Arc Flash Incident Energy - main breaker vs line-side fuse calculation

[Magic Gorge]

For a double-ended SWGR I'm evaluating, the worst-case is with the tie open (non-paralleled). The main breaker trip/delay time is 2 seconds, and reported as PPE level 4. The upstream fuse trip/delay time is closer to 0.3 seconds, with PPE level 2. So, how is it that a fault at the SWGR would extend to 2 seconds without the upstream fuse tripping?

 

[jim dungar]

It is possible the software is not showing the fuse correctly. I don't know how this happens, but I think it happens during editing. I remember having to remove the CB, in this case, and then reinsert it into the one-line.

 

[ron]

Most software use 2 seconds as an assumption that the worker has escaped the area and stops the calculation. I hate that assumption.

 

[D. Castor]

If this fuse is on the primary of the transformer, it's entirely possible. The fuse clearing time of 0.3 sec could be based on a fault on the primary side of the transformer.

Based on the recommendations in IEEE 1584, 2 seconds is used as a maximum fault duration. When you see an arc time of 2 seconds in the program's arc flash report, it means that nothing had cleared the fault by two seconds.

If you look at the time-current curves and compare with the calculated arcing current, it should be easier to understand.