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David[_5_] · January 16th 08, 01:00 PM posted to rec.radio.shortwave

Telamon wrote:
In article
,
wrote:

I have an intense dislike for MOV (Metal Oxide Varistors). 99% of all
the "transient protection" devices available to the public are based
on these devices.

While they work, they are prone to an almost certain dangerous
failure mode.

MOVs can only dissipate "X" amount of energy, after that amount of
energy has been "clamped", MOV will start leaking current even when
there is no over voltage condition. This leads to thermal "run away"
and unless the MOV has a thermal cutout adjacent to it, can result in
fire. I several units that are failing after surviving a close
lightning strike. When connected to the AC mains they all start
conducting several 199uA that rapidly ramps up to 30mA. I have never
allowed them to stay connected to the AC mains because I really do
not want to deal with a fire. Another concern for me is that MOVs
that are failing often produce intense Radio Frequency Interference.

There is at least one company that produces a MOV with an internal
thermal cut out.

http://www.littelfuse.com/data/en/Articles/TMOVPCIMPaper.pdf.

This is not an idle threat. I have seen three house fires started by
MOVs and several electricians I know have seen many such fires.
Fortunately most fires are self limiting and only the plastic case
chars and the fire putters out. But under the wrong conditions the
fire can spread to the structure.

This is all well known. Although they are a class of protection devices
you need to protect them with a fuse so a catastrophic event does not
occur since they are a shunt device with a life span. If the protection
fuse blows you need to replace the MOV.

Use a 15-20A breaker between the mains bus and a grounded MOV. i.e.
mount the PowerMov tabs to the back of the breaker box and treat each
MOV like a branch.

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