Select HRC fuses for transformer

The approximate steps to select the appropriate fuse rating for transformer. (500kVA whether it is cast resin or oil TX).
The basic Principle for Transformer Protection (by using HRC fuses)
A)- Rating equal 150% of the primary current (under no circumstances to be more than 300% - this is exceptional if the adopted secondary fuse rating at LV not exceeding 125% of the secondary full load current).
B)- Compliance to particular conditions related to protection of secondary side.

Accordingly, for 500kVA transformer, in lack of the required information indicated in my previous comments, the rough steps to choose the fuse rating may be as follows:-

a. Primary line current, Ip= 26.3 A.
b. The expected fuse rating = 26.3 X 1.5 = 39.45 A
c. Then the nearest Standard rating is 40 A.
d. Cross-check the Fuse Tripping curve, as per manufacturer recommendation, against the inrush current (= 12 X 26.3 A) to determine the tripping time “t”.
If, t ≥ 0.1 s then 40A will be the appropriate fuse rating.
If, t ≤ 0.1 s then you have to choose the next rating Standard i.e. 63 A will be the appropriate fuse rating.
e. The max. Fault current at the secondary (assume % Imp. =5%) = 20 X secondary current-Is), the counterpart primary current will be = (20 X 500 X 400)/ (1.73 X11 X 1000) = 210 A (More than 3 times of the rated primary current, which is sufficient to clear the fault within the prescribed time).
f. Discrimination / coordination to be ensured with upstream & downstream protective devices.

In light of the above, I would like to conclude the following:
i. To cater the magnetizing inrush current the fuses to be chosen to withstand 12 times the full load current "Ip" for 100 ms (i.e. delay of 0.1s).
ii. Assume the fuse is used as the instantaneous device which with advantage for fault level between HV & LV sides of Transformer, therefore it is deemed as a backup protection for LV side.
iii. Assume the transformer voltage Impedance = 5%, hence the short circuit at LV terminals will be 100/5 = 20 times Full-Load current (Is).
iv. The fault level on HV side is usually high (mostly 500 MVA) and greater than the fusing value which will be cleared quickly.
v. M/s ABB issued a table (based on German standard), for their products, which shows that the permissible protection of transformers on MV – side (12 kV) for cartridge fuses type CEF as 25 A for 250 kVA TX & 40A for 500kVA TX..
vi. As per DIN VDE 0670 P-4, I verified the fusing time for the Inrush current (12 X Ip = 315 A) against the ABB fuse Melting characteristic. I found that at 0.1 s the tripping current for 40A fuse would be about 300A while for 63A about 500A. Although 40 A seems to be feasible, to my opinion 63A is the most appropriate rating.