If a transformer were to choose a primary fuse rating that was higher than its rated current, it would have the ability to see fault current indefinitely. Fuses on the primary side of the transformer are typically chosen based on their characteristics (i.e. time delay vs amount of overcurrent before they blow). For liquid immersed transformers of various rating classes, there are various graphs and tables that define the levels of short time thermal overloads, such as the IEEE 57 series (of which 57.109 focuses specifically on fault current duration). Additionally, they have recently attempted to consider not only thermal damage, but also mechanical damage. All kinds of real-world factors complicate the process, including coordination of secondary side protection with primary side protection, and variables beyond the minimum requirements found in standards.
