Class L fuses showing different incident energies

Question:

In the attached library are three fuses – Bussmann KLU, Bussmann KRPC & Littelfuse KLLU.

Below are screenshots of the results from the three fuses, all using 1200 amp trip. The two Bussmann’s give the same result, which I would expect. What I don’t understand is why the Littelfuse KLLU gives a different result even though it appears as though it should be using the same formula as the Bussmann’s! Why would that be?


Littelfuse KLLU 1200:

Bussmann KLU 1200:

Bussmann KRPC 1200:

---

Answer:

The difference is due to the order of tripping. When KRPC fuse is used, it is the first device to trip before the motor decays. The accumulation of IE stops on the first device as it clears most of the fault (80%). This results to a lower IE of 2.9. With KLLU fuse, it is tripping after the motor decays as seen in the plot below. Thus, the IE is calculated with the motor decay first which results to 8.1, then add the IE of 2.9 when the fuse trips, which totals to 11.