Arc Flash Label on Main Breaker vs Children Breakers

[TM519]

I have only been doing arc flash analysis for about a year, so I still have a lot to learn, and this site has been great with that.

I overheard the following question and I'm hoping someone can help explain, I don't completely understand the language and perhaps that is what is holding me back.

The question was if on an MCC if the main breaker should have a different arc flash label than the children breakers. I am hoping someone can help explain what is meant by "children" breakers, as this is the first time I have heard any breakers referred to that way. I'm assuming they are the smaller breakers inside each cabinet that individually feed each load, and the main breaker is the main breaker leading into the cabinet, which is a much larger breaker than the ones on each individual load. If this is the case, where would the arc flash labels go physically?

I do know the MCC that they are referring to, so I can try to answer any questions if needed as I can pull up the one-line. The MCC is 480VAC fed from a 4160/480 transformer. That transformer is fed from one compartment on 4160V switchgear. There is a 2500AF circuit breaker between the low side of the transformer and the MCC, and then there are smaller breakers (400AF, 100AF as labeled on the one line) in between the MCC and each load.

I appreciate all input.

 

[mopowr steve]

Never heard of “children breakers” before but I assume your assumption is correct.
I believe that the “children breakers can have a lower AIC rating if the panel has/or has been engineered as a series combination AIC rating. Where the main would have an AIC rating equal or greater than the available fault current and the branch circuit breakers have a lesser value.

 

[Hv&Lv]

Your assumption is correct.
we actually use that term a lot in our system modeling.
fuses have children(transformers) transformers have children(services to meters). They are all children to the parent Substation.

 

[wbdvt]

It has been years since I heard someone use that term, but yes they are referring to the individual breakers in each MCC cubicle (bucket).

Unless the MCC main breaker is completely isolated from the rest of the MCC, there is only one label and incident energy value for the entire MCC. If the MCC main breaker is completely isolated, then there could be 2 labels on the MCC. One for the main breaker as the fault there would be cleared by the upstream breaker and a second label for the rest of the MCC as the arcing fault would be cleared by the MCC main breaker. The incident energy levels should be different, probably significantly so.

 

[TM519]

It has been years since I heard someone use that term, but yes they are referring to the individual breakers in each MCC cubicle (bucket).

Unless the MCC main breaker is completely isolated from the rest of the MCC, there is only one label and incident energy value for the entire MCC. If the MCC main breaker is completely isolated, then there could be 2 labels on the MCC. One for the main breaker as the fault there would be cleared by the upstream breaker and a second label for the rest of the MCC as the arcing fault would be cleared by the MCC main breaker. The incident energy levels should be different, probably significantly so.

Thank you for such a clear explanation that all makes perfect sense to me. I've really learned so much since I started coming to this site this month.

 

[paulengr]

Never heard of “children breakers” before but I assume your assumption is correct.
I believe that the “children breakers can have a lower AIC rating if the panel has/or has been engineered as a series combination AIC rating. Where the main would have an AIC rating equal or greater than the available fault current and the branch circuit breakers have a lesser value.

AIC has nothing to do with arc flash. That is for bolted faults.


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[paulengr]

Never heard of “children breakers” before but I assume your assumption is correct.
I believe that the “children breakers can have a lower AIC rating if the panel has/or has been engineered as a series combination AIC rating. Where the main would have an AIC rating equal or greater than the available fault current and the branch circuit breakers have a lesser value.

AIC has nothing to do with arc flash. That is for bolted faults.