Hi John,
1 - NO
2 - NO
The 20 amp fuses in the TIU's protect the outputs. If a short occurs between the transformer and the TIU the Z-4000's should cut power and the TIU fuses would be undisturbed. If a short occurs beyond the TIU's outputs the Z-4000's electronic breakers will react and cut power long before the fuses blow.
The problem you encountered likely has nothing to do with a short circuit or excessive current draw. Rather, the problem is likely a loose connection inside the TIU's binding post. I have seen this on a hand full of occasions over the years and it seems to be caused by improper assembly procedure at the factory. The binding posts of the TIU are held to the case by a threaded nut and lock washer. In recent years they have also applied a drop of lock-tite to prevent the nut from backing off. However, none of these efforts will be effective unless the person installing the binding post first loosens the red or black plastic nut on the outside of the case. If the plastic nut is still tight the internal nut will seem to pull up tight, but will be loose as soon as the plastic nut is backed off. When the post gets loose the wire from the circuit board to the binding post will have an intermittent contact that starts to arc. The heat from the arching melts the case, wire insulation, etc. Even one amp of current draw can heat the post enough to melt the case. The best way to prevent this from ever happening is to keep the plastic nuts tight and use banana plugs on the outside of the TIU.
