In PTW32, Arcflash-study options, under fault current, there is an option so that you can check the box to enable Use equipment specific arc flash equation in protective device library. Any one knows how does that work?

If I select this option, sometime I get very high incident energy at the working distance for the equipment where it could be very low if I don't select this option. However, if I do it manually based on the fault current, distance and clearing time I get from the TCC curves, the result matches what I don't select that option.

Can anyone explain why we have to use specific arc flash equation for some equipment?

It sounds like this might be based on Table 20, Section 9.14 in IEEE 1584-2002. That section/table attempted to simplify the process with generalized equations of circuit breakers for the calculations.

Here is an excerpt from this section:
Calculations were performed for a broad range of low-voltage circuit breakers in order to find those with the
highest values for incident energy and flash-protection boundary.

Back in 2002 it seemed like a good idea but now that we know so much more as we move ahead with the next edition (draft sitting on my laptop) it is almost certain this information will be deleted. It would be best to ask the S/W manufacturer to see if this is what they did but that is my first thought.

...and my usual disclaimer, I'm Secretary of IEEE 1584 but this opinion is mine and may or may not reflect that of IEEE and/or the Working Group.

Thanks Jim! I am not sure why the fuse manufacturer also publish similar arc flash equation as well. IEEE-1584 only covers low-voltage breakers.