Surge Protector Failure

[jack3333]

My question is regarding failure modes for surge protectors (the types used to protect individual equipment like computers, etc.). I examined the circuits in some common surge protectors. In the models I examined, as far as I can determine the "Protection Working" LED will only go off if the thermal fuse is blown. If the thermal fuse is blown, that will also disconnect power to the equipment. So in that way the surge protector will "fail safe". Is it possible that one or more of the MOVs could fail without generating enough heat to blow the thermal fuse? If so, is it very likely? In that case the "Protection Working" LED would still be on, and power would still be connected to the equipment, but the surge protector would not be providing protection to the equipment. Also it appears that if the other fuse, which limits current between ground and the hot-to-ground and neutral-to-ground MOVs blows, the "Protection Working" light would still be on and the equipment would still be connected to power while no surge protection is provided. Is that a likely scenario?

Jack

 

[MacGyverS2000]

Those so-called surge protectors only react to large surges in the real world (i.e., lightning strikes or severe equipment failures)... in those cases the fuse is sure to blow. Minor overvoltages on the line of 20-40V (still a surge, IMO) are often not enough to blow a fuse, but can still damage equipment.


Dan - Owner

http://www.Hi-TecDesigns.com

 

[Noway2]

The company I work for makes some well knownn brands of surge protection devices. One of the features of the products is a measure of the percentage protection remaining. While these are large devices, where the MOVs are typicaly placed in potting compound and sand, the surges DO damage some of the MOVs over time without blowing the entire unit. So, yes it is possible for your protection to be degraded without blowing the fuse.

It will really come down to the I^2T rating of the MOV versus the fuse. Will the MOV fail open before the thermal fuse clears?

 

[jack3333]

Thanks for the info macyvers and Noway. It appears that the "Protection Working" light doesn't really have much value. I did see a circuit for a system which would directly indicate that a MOV had failed. It was fairly complex, so it would be unlikely to be used for low cost, consumer type surge protectors.