Arc Flash Questions 1

[timm33333]

I have a couple of questions regarding arc flash study as below:

1. In arc flash study when we assume that the main breaker of the MCC is isolated from the bus by a barrier, then should we print two labels for the MCC (first label for the cubicle which contains the main breaker, and the second label for all other cubicles of the MCC)?

2. When we perform coordination of phase breakers as part of arc flash study, the settings of phase breakers change which affects the incident energy. Will the coordination of ground breakers also affect the incident energy? In other words, is ground coordination also required as part of arc flash study?

Thanks.

 

[dpc]

1. If you decide that main breaker is sufficiently compartmentalized from the feeder sections that an arc won't propagate, then yes, you will have two different arc flash levels. One for the feeder sections and one for the main breaker section. So at least two labels would be needed.

2. If IEEE 1584 is used, all calcs are based on the 3-phase faults, so ground fault protection has no impact on the calculated incident energy or PPE required. That said, since most faults at least start as ground faults, good ground fault protection can result in reduction of risk of an arc-flash event. But it doesn't change what is on the arc-flash label.

 

[davidbeach]

“In a box” an arcing fault will evolve from SLG to 3LG in around a cycle. Ground protection is relatively immaterial except to the extent that a ground instantaneous may get to the trip decision a fraction of a cycle sooner than a phase instantaneous.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[timm33333]

Thanks. How about the top part of the MCC which has horizontal bus, the conductor orientation in this part is horizontal (HCB). Do we need label for this top part, or is label not required as there would be no work there?

 

[davidbeach]

If it isn’t designed as arc resistant I wouldn’t bother with multiple labels. For non-resistant gear I’d just label the whole thing with the worst section label and be done with it. Put the overall arc-flash responsibility on something upstream and not in the gear.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[cranky108]

More of a note, and it may not apply. But if your MCC has a tie breaker, it might have a higher arc-flash than the typical feeder. You should look at that also.