MOV placement

[IvoS]

Should the MOV be placed before the fuse or after.

[wraper]

Try to think what will happen in a case if MOV fails by conducting current and there is no fuse before it.

[IvoS]

Yes, I am trying to think...... because I have seen it exactly like this in commercially made product. That's why I asked. It makes more sense to put it after the fuse like you wrote.

[wraper]

Yes, I am trying to think...... because I have seen it exactly like this in commercially made product. That's why I asked. It makes more sense to put it after the fuse like you wrote.

The answer is, it's dangerous. Many surge protestors were recalled because they used MOVs without any protection.

[SeanB]

You get MOV units with a built in thermal fuse, so you can have them before the fuse, and if the MOV fails the built in thermal fuse disconnects the power. However having the fuse before does add protection, at the expense of the fuse blowing more often from switch on surges as it ages, or from mains transients blowing the fuse as the MOV does it's job.

[boB]

Many ways to think about this.  For differential voltage surges like short term transient short term high-line voltages, just as you have it is the way to go.

Sometimes the in-MOV protection is just a semi-calibrated blob of solder that melts when it gets too hot.  In that case, a long term slightly overvoltage condition  can make it work as intended.  i.e., the MOV will heat up slowly-ish and melt the solder.  If the MOV has 3 legs, you can add an LED or light to let you know that the MOV is good.
  A high-line voltage that is really making the MOV conduct will just turn the whole MOV to a slag heap with smoke.  The listed (UL,ETL,CSA,TUV etc) products will have you include a fuse, something like your fuse but then  you may have nuisance trips of the fuse.  We had to add a separate low-heat solder melting spring loaded thing to protect our SPDs from high-line voltage for ETL listing.  This will work for rare cases where the over-voltage is "just right" and the MOV heats up enough to melt the solder.  The internal protection is there but was evidently for different conditions which did not meet UL's specifications.

So, for lightning protection, which is a common-mode overload, you would want two (2) MOVs...  One from L1 to GND and one from Neutral to GND.  This protects the insulation systems that are plugged into the AC line.  That voltage is typically MUCH higher than just a high-line AC voltage so the actual clamping voltage of the MOV doesn't have to be quite as precise as the differential mode case.

I'm not sure how well a differential mode MOV works for products or power strips ?  To do a lot of good, the MOV has to be just above the rating of the product they are trying to protect which could be all over the map and if called upon to conduct, they have to be pretty large, physically, to be able to dissipate the power/energy for the time that the over-voltage is there.  Kind of a gamble either way IMO.

boB