A MOV is normally open circuit.
If the voltage across it exceeds its nominal rated voltage it will very quickly clamp it by becoming very low impedance, shunting an overload through it for instance.
When shunting a MOV is dissipating all of the energy across itself and therefore it’s often used in combination with a PTC placed in series with the input. The PTC is slow to act but will heat up and increase in resistance in the event of a MOV and provide the longer term protection.
Using MOV’s in series
- You can dissipate more power / have the MOV’s last longer in an overvoltage event
- You create more creepage distance