SWBD cable insulation deterioration

[rovineye]

During a recent cleaning, we found some internal cabling that had insulation issues (see file). The cabling is from the load cables to the contactors (Cutler-Hammer), voltage level is 6.6 kV, the conactors are pretty much closed all the time so no frequent load spikes to speak of, and loading is well below capacity of these conductors. Initial assesment is low energy electrical arcing, corona discharge, and tracking as a reasult of no outer
semi conducting shield. These cables intended use is listed as temporary installation around damaged cables or for temporary power sources. They come with the cubicles, so I never gave them a second thought. Until now!

 

[rovineye]

A follow-up on what has transpired.

A survey of all the switchgear in question showed that we had these cables installed produced by up to 5 different manufacturers. I say up to, because one set had no manufacturers marks at all. All had the same basic construction and ratings. 3 of the manufacturers cables were deteriorated as previously shown. The other two looked brand new.

Testing showed that the deteriorated cables had not lost any insulating capacity as compared to the brand new cable samples. That was the best news. The cracking had only occurred on the outer jacket.

PD testing did not show a difference between new and old cables from the same manufacturer. But, the point at which pd developed and extinguished was radically different between brands. We use these at 6.6kV, and the 'bad' cables developed pd at voltages as low as 3.9kV, and the good as high as 12kV! Results were consistent across manufacturers batches.

Eaton/Cutler Hammer traveled to my locations and changed out the cables even tho they were out of warranty. They sent large groups of engineers (including the original designer of these contactors), techs and mechanics, obviously very concerned. They now have processes in place to measure pd inception and extinction levels of every cable batch that goes into these products. Last I heard from their engineer he was on the way to Saudi Arabia to check on installations there. Good for them!

 

[ScottyUK]

Thanks for the follow-up rovineye. Interesting that cables which presumably have the same nominal specification should behave so differently. Care to name the good manufacturers? I understand if you don't want to name the bad ones but I always feel that good companies deserve the publicity.


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

 

[rovineye]

The best results were from Okonite and Amercable, and acceptable was American Insulated Wire.

I have to say however, that cables I classify as "bad" may not be bad in other applications. The primary use of these type cables are as a temporary bypass of damaged cables, not permanent as we are using them, and all caution that they should be installed with separation phase to phase and to earth. Installed that way, all these manufacturers' cables may be fine. But we use them long term, tightly bundled and laying on earth, hence the damage resulting from the pd/corona/ozone in all but those least susceptible to pd development.

I wouldn't have any problem pointing out the "bad" manufacturers, but I still have not figured out who made them. They are marked with the cable broker's name, or nothing, but not an actual manufacturers name. Very unusual.

 

[alehman]

I agree. Unshielded cables obviously are supposed to be installed with separation from ground and other phases.

I've seen similar, but not as bad, situations in genset terminal compartments from major manufacturers.

Alan
----
"It’s always fun to do the impossible." - Walt Disney

 

[ScottyUK]

Hmm, 'bad' might have been the wrong word to use. 'Bad' is probably better aimed at the design of the equipment which mis-applies the cable.


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

 

[rovineye]

Point taken ScottyUK. What we have is the end result from customer pressure to build the units smaller (cheaper goes without saying), and most of the folks we work for don't want to hear about the down side.