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Littelfuse Article

Littelfuse Article

Littelfuse Article

(OP)
I am attaching a file that contains:
1. An article by Littelfuse on Misconceptions About Arc Flash Hazard Assessments
2. My EasyPower model of the Littelfuse example in the above article.
3. TCC showing breaker and fuse curves

My reason for posting is to solicit feedback on my EasyPower model and maybe some other people can model it in other software such as SKM, ETAP, EDSA, etc. It would interesting to see those results. For my model, I used an infinite source and adjusted the transformer impedance to match what Littelfuse had for the short circuit current. I also assumed steel conduit and THHN insulation.

As you can see, I did have some agreement on the Main bus (Littelfuse: 135.8 cal/cm2, Me: 107.2 cal/cm2), either way Extreme Danger. Things fell apart further on down the model. For example: Littelfuse at Disconnect Motor 4 has a SC 3.1kA, 9.5 cal/cm2, Me at Disconnect Motor 4 has SC 3.4 kA and 0.2 cal/cm2. So, I am confused on how Littelfuse arrives at 9.5 cal/cm2. I forced the arcing current to be 3.1kA and still was around 0.2 cal/cm2. I have included a TCC showing the breaker, and FLSR and LLSR curves.

Comments/discussion would be appreciated.

RE: Littelfuse Article

Unless you can look at the breaker curves and fuse curves used by Littlefuse, there is no way to compare.

Also, by default in EasyPower, the IEEE 1584 current-limiting fuse equations are turned off and only the fuse TCC is used to determine arc time. If you turn on IEEE 1584 CLF fuse equations, the results will be different.

When it comes to arc-flash, Rule #1: Not an exact science.

RE: Littelfuse Article

(OP)
True about the current limiting fuse but I don't think the currents referenced are in the current limit region. I also used the 100% and 85% currents to calculate. If you get the time from the TCC and input that into the formulas in NFPA 70E, one still does not arrive at the ~9 cal/cm^2 figure that Littelfuse has. I expect to have a conversation with them next week and this article is from 2007 and is not available on their website anymore...hmmmm...

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