Re potting Magneblast breaker bottles

[Thedroid]

What are the recommended materials for re potting GE Magneblast breaker bottles. Ours are showing signs of cracking and seperation between the bar and potting compound. We were smelling ozone, and called in a specialist. He chiseled out the old potting compound re wrapped the bars with silicone tape, and bought some tar from Home Depot to fill the bottle up with. We had a few bus joints that showed signs of partial discharge also. The joints were enclosed within a plastic case tied together with string and filled with a tar like compound. These cases were removed, the bolts retorqued, and were covered with silicone tape. The cases were not reinstalled.

Is there a special compound that should be used, or is roofing tar sufficient? We would like to inspect the bus during our outage, but are unsure whether to use the same contractor.

 

[oldfieldguy]

That's an interesting one.

In the absence of hard data, I seem to recall that the original potting compound was derived from pine tar, and the roofing compound you're likely to get from Home Depot is of petroleum origin, but as long as it does not contain conductive fillers such as carbon black, it should be sufficient.

I think I'd have created a test sample for testing breakdown voltage out of a couple of electrodes and a bit of both compounds and then subjected them to elevated test voltages, but that's because I worked with some folks who were trying to keep archaic equipment going.

The old guys who used to KNOW about potting compounds and such are getting hard to find. Sounds like you found one.

Please keep us informed of further activities.

old field guy

 

[dpmac]

Many of the breaker rebuilding shops rebuild/retrofit Magnablast bottle assemblies. I am sure Zog or others will fill you in regarding the potting compound. I would be more concerned about the compound viscosity being used then its electrical insulating properties. You don't want the compound seeping/dripping onto the breaker over time.

I am not convinced MC switchgear or bus joints were ever "PD Free", or meant to be, when they were new. Many mfg did go to great lengths to do the best they could to insure bus joints made in the field would last. Dux Seal, Solorite with semicon tape were mostly used. ITE used two hard plastic covers with a hemispherical shape on each that was painted with a semi conducting paint. The bolted joints had thin copper "tangs" that made contact with painted semi conducting surface. This would act as a "corona ring" of sorts. The covers were simply taped together in place. No filler at all. This arrangement worked well and accomplished much more then fillers and tapes. The basic rubber boots on MC gear joints used today make no attempt to "suppress" corona or PD as far as I know.

I am sure the experts will help me out here.

 

[Zogzog]

I assume the OP is asking about the pole peices themselves, there are no "bottles" on a magneblast. We actually manufacture replacement poles for these, I havent done a feild repair in 10 years on the poles, I just replace them with new ones. I can't recall off the top of my head what material we used on theat last repair (One of those 36 hour straight jobs) but it was not something you get at HD. Let me go look at the job again and see if I can find what we used. I think it is in the AM maintenece manual.

 

[Thedroid]

I'm talking about the primary disconnecting devices in the switchgear, not the breakers. I found reference in the manual to GE D50H49 compound, and the procedure for heating and pouring it. I can't find any other info on this compound online, so I'll probably have to call GE and see if they have a suitable replacement.

 

[Thedroid]

I should also say that this switchgear was commissioned in 1959, and has ran very reliably so far. We have an opportunity to inspect the primary side of the system for what is probably the first time in 50yrs, and would like to be prepared to restore these joints if it is deemed that further inspection is necessary.

 

[Zogzog]

Ah, those bottles, my mind was thinking about vacuum bottles for some reason, wasn't thinking about the cells. I think you can still get that stuff from GE.

 

[Thedroid]

I tried emailing GE, with no results. Is this a common repair on this old switchgear? The procedure out of the manual is pretty straightforward, but I need to find a source for the materials.

 

[dpmac]

As mentioned earlier. One of the problems typically encountered is the dripping of the "Tar" or "pitch" through the bottle assy. onto the breaker pole piece and breaker. I have been told that good results can be had by removing the old tar out, cleaning the assy and putting about an inch (1") of fast dry epoxy in the bottom of the assy, then simply refilling with the original removed tar after melting into a liquid form. Messy but functional. The epoxy prevents leakage of tar from dripping onto the breaker over time. I would imagine one could use G&W Novoid compound with excellent results also. Just my opinion of course.

http://www.gwelec.com/support/msds.cfm

 

[Thedroid]

Still haven't heard back from GE. The Novoid looks like it would be a good substitute for the original compound.

 

[dpmac]

A few further questions if you don't mind:

When you wrote about finding "evidence of PD" does this mean you found visual or audible indications? Can you describe what you found associated with the bottles and joints other then shrinkage and cracking of the compound. What is the voltage class of the equipment?

I am unsure of the risk vs reward for your situation. Are you removing the bottle assemblies from the switchgear to perform the re-potting operation? This would be very time consuming. You run the very real risk of altering the alignment of the primary tulips relative the breaker. This would be catastrophic should you drive the breaker up into the mis aligned primary disconnect and break the porcelain housing.

Thank you.

 

[Thedroid]

The smell of ozone had been real strong over the last few months, and after changing breakers and inspecting the load side, we decided to open up the line side bus overs. There was white powder, and discolored insulation surrounding the cracked insulation. Also, one of the bus joints that was inspected was very loose, and also showed signs of overheating. I realize the risk, and would very much like to stay out of this gear, but it might need more attention. I think the general consensus is to repot the bottles still in place.

 

[Zogzog]

Why don't you just replace the bottles?

 

[Thedroid]

Cost would be too high, and would also require disassembly of the switchgear.

 

[Thedroid]

Finally heard back from GE. They recommend using 3M epoxy and 3M 130 splicing tape instead of the original potting compound. The hard part will be figuring out how to clean out the old compound in the event we find a problem. As far as I know, removing them would require disassembling the main bus, and this is not something we are interested in doing.

 

[oldfieldguy]

droid...

Doesn't sound like you have a lot of choice. If you don't do something, the bus will eventually disassemble itself.

old field guy

 

[Zogzog]

Thedroid, just in case, I stock spare magneblast cubuicles and breakers, you know, just in case

Do you know specifically which bottles have PD?

 

[Thedroid]

No, but the two compartments that were inspected both had issues with the potting pulling away from the bars. Two of the bolts holding the bus joints together were also very loose. We plan on removing the joint covers, torquing down the bolts, and wrapping them with 130 tape. If any of the bottles look suspect, then we will have to decide what to do from there.

 

[Thedroid]

We intend to go through with the repair here in a few weeks. We are going remove the old cotton/varnish from the bolted joints, clean, retorque, and recover with raychem HV tubing and sheets. Any of the potting compound that looks pulled away or deteriorated will be replaced with 3M Scotchcast 4 epoxy resin. Does anybody have any other suggestions?

 

[ScottyUK]

Consider that you will never be able to remove epoxy in future if you ever needed to, at least not without destroying the equipment.


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