Fusing Issue

[sswing11]

I've inherited the re-design of a machine that implements two 5KVA ballast transformers. The present over-current protection is provided by midget-style, rejecting, Type CC, Slo-Blo fuses. My experience tells me that inductive loads are better protected by Type J fuses, but I'm new to this sort of large transformer application. Any comments?

 

[coconutalley]

sswing11,

I don't have experience with fusing ballast transformers but
I have worked on high current pulse applications.
Typically, you'd want to start with NEC requirements to protect the transformer and wiring [is it 125% of maximum?].
Then look at your application and see if this is going to cause premature trips. Look at the I square t chart and see where the load current is in relation to the trip curve. In high current pulse low frequency applications the RMS current might be lower but the peak-peak current could be very high. This has been known to cause electro-mechanical and thermal stresses on fuse elements causing premature fuse failure. The solution ended up with twice the RMS rating and bigger gauge wires.
Typically slo-blo is chosen for inductive loads but different fuse manufacturers has different names for similar types. Best to compare I^2t charts. You can use a fast acting fuse in place of slo-blo but you would pick a very high trip current to avoid premature trips.