broken / arc-damaged lug on motor surge arrestor ground jumpers 5

[electricpete]

Attached are some photo’s of something we found recently in 13.2kv motor terminal box

http://electricpete1.tripod.com/linkfile.htm

(link to 3MB powerpoint)

On slide 4 you can see the overview of the terminal box. The surge arrestor cases are each insulated from ground. The surge arrestor ground terminals are jumpered from C phase to B phase to A phase to ground.

On the jumper between A and B phase arrestor ground terminals, we found a broken lug at the B phase end. It was completely broken between the tongue and the barrel. There was some discoloration present. Close examination of the broken surfaces reveal pitting which seems to me related to arcing.

What do you guys think about this? My theory is that the connection broke from mechanical causes and then while it was open but in close contact, arcing occurred due to potential difference between A phase side (at ground) and B phase side (floating between B and C phase voltage due to surge cap capacitor voltage divider). Mechanical causes we are looking at include vibration (we have noted heavy vibration on outside of term box for this whole family of motors). Or possibly over-rotation of the lug while tightening the connection. Does this sound plausible?

Any other thoughts?

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[electricpete]

Here are the pics in much smaller form (around 60k each):

slide 4 terminal box overview:

http://tinypic.com/e9j59y.jpg

slide 1 both halves of lug

http://tinypic.com/e9j57p.jpg

slide 3 barrel of lug

http://tinypic.com/e9j5f6.jpg

slide 2 tongue of lug

http://tinypic.com/e9j5ed.jpg

Any thoughts?

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[electricpete]

We have two competing theories of how the original mechanical failure would have occurred.

Teh installed orientation as shown is vertical terminal with lug and cable in the horizontal plane.

Theory A is cable vibrating vertically flexes the lug tongue at it’s weak point and creates a fatigue failure. We know we have high vibration in this terminal box as measured on outside of terminal box with machine running.

Theory B is that the lug was stressed when initially installed due to overrotation. If you looked from above you could see the ramaining tongue portion of the lug was rotated about 10 degrees past where it would need to be to allow the jumper to go in a straight line to the opposite terminal. And it appears the jumper did not have mcuh slack. Possibly vibration also played a role in theory B by allowing initial stress from tightening to relax and retighten leading to eventual fatigue.

The difference between theory A and B would be orientation of bending of the lug. If we picture the lug has a strong direction and weak direction similar to a 2x4, theory A involves bending in the weak direction, while theory A involves bending in the strong direction.

Apparent problems with theory A – why don’t we see evidence of fatigue cracking at the opposite lug or at the point where conductor endters the lug barrel?

Apparent problems with theory B – Somehow I expect the lug would have buckled one corner up and one corner down if the bending was occuring in this plane (don't you think?). Also as noted above this involves bending in the stronger plane.

Any comments on theories A or B or other theories, or other info you think should be obtained? Note that the actual failure surface of the lug cannot be examined in detail since it apparently acred after failure.

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[jghrist]

I don't like theory B for the reasons you give - I would expect some distortion if it were bent in the strong plane.

Another theory: The lug burnt off by high surge current. Maybe it was weakened by vibration fatigue before the surge. This would explain the burn marks on the cabinet case better than arcing after breaking off. I wouldn't expect a lot of current from arcing to equalize the potentials.

Was the broken lead lying on the cabinet? There looks to be burn marks both on the cabinet and on terminal. A metallurgist may be able to determine if the failure was because of burning, vibration fatigue, or a combination.

 

[electricpete]

Suprisingly enough, the lead is reported to have been found laying exactly where it is shown in slides 1 and 4 with a few tenths of an inch gap between the broken halves.

Your theory that the conductor broke during electrical transient is worth considering. But if the surge current was so tremendously high as to melt a connection, I would have expected to see some sign of overheating elsewhere... but I didn't. Maybe because that specific point was higher resistance due to fatigue? I have a hard time believing that, since temperature for discoloration of insulating materials is so dramatically lower than temperature for melting of metals.

A few other wandering thoughts

I suppose a fairly large surge of current passes through the cap and jumper every time the motor is started or secured due to the high dv/dt.

I believe the reason that the surge cap canisters are insulated from the term box is to facilitate dc insulation resistance testing of the motor. Surge caps have to be isolated during this test since they would take forever to charge up. It’s much easier to break the connection at the ground side of the surge caps for meggering the motor rather than at the energized side because the energized side requires Raychem tape (based on not enough clearance for 13.8kv in air). This configuration is unique at our plant (other motor surge cap cases are grounded) but I don’t think it would create any problems that would have contributed to this failure.

We plan on doing capacitance test to verify the integrity of the caps. I don't know of any other good tests for caps, short of a Doble test.

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[wschel]

electricpete,
Has phase A cap been replace or removed? I have seen people bend lugs to get the conductor out of the way while replacing equipment. That could weaken the lug at just the point where it failed.

 

[ScottyUK]

If your theory about the lug having rotated past the nominal position as it was tightened is correct, this would have presumably put some tension on the conductor. Add in vibration and you have the potential for mechanical resonance. The conductor would have to be taut for this to occur, like a guitar string.

If resonance is at work, the lug at the other end may show some sign of weakening too.


----------------------------------

One day my ship will come in.
But with my luck, I'll be at the airport!

 

[RalphChristie]

Definitely mechanical failure on some way. We have experienced it a lot on some of our equipment that use similar lugs. Most of the times it is on older installations and the conductors are not supported in some way.

The point of failure is always on the smallest part of the lug. That is the point where the lug can be bend the easiest. In my opinion someone bend it while working there, and with vibration it weakened. The lugs that we use on this moment seems to be thicker than the lugs on your photo's (We have used similar lugs and have changed it) and it seems as if the failures have decreased.

Just a side note: Maybe nothing wrong with it, but I do not like the way the lugs are crimped very much. I would rather use a hexagonal crimper to do the job. But, this is just a personal opinion.

[red]Failure seldom stops us, it is the fear for failure that stops us - Jack Lemmon[/red]

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[electricpete]

I agree on the crimping - we like a crimp that compresses all the way around rather than just at one point if possible. This was vendor supplied equipment, not done by our guys.

One other thing I am thinking about is that the guys sometimes climb into the terminal box to clean it and determ for testing. Maybe someone stepped on that jumper close to the B-phase side.

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[itsmoked]

Consider that the lug was allowed to rotate during tightening. Typically the person who doesn't realize or care that this might be a problem would then allow it to continue until the cable itself stopped it from turning. This IMHO, would then mean that that one "side" of that lug would be under compression (in the horizontal plane) while the other, (closest in pic) side, would be in tension. Any vibration, in any plane, that then occurs will eventually start a tear in the lug at the tension edge. As the tear progresses the resistance of the lug will go up. This will subject the lug to abnormal heating which will oxidize and em-brittle the lug. This will lead to more rapid "tearing" until failure which will probably come during an electrical event that provides your arc pitting evidence.

 

[fsmyth]

Well, there you go, Scotty, 'smoked --- another
professional category: "Cable Tuning Consultation".
I think I will add that to my business cards...
"Expedited Service - Resonance Guaranteed to +/- 2%"
Or maybe a new market: anti-resonant dampers for
cables AWG2 and up....

Gotta cut down on the coffee.

That does sound reasonable; a simple vibration-induced
mechanical failure to a high stress point.
<als>

 

[swgrmfg]

E-Pete:

The cables interconnecting the caps are too taught. A semi loop on the cable is required to relieve mechanical stresses - specially during transportation. The pitted break is evidence of high frequency vibration. Couple that with a pre-stressed crimp lug you get your textbook photo. Lucky it was not on the line side. I have pictures of those occurences on record. (Incidentally, the same type of lug!)