Arc flash in main switchgear.

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Arcflashnewbie

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Ok, here is the scenario:

480 comes into a switchgear. Incident energy level is 0.72 cal/cm^2 above the main breaker. After main breaker the IE is 17 cal/cm^2. On the buss of the switchgear one assumes the IE is 17 cal/cm^2. Now, if a bucket has a higher IE (let's say, the bucket for building buss 2 has an IE of 58 cal/cm^2) which sticker do you place on the switchgear?


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Arcflashnewbie said:
Ok, here is the scenario:

480 comes into a switchgear. Incident energy level is 0.72 cal/cm^2 above the main breaker. After main breaker the IE is 17 cal/cm^2. On the buss of the switchgear one assumes the IE is 17 cal/cm^2. Now, if a bucket has a higher IE (let's say, the bucket for building buss 2 has an IE of 58 cal/cm^2) which sticker do you place on the switchgear?


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I do not see any credible mechanism other than motor contribution or current limiting fuses to explain a higher IE after the main breaker than before it. Who did the calculations?

I would say that you either put a separate sticker on the one bucket or rate the whole switchgear at the highest IE.
And again, I am having a problem understanding the higher IE for one bucket. Higher for one bucket than for another bucket, sure, since it depends on the exact trip curve of the OCPD, but higher than the incoming IE when there are no transformers involved just does not sound right.

PS: I looked at your profile, and if there are job openings for arc flashes, I may change my profession. :)
 
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The bucket rating should be based on the maximum possible incident energy available from conductors within that bucket.

Since the line-side conductors on the bucket MCP have the same available fault current as the MCC feeder bus, that is your incident energy for the bucket.

I also agree that it seems very unusual to have a 2,350% increase on the load side of your main: That would be a heck of a motor contribution.
 
Rarely do you get labels for individual buckets. Are you trying to put the label for the 'load' on the feeder bucket?
It is entirely possible for you to have one IE level at your main equipment and then have an extremely higher IE at a downstream piece of equipment.

Have you had training in label installation?
 
I have been doing electrical system studies for a number of years and have not seen this situation. Normally the line side of the main circuit breaker would be very high as the protective device would be the protective device on the primary side of the transformer. Typically, this would be higher than the load side even with motor contributions.

Since the main breaker is typically in the same enclosure as the feeder breakers, the main breaker cannot be used for arc flash for the MCC. Therefore, the rating of the entire unit is taken as the incident energy with the fault being interrupted by the transformer primary protective device.

I will send you a PM if you want to send me the report to look at. I am very curious.
 
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