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

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

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

(OP)
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.

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

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?  

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

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?

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

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

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

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

(OP)
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.

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

Are all three phases in the same conduit?  

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

(OP)
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.

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

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

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

(OP)
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?

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

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.

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

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.  

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

(OP)
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.

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

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

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

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.

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

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

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

(OP)
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.

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

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.  

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

(OP)
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.

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

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

Where are you located?

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

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.

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

(OP)
This plant is in Davenport Iowa. I work out of the corporate office in Madison WI. The incoming 13.8kV is 3 wire and we step it down to 480V 4 wire at the delta/wye transformers.

I'm guessing earth ground is being used as the return ground path for the utility but I am working on finding out for sure.

Hopefully the IR scanning will tell all and I will be able to get this issue resolved.

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

(OP)
I just looked at some thermal images of the termination of the MC cable conductors to the overhead utility conductors.
I'll try to describe this properly so it makes sense.
I call the utility conductors A, B and C phase layed out from left to right.
I call the three parallel conductors 1, 2, and 3 layed out from closest to me to farthest away from me. so this forms a 3 X 3 matrics.

The IR scan found three hot spots, one on each phase.
At connection A2 the differential compared to the other A phase connections is about 65 deg F, with a temperature of 135 deg F.
At connection B1 there is a 430 deg F differential with a temp. of 515 deg F. (this is bad)
At connection C2 the differential is about 70 degF with a temp. of 146 deg F.

So there are deffinitely some issues here and they will be addressing these as soon a possible.

One thing they told me is that all of the connections to the utility from the cable conductors are pigtailed using split bolts. Is this a common practis?

One other thing I just found out is that there is evidence of arcing from the armor on the MC cable and the cable tray. I'm thnking Maremite's comment about a high impedence ground fault might account for this. Any thoughts?

Again thanks for all the feedback. Long term we will be replacing all this cable I just hope that nobody or nothing gets hurt before we get that done.

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

515 deg F??  

I'd be making immediate plans to de-energize this feeder.  This is not something you can wait to deal with.  You really have a responsibility to get this taken care of ASAP - like today.  Or stand a 24x7 fire watch.  

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

(OP)
They will be repairing these ASAP and keeping an eye on them until then.

One question I have has to do with the cable that is running hot. I don't think fixing these terminatins will fix the problem with the entire cable being hot, I think there are some issue with the cable still, I also found out that these cables are at least 40 years old.

When they reenergize this cable, is there a big risk that  the insulation will fail (I'm assuming at this point that the insulation isn't in the best of shape on that cable that has been running hot)?

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

Well, the heat should keep it dry, but a 40 year old 15 kV cable is probably on borrowed time even under good conditions.  The less you mess with it (until you replace it) the better off you will be.  

If the insulation is as hot as it sounds, it may have already basically failed.  

I'd budget to replace the whole thing.  

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

Maybe the cable impedances are not matched, causing the bulk of the A-phase current to flow on A2. This could be caused by significantly different conductor lengths (all parallel conductors must be the same length so impedances are equal), or there are one or more bad connections causing unequal current division.

Another possibility is that for some reason you have high shield current, causing a cable shield to overheat. This could be caused by improper grounding.

And what dpc said about shutting it down... Now, before something bad happens.

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

(OP)
I appreciate all the input and am wondering if any of you have the name of any experts out there that I could bring in as a an advisor. I'm looking for some one who has a lot of experience with this type of situation.

Thanks, Hopefully it won't be long and this will be resolved with out incident.

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

If this was a design problem I would expect all three cables to be hot.
I suspect that you have bad terminations diverting current away from one phase in the hot cable. The unbalanced current could explain both the heating and the marking on the cable tray. The diverted current will be causing an unbalance in the other two cables also but the unbalance will be divided between two cables and the effects will not be as severe.
The first step is to have the crew clean and redo the terminations at both ends.
If you suggest to the tradesmen to take the time to do a good job cleaning the terminations, and when in doubt, take the time to do it right you will probably be succsessfull.
A couple of tips;
Avoid bending the old cables. For overhead connections use a rope to suppoprt the cable close to its normal position rather than letting it hang down.
If you are connecting copper to aluminum outside, the aluminum must be above the copper. If the copper is above or beside the aluminum, rainwater will react with the copper and cause corrosion of the aluminum.
Use anti-corrosion compond such as Penetrox or No-alox.
respectfully

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

You haven't said one important thing. Is the cable oil insulated? The old cables have paper/oil insulation so the common problem in them is for oil to evaporate or leak if you have leaking problem somewhere. I run on few such problems. The solution is simple, you need to pour oil again in it. If it is oil insulated cable than I can bet 9:1 that it is the problem because it is normal with such cable when it losses oil. You need to check this fast because paper isolation is will break down very fast.

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

(OP)
This is not oil insulated cable. How the heck would you refill 300 ft of cable if it was oil filled? That just sounds funny to me.
Not sure what you comment, "you haven't said one important thing" means.
Any way we reterminated the connections from the MC cabel to the overhead wire and things seem to be fine for now. We will replace all of this cable in the near future but for now I think the imediate concerns have been addressed.

Thanks to all of you for your input.

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

The cable armor heat has disappeared? There's no unusual heat from an IR scan now?

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

(OP)
When we put loads back on the system the temperature of the suspect cable was reading about the same as the others. I kow when it was reading hot they had about 400A at on the primary and when I took the readings after we fixed the terminations they were only at about 300A. So they certainly were not up to full load or even normal load yet. They were also only running for a few hours when I took the last scans so I'm not sure how long it takes for the temperature to stabalize.
Keeping my fingers crossed that we don't see the same temperture difference in the cable as we did before. They are suppose to take readings again this morning and then on Thursday. We will keep an eye on it until we actually replace all of it in the fall.

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