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 Post subject: Generator Supplied Panels
PostPosted: Tue Jun 24, 2008 6:50 pm 
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Joined: Thu Jan 10, 2008 8:49 pm
Posts: 480
Location: New England
I have some 208/120 and 277/480 breaker panels that supplied both from the grid and from a generator via a automatic transfer switch. The generator is 250kw with a MCB, the panels when fed from the grid are all below 4.0 cals/cm2.

I need to label the panels with Arc Flash labels. Since the normal service will typically takes place when the panels are on the grid I want to just use 4.0 cal/cm2 and Level 1 PPE.

I have some doubt if the MCB on the generator would even trip. If it does it would take a very long time, like 30 seconds. Using this time value in the IE formula would no doubt result in something higher than 40 cals.

I'm thinking about just using the normal grid conditions for the label, 4.0 cal/cm2 and Level 1, and adding a line to the label, "Caution - Obtain Live Work Permit when Generator supplied"

Any opinions or suggestions welcomes.


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PostPosted: Wed Jun 25, 2008 9:37 am 
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Joined: Thu Jul 19, 2007 7:54 am
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Location: St. Louis, MO
IF the generator is sized correctly, chances are the hazard will not be much higher than the grid, especially if generator decay is taken into account.

I would suggest performing the calculation both ways, but use a maximum of 2 seconds for the generator feed. This should be very conservative. Also calculate short circuit currents from the generator using the generator internal impedances.


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PostPosted: Mon Jun 30, 2008 8:25 am 
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Location: New England
The generator is 250KW or 312KVA @480V. So this is 375A at Full Load. Impedance is 15%, so fault current is 2504A. This would result in an arcing current of 2010A. Generator MCB is 400A. This would be a multiple of 5 at 100% and 4.3 at 85%. On the trip curve this equates to about 80 secs for the breaker to trip. Of course I doubt the generator would produce this much current for this duration. This is what is hard about generators. If I take 2 seconds and put it in the calculator I get a value of 11.7 cal/cm2 or PPE level 3. Under normal grid power the panels are all Level 1, less than 4.0 cal/cm2. So again I am thinking to just put a label that says no work without a permit while on generator.


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PostPosted: Mon Jun 30, 2008 11:27 am 
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Location: St. Louis, MO
With the verified calculations, your method would probably be preferable, especially with a two category jump which put the PPE class above Cat 2. This also assume that the system is rarely on generator feed. If you were often on generator, I'd say go ahead and post Cat 3.


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