I am doing an arc flash assessment and have came across a strange observation. Wanted to get some confirmation from others experience....

I have a LV system (0.415kV) fed by transformer (3.3/0.415 kV) protected by a 250A HRC fuse (GE K81 PEX) for shortcircuit on HV feeder. For a fault level on 0.415V LV side (~19kA 100ms symmetrical rms), the fault current seen by fuse on HV is around 2kA. As per the pre-arcing time vs. prospective current graph of fuse, it will melt in 4s. This gives a hazard category of X

My take on the observation is: Fault current on LV is much less than the switchgear rating (63kA), which when reflected on primary would be even less ..... hence fuse take more time to trip this giving high hazard category.

Want to confirm I am not doing anything wrong. And such observations with LV are usual???

Energy & Time

You have just understood the relationship to the release of energy, and the time it takes to clear it.

Higher categories can be achieved at lower current levels when the time it takes to clear is excessive. Just like the sun, you can get a second degree burn if you are exposed too long.

Yes this is a typical problem for industrial plants, can be mitigated but not easy to do and expensive.

There is always the 2 second rule in the IEEE 1584. If you use this as a cut off time it may offer lower IE.