After re-reading the wording in my last comment, and the subsequent response, I wanted to clarify a bit. I didn't mean to imply that the choice of primary fuse rating would somehow allow a transformer to indefinitely see fault current at some high value beyond its ratings. Primary side fuse choice, and the characteristics of the fuse (e.g. time delay vs amount of overcurrent before it blows) is in turn based on the transformer characteristics. Transformer related standards, such as the IEEE C.57 series (C.57.109 in particular for fault current duration) have all sorts of graphs and tables setting levels of short time thermal overloads for liquid immersed transformers of various rating classes. More recently they've tried try to factor in not only thermal damage but mechanical damage as well. For the type of transformer we've been discussing, a 2x rated current overload might be allowed for 30 minutes, while a 25x overload might only be allowed for 2 seconds. As always, there are all sorts of real world complicating factors, including coordination of primary side protection with secondary side protection, and product specifics that might go beyond standards minimum requirements.
At this point, I'm already beyond what I feel comfortable commenting on. Anything more and I'm likely to open my mouth only to change feet, so best that l leave it here.
