how to calclate do fuse rating to be used

The selection of source-side fuses for transformers needs to take into consideration a number of factors … the most significant of which are: sizing the fuse slow enough to allow for magnetizing and cold load inrush currents; and sizing the fuse fast enough to protect the transformer from excessive mechanical and thermal stresses that occur during faults and overloads.

The fuse(s) should be selected to protect the transformer from excessive thermal and mechanical stresses per ANSI/IEEE Standard C57.109.

The fuse(s) should be selected slow enough to allow for the cold-load inrush currents and magnetizing currents that happen when the transformers is energized. One rule of thumb for drawing a magnetizing current curve is to draw a curve through the following points:

• 1 second – 600% of name-plate full load
• 0.1 second – 1200% of name-plate full load
• 0.11 second – 2500% of name-plate full load

If the load will be connected to the transformer during re-energization, cold-load pickup must be considered also. This is typically represented by a curve though the following points:

• 100 sec – 200% of full load
• 10 seconds – 300% of full load
• 1 second – 600% of full load

You will still need to select a fuse style suitable for your application, bay-o-net, cutout-mounted expulsion fuse, power fuse, full-range current limiting fuse, etc.

IEEE Std C57.109-1993, IEEE guide for liquid-immersed transformer through-fault-current duration.