fuse VS MOV for overvoltage protection

[m4rtin]

As I understand, ATX PSU's usually have both fuse and metal-oxide varistor(MOV) for overvoltage protection. Do they serve different purposes? Or are there just two layers of overvoltage protection?

[M. András]

fuse is for overcurrent, what the mov will blow when the voltage reaches its treshold voltage

[Kremmen]

A MOV will short overvoltages. It is effective against short spikes but sustained surges can cause it to absorb more energy than it is capable. For that reason there must always be a fuse on the supply side of a MOV, to break the current in such a case. The energy capacity of the MOV should match or exceed the rupture energy of the fuse to avoid the MOV exploding.
So, as Andras stated the fuse breaks overcurrents and the MOV absorbs voltage spikes. The voltage margin should be sufficient to avoid MOV conducting under normal circumstances. For a say 230 VAC circuit, a 275 VAC MOV is usually specified.

[jeroen74]

If possible, use a MOV that has a built-in thermal fuse. After every surge a MOV slightly degrades (the voltage at which it allows 1mA to flow lowers) and it's very possible that after a small surge it becomes low impedance, not low enough to blow the fuse, but low enough to act a nice little heater that dissipates a couple of watts. Can set your house on fire. A MOV can literally glow red hot. And explode very nicely  covering the whole innards of your widget with black stuff.

IIRC in certain territories high mains is a frequent occurrence; that's a condition where the mains voltage is way too high for a couple of minutes.

[ptricks]

Fuses will not work for over voltage they are over current devices when used by themselves. MOV are voltage sensitive and will basically short the power and ground together in an attempt to absorb the surge, that shorting causes current to increase , and if the current increases long enough the fuse will blow.  Fuses also serve another function besides blowing when a MOV clamps, they limit current for the entire supply. If something blows in the supply like a FET , the MOV will do nothing but the increase in current from the shorted FET will blow the fuse.

Years ago when I went to college they taught us to design power supplies that were current limited on the input using a resistor sized for the desired output, we also designed supplies so that individual circuits within the design had resistors limiting the power to those sub circuits, these supplies were very safe because regardless of what happened in the supply nothing could exceed the rated current.  Now with everyone energy conscious they don't recommend these designs because of the power lost in the resistor.