MOV (Metal Oxide Varistors) are critical components required in most power supplies when compliance with surge requirements is a must. The surge withstand voltage will depend on the size of the bulk capacitors (if any present), series components (like inductors for EMI filters) and also to a certain degree on how the layout is made.

The rated voltage of a MOV will determine when the MOV will start conducting current, and therefore it is critical that the MOV is rated for a voltage that is higher than the maximum allowed input AC voltage.