What could cause a 15kV MC cable to be very hot to the touch?

[lookingtolearn]

This situation has 3 parallel 3/0 15kV MC cables feeding a productin facility. One of the three cables is very hot to the touch (I couldn't keep my hand on it for more than 10 sec.) while the other two are not even warm. I'm not sure of the age but probably 30+ years.

My first thoughts are bad terminations, but 2 0f 3 conductors of one phase would have to be bad to cause only one cable to get hot. that seems unlikely.

Does anyone know of any other cause for a cable to be hot the entire length. If ther were insulation issues would that cause the cable to get hot?

Any help would be much appreciated.

 

[dpc]

First of all, I would never put my hand on an energized 15 kV cable.

Second, have you checked the currents in the cables with a clamp-on ammeter?

How far from the termination were you touching the cable? A hot termination will conduct heat down the conductor very nicely.

Do you have the capability of doing infrared scan of the conductors and terminations?

 

[BJC]

Check all the ground connections and the shield connections at both ends of all the conductors. Draw a pictures of the cable connections and ground connections.
Is it type MC cable?

 

[ScottyUK]

Image currents in the screen? Check for unintentional bonding at the glands.


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Sometimes I only open my mouth to swap feet...

 

[lookingtolearn]

The cable is MC cable and it was hot the entire length of the cable, 150'+

They are planning on doing an infrared scan this week.
The load seems to be balanced over all but current readings have not been taken on each individual condustor.

There are 3 conductors per phase and the phases seem to be ballanced.

If a ground or shield connection would be bad would that cause only one cable to be hot? Unless there is a ground fault there shouldn't be any current in the grounding conductor.

Thanks for the feedback, keep it coming.

 

[dpc]

Are all three phases in the same conduit?

 

[lookingtolearn]

This is Metal Clad cable with 3 conductors plus ground in each of the 3 parallel cables. So each of the 3 cables has all 3 phases. There is no conduit, the MC cable lays in cable tray.

 

[dpc]

So what is getting hot - the conductor or the armor?

 

[lookingtolearn]

It's the armor that is hot. Because I don't have access to the individual conductors, I don't know which conductor is hot. I'm thinking it has to be a conductor generating the heat but I can only touch the armor so that is what I feel as hot. Is there anything that would cause the armor to get hot? Some EMF coupling of some sort? But why only one cable and not all three?

 

[BJC]

You need to draw the picture of the system to understand it.
You could have one phase of one conductor in the hot cable disconnected on one end or the other ( or it could have a high resistance connection.) That would mean 2 phases in the armor enclosure of a cable. If the system is not fully load the other two cables would carry the load.
Normally the current in the three conductors will even out in the A-B-C conductors for a Zero sum. If there not zero they will cause inductive heating.

Something isn't right, so something is wrong. Start looking systematically, as I said a detailed diagram is a good way.

 

[dpc]

As BJC says, you could have some circuit condition that is inducing current in the armor, or for some reason all of the imbalance current is flowing in the armor. You need to do as much inspection as you safely can and sketch up what you have. If you can take some current measurements that would be very useful. If you can check the phase currents in the hot cable, you will know if the heating is from the conductors or the armor.

 

[lookingtolearn]

This is all great advice and I will try to get as much of it implimented as safely allowed. Hopefully something will jump out as being the problem.

Thanks again to all of you for your help, it's much appreciated.

 

[BJC]

I would be a cheap lunch an IR scan of the terminations will be telling.

 

[alehman]

PLEASE BE CAREFUL. If there is something causing current to flow in the armor, it is clearly wrong. If the armor were to come open or disconnected from ground at some point, it may well become energized.

 

[Marmite]

It's possible that your cable may have developed a high resistance ground fault which is causing current to flow in the armour. This combined with poor armour/sheath bonding at one end due to corrosion, missing bonding, or the fact that the bonding has burnt off would cause current to flow back along the armours. However, in my experience on high voltage cables ground faults do not last for long in this incipient state before they give themselves up as permanent faults. I've seen it more commonly on low voltage buried paper lead cables where the lead has literally melted and disappeared. Obviously if you do have a ground fault then the protection should have seen it.
Regards
Marmite

 

[lookingtolearn]

The armor on this cable does not have a jacket or outer protective covering so the armor is in direct contact with the cable tray. Wouldn't this ground any current flow through the armor, assuming the cable tray is properly bonded? Of course that might be a faulty assumption, because obviously something is wrong.

When the utility runs it's overhead feed they don't have a grounding conductor do they? In this situation the utility protection (On the pole) is about 1000 ft away from where the transition from the overhead to the parallel MC cable takes place. I don't think there is any GF protection on the utility end but I don't know that for sure. I'll have to check that out.

 

[dpc]

Utility distribution feeder will normally be grounded and there will generally be a ground/neutral conductor brought from the substation along with the phase conductors.

 

[lookingtolearn]

I only recall seeing the 3 phase conductors on the poles for the overhead feed to the plant. Would the ground be run as a 4th overhead wire from pole to pole? I'm not at this facility so I'm going from what I remember seeing.

 

[dpc]

Yes, but usually lower on the pole and with smaller insulator.

Where are you located?

 

[alehman]

3 wire or 4 wire primary depends on the load and the location. If loads are all 3-wire, only 3 wires required. For insulated cable there is normally a ground or neutral. The cable shields (or concentric neutral) may be used as ground and a separate ground may be run.

For many years overhead circuits sometimes used earth as the return path for single-phase loads. But in many areas that is no longer permitted. New overhead installations in the U.S. with line-neutral loads must have a neutral/ground conductor.