Overdutied Breakers 3

[controlnovice]

Previous EE had a short circuit study completed by an outside firm. Resulting study found ~30 breakers that were over-dutied, their SCC rating was less than the available SCC at the bus.

Example: SCC available at 480V MCC bus is 33ka. Breaker feeding the MCC is rated at 25ka.

When I reviewed the study, they had incorrectly input some of the breakers. Some of the breakers are GE AKU-25 with fuses, which bumps up the breaker rating to 200ka.

But what about the molded case breakers feeding the individual loads from the MCC? If the SCC on the MCC is 33ka, the molded case breakers are 14ka (but the rating on the cubicle door indicates 25ka - someone here explain that?).

Do the molded case breakers need to be changed as well? If there is 33ka available on the bus, wouldn't it be available on the load side of the molded case breakers? Or, since the main breaker has a current limiting fuse, does that also protect the molded case breakers?

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[rbulsara]

In the USA, many molded case circuit breakers are 'series rated' with their main breaker and/or fuses to provide higher SCCR than their own rating. These series combinations are determined by testing (UL) and are listed as such.

Look up the manufacture's data or ask a rep.



Rafiq Bulsara

http://www.srengineersct.com

 

[dpc]

If you are talking about combination motor starters, the complete starter unit can have a short circuit rating that is higher than the SC rating of the molded case breaker. It's MCC Magic.

 

[controlnovice]

Forgot to mention in the first posting:

The breakers are rated 14ka. There is a label on the cubicle that the combination is rated for 25ka.

Our MCC has available SCC of 33ka.

Looks obvious to me that they need to be changed.

Am I missing something?

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[rbulsara]

What does the main label for the whole MCC say?


Rafiq Bulsara

http://www.srengineersct.com

 

[dpc]

Does the MCC have a main breaker?

Google up "series rating" and you can see it MIGHT be OK as is, if the breakers in the buckets and the upstream breaker are listed combinations with a series rating exceeding your 33 kA.

Without the series rating, then yes, it is a problem. But not as uncommon as you might think.

 

[controlnovice]

rbulsara:

MCC has SCC rating label of 25ka also.

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[dpc]

Then you need to check the next upstream breaker to see if a series rating might apply. If not, you have a problem.

 

[rbulsara]

OK, your assessment appear right. As dpc said you may want to check series rating with the main, if it exists. If too many small breakers to replace, you can explore (compare costs) installing a fused switch as the main, if the breakers are series rated with class L fuses.

Rafiq Bulsara

http://www.srengineersct.com

 

[controlnovice]

Just found they are not series rated with our main (main is in switchgear, not MCC - my bad earlier).

Looks like replacement.

Having overduttied equipment puts us in the "Do not open, even with door closed" (essentially) level for Arc-Flash. Definitely can not work on when energized. (that's just my best guess from experience. haven't run study yet)

What do we do in the meantime (until we can get funds and correctly rated cubicles) about maintenance?



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[rbulsara]

Maintain only when de-energizedm which all should be doing when at all possible.

I not sure where people are getting the idea that an arc flash analysis and PPE makes it safe to work live. PPE only reduces 'possibility' of the burn injury to a treatable level (level 2, which is curable but most painful level). All other hazard like those from pressure, shock and shrapnel still remains.

One can still die, if he/she cannot separate from a live part in time. Arc flash calcs assumes that the person is not in direct contact with the live bus or immediately separated after the contact.


Rafiq Bulsara

http://www.srengineersct.com

 

[dpc]

As Rafiq says, the gear should be de-energized before operating any devices, and the area around it should be roped off.

But don't confuse arc-flash with short-circuit interrupting issues. Even with adequately rated breakers, the arc-flash hazard can still be quite high. It just depends on the upstream device, the clearing time, and the arcing current.

Underrated breakers are another issue unto themselves. There is no arc-flash calculation or rating related to failure of underrated breakers.