps spread out heat particularly if your soil presents a lot of heat resistance. If your soil is sandy you most definitely need to use concrete to spread out the heat.

Also, where nylon or steel rope saws into elbows is if the elbow has too small of a radius OR the rope is too small in diameter. We use nylon rope for a lot of wire pulling but it is either Jetline twine or more like 1/2 inch. The problem with high strength ropes is to use a smaller diameter ropes which creates more sawing pressure at elbows. If the sawing pressure is low enough sawing does not occur.

You can also use polypropylene rope if you would rather use a large diameter rope but be very careful of what you buy. The polypropylene rope that is rated for installing and removing submersible well pumps is very good stuff. There are basically 2 grades of polypropylene rope which is the really good stuff that is rated against rotting due to water exposure and cheap crap. The cheap stuff also tends to break without warning because the fibers are of unequal strength and other quality problems.

When I was moving a factory I liked to use 1/4 inch polypropylene rope for up to 100 amp wire pulls and 3/8 nylon for 200 amp wire pulls. The latter was pulled in using a set of sheaves that redirected the rope so that we could use a forklift truck. Any vehicle with a torque converter makes for an excellent wire pulling machine.

 

[tefaber]

Thanks for all the information, it has been very insightful. One more question. Are there any possible solutions to this problem besides removing the cable/elbows altogther. Can the IR effect be minimized by insulating the elbows? Perhaps some type of backfill? Any thoughts?

 

[lengould]

If there's any way to make a longitudinal cut in the elbows-in-place, you might not need to remove them. Requires that they not be touching each other, and have no magnetic or conductive contact with each other. The cut needs to be wide enough to ensure that eg. backfilling etc. won't force the edges together. Best would be to cut in the back of the elbows, eg. not where pulling/or/tight wires would contact the cuts.

Note to others, this is certainly not recommended practice except possibly as a one-time emergency fix for this particular situation.

Pechez les vaches.

 

[davidbeach]

lengould is correct that cutting the elbows, parallel to the conductors, will solve the problem. Getting to the elbows to do that much seems to be most of the work necessary to remove and replace the steel elbows with PVC. The problem is not an IR effect, it is an electromagnetic effect and will be entirely unaffected by insulation between the conductor and the elbow.

The only way of eliminating the effect without removing the elbows or cutting them would be to short each elbow to the conductor enclosed, at both ends of the elbow. That would eliminate the heating, but then you would need 15kV insulation on the outside of the elbow. THIS IS NOT A RECOMMENDED SOLUTION.

 

[dpc]

tefaber,

It might be possible to remove the conductors and re-pull them such that there are A-B-C phase in each conduit, depending on the conductor size, conduit size, etc. I have done this before for fairly short runs where the client had three conductors per phase. It's a lot of work, but if the conduit is embedded in concrete, it might be worth thinking about. You might have to re-conductor to get a configuration that works with the existing conduits.

Splitting the steel elbow as mentioned would solve the heating problem, but then you would have created
other problems. If you're a consultant or engineer-of-record, you should try to steer them toward a permanent solution that complies with NEC.

If they choose another path, then at least you tried.

 

[tefaber]

I agree, it should be fixed the right way. The city had used this installation on their 4kV system as well, but they haven't had any problems with it...only the installations on the 13kV system. Do you think the city can get away with the 4kV installations due to the reduced voltage? Any thoughts? Thanks again.

 

[lengould]

Heating due to induction should be entirely a function of amperage, voltage should not be a factor in what temperature the elbows come to, though it may be a factor in whether the resulting heat breaks down the insulation. Likely the lower voltage runs can put up with the added heat load for some reason, ie. not loaded so close to their current limit, less current per unit elbow diameter so lower temperature, different cable construction etc.


Pechez les vaches.

 

[dpc]

What lengould said. Induced current is simply a function of net current within the conduit.

I would be very cautious of the 4 kV system if it is installed the same way. You'd want to recommend fixing them both, since both are NEC violations.

 

[Skogsgurra]

Interesting, indeed.

We have yet to know what currents were involved in the 13 kV and the 4 kV systems.

We know that there were not more than 60 percent of rated current in the 13 kV system - which failed. How many amperes would that be?

And how many amperes are there in the sucessful 4 kV system?

Please, tefaber, we need that information to be able to judge this case correctly.

Gunnar Englund

http://www.gke.org

 

[tefaber]

Right. Sorry - the 13kV circuit is 750URD rated at 535 Amps and was pulling about 60 percent of that at the time of the fault. The 4kV circuit used 500kcmil cable, rated at 430 Amps. It is pulling around 80 percent.