Prot_A and Prot_B are triggered low in a fault condition by several parts of the circuit.
They operate the relays in the bottom right corner of the schematic.
The transformer winding is connected via diodes to both, so it makes sense that if the transformer overheats, and the inbuilt thermal switch connects this winding to ground, both Prot-A and Prot_B will be grounded, thus triggering both relay circuits and saving the planet.
Most transformer thermal fuses I have seen are non resettable normally closed, you now need a new transformer should it open type.
It is possible Peavey had a rather better idea in this case, and put a normally open one in there that would trigger the protection circuits should it overheat, then when it cools down you can do the same thing again until you get fed up with it.
Dunno but it kind of makes sense, where the non resettable ones just don't.