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.


Terry