And transistors are there to protect the fuses
I once saw a cascade of blown fuses ranging from a 3A one in the plug to a 30A MCB protecting the subcircuit. I find it difficult to believe that the energy let through by the lowest rated fuse (usually quoted as I*I*t) was enough to trip or blow the bigger ones but that's what happened.
The original fault was a mains filter cap that exploded in a colleague's face. Fortunately he was unhurt. This all happened at BBC Kingswood Warren way back in 1974.