Trouble understanding overvoltage protection circuit
When the current in R5 gets high enough to achieve a voltage drop of 0.7V, Q1 turns on reducing the current in Q3 and limiting it at 0.7/R5=1.7mA (current limit to protect U1 and U2)

Turning on Q2 will short circuit 220V over R1, and if it is something like a ptc resettable fuse it will "block" the overvoltage situation.