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 Post subject: Line Side Main Breaker >40cal
PostPosted: Tue May 26, 2009 7:52 am 

Joined: Tue May 26, 2009 7:35 am
Posts: 1
Location: Fredericton, NB
I recently completed an arc flash study where the line side of a main breaker in 480V metal enclosed switchgear has an arc flash hazard category of Dangerous because the upstream protection is located on the primary side of a step down transformer. The remaining portion of the 480V switchgear including the load side of the main breaker, three feeder breakers as well as the vertical and horizontal bus all have a category of 3 because the main breaker is able to clear the arcing fault in a timely manner. The switchgear consists of two sections, an incoming 32" wide section housing the main breaker and a 22" wide section containing three feeder breakers.

What if a person is performing electrical work in the 22" wide feeder section, and an arcing fault incident occurs on the line side of the main breaker in the incoming section that is labelled as category Dangerous?

Altough the probability of this happening is low, I would like to determine how the code addresses this scenario or identify what the best practices are for this situation. Currently the interpretation of the code and best pactices are unclear to me when applied to this example. If the worker is not directly working on the line side of the main breaker does category Dangerous and associated flash protection boundary still apply? Should the 480V switchgear be labelled on a cell by cell basis or should it all be labelled as the highest category? For example, should the incoming section be labeled as Dangerous with the feeder breaker section labelled as category 3 or should the entire switchgear assembly be category dangerous? Note that my calculations show that the flash protection boundary associated with the main breaker being category dangerous, encompasses the entire switchgear assembly.


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PostPosted: Tue Jun 02, 2009 2:19 pm 

Joined: Tue Jun 02, 2009 1:40 pm
Posts: 19
1. OSHA says don't work on it live unless there is no other way. Based on the danger to the equipment (not even counting the personnel) why take the chance?

2. Use current limiting fuses in the primary side of the transformer, this will reduce the hazard.

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