Insulated Busbars - Pros and Cons

[SparkyLarks]

I'm ordering new 400V MCC's at the moment and was planning to go with Air insulated and supplier is pushing Insulated busbars.

Basicialy just a plastic cover on the busbar.

I'm of the view if sufficient spacing in used in the construction then the insulation does not offer much extra protection. The weakest points are the busbar connections and the starter stabs which need to be uninsulated.

I was wondering
1) Does the insulation offer any major advantages,
2) If there are any disadvantages associated with the insulation?

Regards
Mark

 

[dpc]

Try dropping a screwdriver across your bare bus while it's energized and you'll get a new appreciation of the advantages of insulated bus. It's much safer to work around, reduces faults from rodents, etc. In the US, it's become the norm for most MCCs.

 

[desertfox]

Hi

Can't see much of an advantage in having insulated bus bars if the existing board didn't have them to start with.
I agree with dpc about vermin straddling adjacent bus bars however if vermin are that big a problem then the area's where the bars are joined should also be vermin proofed.
In the UK if there was any chance of dropping a tool onto live bars, the board would be isolated to prevent that from occurring.
Finally though, if your board had air insulated bus bars and you replace them with insulated, how would that affect the cooling of the bars?
I ask because the switchboard when originally designed would have undergone a temperature rise test, which would have included the air insulated bars, I'm not sure how changing the configuration would effect the type test certificate.

 

[desertfox]

here is an article

http://www.docstoc.com/docs/26466052/Bus-bar-insulation-system

 

[marks1080]

Also consider the concequence of actually have a legitimate phase to phase fault. Run a probability analysis for this scenario. You may quickly find yourself willing to fork out a little extra money today to save bundles more in the future.

 

[ScottyUK]

Wastewater, sugar processing and refinery applications tend to have problems with silver or tin whiskers Insulated bars go some way to preventing this problem from forming conductive crystal between conductors.

Desertfox - things do get dropped, or come loose, or get left behind by careless maintenance guys. Insulated bars provide a layer of protection against a major event, just as commissioning tests provide a layer of protection, and arc-rated boards provide another. If one layer lets a fault through another layer catches it, unless you only have one layer of protection: then you're in trouble. Good point about the type tests if this was a retrofit.

 

[desertfox]

hi ScottyUK

Yes fair point about stuff being left behind and I agree the insulation would offer some protection.

 

[SparkyLarks]

Thanks for the views.

It does tend to confirm my own feelings. It offers a layer of protection, and given the facility is an alumina plant, The risk of tracking due to alumina dust is a reasonable risk. The plant is 30 years old and some boards have insulted busbars some do not. There has never been a rodent issue. There have been a number of boards fail due to over heating, and one where the stab blocks failed earlier this year, but that was a 30 year old board in a very damp and dirty environment. Most of the other boards are in clean dry substations.

The board is a form 4 boars so the busbars would not be exposed unless the board was dead. The plant has a good culture of isolation and documented double checking procedure before reenergisation, so I am not too worried out a screwdriver but things can become loose or fall out.

The question has also been asked about retrofitting heat shrink insulation on the existing uninsulated busbars. My view and it seams to be echoed was that that would require retesting of the board and it is probably better to leave it alone, or get the OEM back to do the work, which is likely to not be the most effective way to spend the available budget. Especialy in the case of arc rated boards with good commissioning procedures following work on the busbars

 

[podobing]

SparkyLarks,
I work in the mining industry where things are sometimes a little dusty, however not like it used to be. Our equipment gets moved from place to place, and the ability to withstand vibration is put to the test. At one of our sites, some people were doing some mechanical repair work on an underground conveyor belt, and they had the system locked out and tagged out properly. It was a 480 volt system, 1200 kVA transformer (I believe). Sometime while the work was going on, one of the stand-off insulator spacers between the busbars broke (unthreaded or ??), allowing two busbars to contact one another. When they went to restore power to the conveyor belt using a molded case circuit breaker, the breaker blew up, severely burning two employees. Since they, they have required insulated busbars on all such equipment.

Note: The circuit breaker was properly sized, but age, dust and usage take their toll on everything, including me.

Long story short - in addition to preventing "dropped tool" accidents, the insulated busbars can also provide back-up protection against other failures such as insulators, mechanical fasteners, and/or circuit breakers. Just like anything else, the bus bars would have to be sized larger because you are now putting a blanket on them.
Dave

 

[rcw retired EE]

We used to buy non-insulated bus MCC's because they cost less. After a couple of burn downs we switched to always specifying insulated bus MCC's. We have not had an MCC bus failure in the last 20 years. It could be coincidence.