OK thanks. this is starting to come together for me. It does seem to be more than simple arithmetic tho. Say a 50 KVA transformer has a 6 amp fuse on a 13.2kv primary, it would theoretically limit the secondary to a bolted ground fault of 650a or so on a single 120v ungrounded conductor but we know the momentary AIC might be ~10ka depending on the impedance of the service conductors. I assume some of this is just the electrical inertia of the collapsing field but there is probably trip curve that carries an overload for a while.
From a practical standpoint we still have to consider a service conductor as unprotected. It would be interesting to see what a fault would need to look like to blow the primary fuse and what else might burn up first.
I guess I spent my 30 years in this trade on the load side of the service point and just took the PoCo side as magic.
