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 Post subject: hrc question
PostPosted: Sat Apr 24, 2010 6:44 am 
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If the switchgear that feed mcc are rated at hrc 4 does this mean the main breakers at the mcc are rated at hrc 4 ? And would anyother part of the mcc be rated the same , like a tie breaker ?


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PostPosted: Sat Apr 24, 2010 8:22 am 
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PUCKMAN wrote:
If the switchgear that feed mcc are rated at hrc 4 does this mean the main breakers at the mcc are rated at hrc 4 ?


For all intents, once you have some amount of conductor between one location, your switchgear, and another, your MCC, is is possible that the two locations will have different HRC levels. I have seen situations where the HRC has gone up, almost as often as it has gone down.


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PostPosted: Sat Apr 24, 2010 9:18 am 
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And please keep in mind that if there is a risk of arc flash on the line side of the downstream device, the clearing time of the upstream device should be used for the IE calcs and labels.


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PostPosted: Sat May 29, 2010 8:19 am 
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You need to clarify a little on the complete design.

If the switchgear that feeds the MCC is rated 4, but the MCC is fed by a power circuit breaker in that switchgear, then the MCC will be rated based on the calculation after the feeder breaker. Everything in the MCC is rated the same as the incoming since both the horizontal and vertical bus and tops of the mcc bucket disconnects are all that rating.

If the MCC is fed directly from the switchgear without a feeder breaker and the breaker is in the incoming section, then the incoming section is rated the same as the switchgear but everything after the breaker is rated based on the calculation after the breaker. In that case the horizontal and vertical bus will most likely have a lower rating. The only issue with this is the design of the MCC. The incoming section must be fairly isolated from the balance of the gear with sheet metal, baffles, etc. so that the balance of the MCC can be considered isolated from the incoming section.


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