# How can you check that MOV is damaged?

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##### Banned

1.
If I use a thermally fused MOV (the fuse gets open when temperature is too high), how can i know when the internal fuse in the MOV got open?
http://www.littelfuse.com/data/en/Data_Sheets/Littelfuse_Varistor_TMOV.pdf

2.
What does that picture mena?
And how can i conclude from it when the fuse gets open depending on temperature?

3.
Does anyone know about a MOV that comes with PTC tight to it?

Thank you very much.

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##### Banned
The graph is pretty self explanitory. Between 85 and 125C you have to linearly derate the value you chose for your fuse.
So if you use that fuse in a 105 degree environment you have to derate it's current value by 50%. So if you were planning on using a 2amp fuse, you need to use a 4amp fuse, or you're going to get false trips.

##### Banned
Oh I see.
Thanks.

You know how can i know if the fuse turned out to be a cut-off?

##### Banned
Not a clue sorry =)

#### Willbe

##### New Member

1.
If I use a thermally fused MOV (the fuse gets open when temperature is too high), how can i know when the internal fuse in the MOV got open?
Feed the MOV from AC through a high value resistor so that you get max 1mA or 10mA current through it. Look for the typical V-I curve for an MOV on a scope; if the fuse is open you will just see an AC waveform on the scope.

". . .how can i conclude from it when the fuse gets open depending on temperature?"
The graph you show doesn't tell you.
Fuses [and coils and semiconductors] have an I squared T rating; if a 1A fuse opens in 3 sec at 2A, then its rating is 12 amps-squared-seconds. This formula applies for part of the "trip" or "clearing time" curve.
These are on the Web but are hard to find.
Of course, a 1A fuse holds 1.1A or so indefinitely so the curve doesn't apply at this end of the curve, and at enormous current the clearing time also deviates from the formula.

It's I squared because heating in a resistor is proportional to I squared, and the ultimate temp rise which melts the fusible link is proportional to time.

Fuses also have an "interrupting" current rating. A residential 120v/240v supply may have a short circuit current of at least 20,000A, so a 15A fuse for this application has to be able to safely interrupt 20,000A.

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