Arc flash analysis on circuits with fuses

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EC Dan

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I'm doing an arc flash analysis on a circuit with a fast acting fuse (Siba URE series). I'm used to doing these analyses on circuits with breakers but it seems like my strategy needs to be a bit different with fuses. One potentially complicating factor is there are four fuses in parallel (these are installed in a power controller). Can I get some input into my methodology:
  1. Determine bolted fault current at destination downstream of fuse
  2. Determine arcing current for the same location
  3. Divide the arcing current by four (number of fuses in parallel)
  4. Use that fractional arcing current to get the maximum let-through current per fuse based on the fuse cut-off current curve and the clearing time based on the fuse TCC
  5. Multiply the maximum let-through current per fuse by four
  6. Use that resulting value as the maximum arcing current with the clearing time to calculate incident energy
I get very small arc flash boundaries even on circuits with 35 kA available fault current, like 1-2 inches, but I guess that should be expected for fast acting fuses? Please let me know if I'm oversimplifying or there's a flaw in my methods.
 
If you have a time-current curve from the manufacturer for the four fuses in parallel, you can just use that and the total fault current. The maximum let through current is generally ignored for arc-flash calculations. If the fuses are in their current limiting range, the clearing time will be extremely fast and the resulting incident energy low. If the fuses are not in their current limiting range, the maximum let-through current is not relevant.

You don't mention how you are calculating the arc-flash, but in IEEE-1584, there is an alternative method for calculating incident energy downstream of CLF, but only certain sizes and type of fuses were tested. This is covered in an annex of the current IEEE 1584. Assuming you are using some type of software if you are using the latest version of IEEE 1584 (2018). If you're doing it by hand, you have my sympathies.
 
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