Indeed, the AVR proceeds to a comparison between Uh1 and Uh3 in order to determine whether or not a short-circuit has occurred.
This function allows to increase the excitation current to ceiling in order to provide 3xIn in case of a short circuit on the generator outputs.
When the fuse on Uh1 blows, the AVR interprets the event as a short-circuit situation ( no UH1, still UH3).As a consequence is to observe an under-voltage if the fuse blown, replace the UH1 fuse allow to solve this issue
After 10 s there is an automatic excitation shutdown between 0.5 and 0.8 A.