Q: Maximum fuse rating before a MOV (varistor) I just watched some scary videos of MOV's catching fire and I've flee'd here to ask you guys for help. 
I understand that it's a good idea to put a fuse in front of a MOV, considering it fails short-circuit. However, is there a maximum fuse size that can be used before the MOV? 
I'm pondering the possibility of a MOV failing partially short-circuit which then runs away to become a fire hazard. i.e. the short might be enough to blow a 1A fuse but not enough to blow a 13A fuse. 


*

*Would it be safe to rely on a varistor to blow a standard, household 13A fuse found in a mains plug?

*Would it permit too much power? 


A: Commonly, for example in multiple outlet powerboards that include a MOV for surge protection, a thermal fuse is also used.
The thermal fuse is in contact with the body of the MOV.
When the MOV starts to fail, its power dissipation and temperature increases, and the thermal fuse opens in that case.
Often, an indicator LED shows that the thermal fuse is OK. When it goes out, it means that MOV has failed, the thermal fuse has blown, and surge protection is no longer available.

A: Choosing the right fuse when using a varistor is not really different from choosing a fuse when not using one. 
A fuse should be rated according to the expected maximum current consumption of the protected device. A fuse should be as small as possible, but as large as required for the device to operate without triggering it during normal operation.
A fuse should blow as fast as possible in case of a failure to minimize damage. If startup inrush current or sporadic current spikes are to be expected, a "slow" fuse will help. It is common to have a pretty conservative safety margin like factor 1.2-1.5 to prevent a fuse from failing too early, but it really depends on the type of fuse and the application.
There are potentially more consideration and its not really a subject as trivial as it seems at first.

A: @coates - I just realized I was thinking of thermistor inrush current protectors with regards to the load current. The MOV protects the load from excessive voltage, but when it fails shorted there won't be any excessive current in the load, just in the wiring upstream from the MOV - I.E. your house wiring. So the 13 amp should be ok in all instances provided the circuit breaker rating is not lower than that.
