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Largest Calculated Incident Energy?
https://brainfiller.com/arcflashforum/viewtopic.php?f=33&t=4391		 Page 1 of 1
Author:  Jim Phillips (brainfiller) [ Sun Jul 16, 2017 1:32 pm ]
Post subject:  Largest Calculated Incident Energy?
Since 130.7(A) Informational Note 3 referencing the 40 cal/cm^2 threshold is disappearing, I thought it might be interesting to compare notes on the largest calculated incident energy that anyone has seen.

If a 2 second cutoff was used (IEEE 1584-2002), the incident energy would of course be lower.

Here we go:

What is the largest calculated incident energy that you have seen (cal/cm^2)?
Less than 40
40 to 75
75 to 100
Greater than 100
Author:  wilhendrix [ Mon Jul 17, 2017 8:26 am ]
Post subject:  Re: Largest Calculated Incident Energy?
The highest we've seen was north of 160 calories at 2 seconds. The problem was the original fuses installed were 3000 amps.
Author:  tish53 [ Mon Jul 17, 2017 9:03 am ]
Post subject:  Re: Largest Calculated Incident Energy?
I have encountered many initial calculations over 40 but through fuse changes or CB setting adjustments able to get all lowered to around 18 and most to below 8 cal/cm2. Most dangerous spot for us is a long cable run to a sump pump and the short circuit current drops to such a level that it takes a long time for the fuse or CB to operate
Author:  bbaumer [ Tue Jul 18, 2017 3:47 am ]
Post subject:  Re: Largest Calculated Incident Energy?
This is the highest I can think of at the moment. It's from a study I barely started modeling on a building with dual services, one 4000A and one 5000A, 480V services with tie that is kirk keyed so you can't close both mains together. I quit on the study until such time I can schedule a shutdown in order to survey these boards. I'm not comfortable opening them up live to verify cables and fuses as there are absolutely no barriers between sections so a fault at the line side of the main can blast through all sections unimpeded. You can see we've already greatly reduced the main fuse sizes from what were in there at one time due to actual load in this building being far less than the switchboards are rated for.

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Author:  jellison [ Tue Jul 18, 2017 3:52 am ]
Post subject:  Re: Largest Calculated Incident Energy?
Fire Pump :o
Author:  bbaumer [ Tue Jul 18, 2017 5:49 am ]
Post subject:  Re: Largest Calculated Incident Energy?
jellison wrote:
Fire Pump :o


Yep, this is often an issue for us. We have dozens of fire pumps on campus and most of them have normal power fed straight from the pad mount secondary with no OCP and the emergency fed from a generator with with a 2 second limitation.

I've gone to adding a fusible disconnect on the normal power feeder sized to handle locked rotor current which helps keep the incident energy down.
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