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]

Was bend radius OK? Also, was the cable pulled with excessive force through the bend?

Gunnar Englund

http://www.gke.org

 

[davidbeach]

It was the electric heating caused by steel elbow enclosing a net current. That steel elbow will run very hot in the situation you describe. Either the steel elbows have to go, or you have to use smaller conductors and pull all three phases in all three conduits.

 

[dpc]

I have seen this happen before, so I would tend to agree with the cable supplier. Cable insulation melted in steel elbows causing a fault.

I have also gotten calls from clients asking why the conduit elbows were too hot to touch.

This would seem to be the most likely cause of failure unless you have a better candidate.

As I'm sure you know, if all three phases are run in one conduit, this is not a problem because the net flux is close to zero.

 

[rbulsara]

davidbeach is right..not sure what 'net' current means, but running single phase conductor in a metal conduit will induce currents in the conduit/elbow to heat up.

 

[tefaber]

I believe the installation was okay. Another specific - the manufacturer said that by installing cable in this fashion (one cable per conduit), the cable will be derated by up to 50 percent. He said if three cables had been installed per conduit it wouldn't have been a problem since the magnetic fields would have cancelled out.

 

[davidbeach]

net current is the phasor sum of the current(s) flowing in the conductor(s) in question. With one conductor, it is the current in the conductor. With three phase conductors (and a neutral if applicable) the net current would be the phasor sum of all conductors and should be very close to zero.

 

[itsmoked]

So this is caused by the three elbows interacting as a crummy transformer? Would this still happen if an indiviual phase was in a steel bend 30 ft from the other phases?

 

[davidbeach]

itsmoked, it's three independent transformers, proximity does not play a role. Check NEC 300.20.

 

[ScottyUK]

Tefaber,

Davidbeach is right on the mark.

You ask if anyone has seen anything like this: We had a similar situation when someone installed a steel Unistrut frame supporting the line connections to a 56MVA generator and through lack of knowledge managed to form a closed loop around each phase. The support was running so hot that the galvanised finish was failing and the underlying steel was coloured by oxide.

You may also encounter seemingly strange brass or aluminium fittings on single core cables. Aluminium cable armour, aluminium glands, brass gland plates are all typical for single core AC cable installations. These components are used to avoid the magnetic induction problem which you have observed. Single cores are usually installed in a trefoil group to minimise the external magnetic field and are cleated together to withstand the large electromagnetic forces which exist during fault conditions.



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If we learn from our mistakes,
I'm getting a great education!

 

[itsmoked]

Thanks davidbeach.

This has nothing to do with the *bend* either, only that it is a closed magnetic path around the cable?

 

[davidbeach]

itsmoked, yep, the bend is irrelevant; somebody thought it would make a stronger system to have steel els in a phase segregated system - big mistake. They just needed to learn that pulling with a steel rope won't burn through PVC els like nylon rope will.

 

[BJC]

A steel rope or cable will saw through PVC and steel elbows. Bare steel or nylon coated the results are the same. Been there seen it happen. In addition to being a good saw both steel and nylon store a lot of energy in streaching and are very dangerous. Large diameter ropes with parallel strands covered are the only way to go. The local electrican store has non- streach ropes.
One you saw even a little nick in an elbow you cable is compromised.

 

[dpc]

tefaber,

This really is not a good installation and violates NEC Article 300 as David Beach has pointed out. It needs to be corrected.

There is no provision in the NEC for derating a conductor due to magnetic heating of the enclosing raceway. The conductor is supposed to be installed in such a way that the magnetic heating does not occur.

Do the city and yourself a favor and make sure it gets corrected. Let us know how it works out.

 

[davidbeach]

A steel rope or cable will saw through PVC and steel elbows.

Not true. A steel hard line allows pulls through PVC elbows without burn through where a nylon rope would burn the PVC. Now, if the bend is too tight it won't matter what is used for a pull-line, you'll have burning.

There is much less friction between steel and PVC than there is between nylon and PVC, and the steel can disperse the heat much better than a nylon rope. The stretching of the nylon rope only further compounds the problem.

Check with the cable manufacturer's for their recommendations for pulling and you'll find that they recommend steel rope.

 

[BJC]

Have check many times and pulled many miles of cable. The only advantage to steel cable is when you saw all the way through a steel elbow the induction heating problem goes away.

 

[itsmoked]

hehehe Good point.

Thanks for those clarifications davidbeach!

 

[Skogsgurra]

Excuse me. I'm not convinced.

That induction heat thing is so well-known that I thought it could be excluded. Also, we have not got any information about current in the cables. Only that "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"

The OP has already contemplated the effect of induction heating and asks for comments. A cable carrying 60 % current causes about 36 % heating and since the elbow probably is surrounded by dirt, I think that the cooling is adequate.

Many questions remain unanswered. The main question is: What is the rated current? Other questions are: Bend radius? Vertical stretches that put constant pull forces on the cables? Surrounding material? Dimensions of cable and elbow? Elbow materiel? (Sheet steel, stainless, cast iron?) Dimensions, current, material and cooling have an influence on temperature rise. Many installations are OK even if there is one phase per steel tube or elbow.

I find it too easy to say that magnetic heating is the cause. It may very well be so, but that effect is so well known by all people in the trade that a (conscious) violation seems unlikely. The OP does hint that there might be some other effect. Let's find out if there is.

There might even be a short-term overload that has hitherto gone unnoticed. A 1 hour peak recorder does not tell you the whole truth.

Gunnar Englund

http://www.gke.org

 

[TheBlacksmith]

We did case studies with mock ups for our project and the heating in very short runs of steel conduit (bulkhead penetrators) with single phase cable was surprising. All of our penetrations are specified as "non-magnetic".

 

[davidbeach]

skogsgurra, 60% current equating to 36% heating would apply to I^2*R type of heating of the conductors. In the case of induction heating due to magnetic enclosures around a single conductor, the heating has a multiplier of much more than one. If all three phase conductors passed through one steel elbow there would be no heating (if a balanced circuit), so we have far more than 36% heating.

Yes, it is a well known issue, but apparently not by who ever did the installation. With those elbows in that installation they will always have problems, whether there are other contributing factors or not. There's not much point trying to figure out the minute contributions to the cable heating (which would be relatively uniformly distributed if it were overload related) while the significant source of heating remains.