MOV failure in surge protector?

[JHZR2]

For a while we had our chest freezer in the basement moved to an alternate location, and had it connected with an extension cord (of appropriate ampacity). We were putting it back in place and disconnected the cord to see this:

I looked at the cord and it had this:

I'm sure it wasn't dirty like this when we put it in. My wife did the pull out of the cord and she said the dust came out when she pulled the cord.

Note the lights both still show ground and protection (red led is a bad choice, but there is a correct ground and polarity of wiring in the outlet with appropriate sized conductors and a short run to the source.

Is this what it looks like if an MOV fails? Perhaps a surge to ground? We haven't seen any indication of any issues, nor do we have issues with the freezer or anything else.

 

[bullwinkle]

Looks more like a bad connection or the protector couldn't handle the starting amperage of the freezer's compressor. Usually when an MOV fails, it's a sudden pop, like a fuse, doesn't get melted like that. Why have a spike protector on a chest freezer? They usually don't have electronic components anyway (unless they're sticking circuit boards in them now), usually just a temp control & compressor.

 

[JHZR2]

There's nothing melted, just that dirty dust.

I dunno. I put one on our old refrigerator when the circuit board went bad, and since I bought a bunch, I just put one on...

 

[Quest]

MOV has a finite service life (duty cycles).

If yours popped, replace the unit with fresh new MOV-equipped power bar or adapter to obtain the necessary protection.

Q.

Oh and BTW: cheeply made ones (MOVs from mostly taiwan or china) will pop like that, with a loud pop and some dark sooty smoke coming out.

High quality ones from Matsushita, Siemens, Vishay, etc. will simply just die w/o popping.

...In case you wonder

 

[JHZR2]

Maybe Ill contact TrippLite and see if they want it for a post-mortem...

Actually, it still works just fine as far as allowing power to pass through to operate the freezer...

 

[JetStar]

If you don't plan on using it anymore, cut it open and see if the MOV is in fact smoked. Take pictures for us.

 

[spasm3]

Any chance of lighting having run in on it? I mean carbon streaks are ususally from arching.

 

[Quest]

http://www.google.com/patents/US7593209

Please refer to section2, from paragraph 2 and until the end of the section.

(3, 4 and 5th paragraphs cited):

Destruction of an MOV will not affect the normal operation of a surge protector because MOVs are shunt elements. Ironically, poorly designed surge protectors give some indication of destruction by appearing singed, whereas well-designed surge protectors do not appear damaged after destruction.

To avoid a fire hazard, thermal protection schemes are used to prevent the MOV from reaching its thermal destructive level. The use of thermal protective fuses allows the use of LEDs indicating protection and fault operation. Also, it is very important that ground and neutral wires are correctly connected. Since MOVs are surge diverters, if the neutral wire is not connected then the destructive energy cannot be diverted away from the sensitive load.

It is possible that after much surge suppression, the thermal limit of the fuse may not be reached, but the MOV may degrade and fail. Conventional surge protectors do not indicate that such a failed MOV is no longer in service. Such conventional surge protectors will indicate failure only if the thermal fuse has failed.

 

[Quest]

Lastly, here are some MOV photos:

http://www.zerosurge.com/technical-info/truth-about-movs/

A well executed MOV layout, should also includes thermal fuse in between the MOVs to protect them (I have such layouts in my APC server rack power distribution bar), amongst a few other things. Unfortunately, over 85% of the so-called surge protectors out there sold nowadays do not come with proper execution/implementation of MOVs, and it's typical for them to appear sing'ed when popped opened.

The cheaper the power bar, the more catastropic the popping will be.

Lastly: beware of those cheep, countefeited UL listed power bars for they may not have even past the basis fire hazard resistance (on the plastic casing materials used) in their power bar, and when the MOVs suffered from thermal overload and popped due to surge, they will cause a small fire to the plastic casing, which may ended up burning down the house.

Don't say I have not warned you folks.

Q.

 

[Quest]

Lastly, the subject of thermally protected MOVS (also known as TMOVs), by Littlefuse:

http://www.newark.com/pdfs/techarticles/littelfuse/Littelfuse_ThermallyProtectedTMOVVaristors.pdf

Q.