Overdutied Breakers
Overdutied Breakers
(OP)
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?
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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RE: Overdutied Breakers
Look up the manufacture's data or ask a rep.
Rafiq Bulsara
http://www.srengineersct.com
RE: Overdutied Breakers
RE: Overdutied Breakers
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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RE: Overdutied Breakers
Rafiq Bulsara
http://www.srengineersct.com
RE: Overdutied Breakers
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.
RE: Overdutied Breakers
MCC has SCC rating label of 25ka also.
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RE: Overdutied Breakers
RE: Overdutied Breakers
Rafiq Bulsara
http://www.srengineersct.com
RE: Overdutied Breakers
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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RE: Overdutied Breakers
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
RE: Overdutied Breakers
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.