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

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.


Terry

I used to use them a lot in Texas (lightning). They'd always be in a
metal enclosure and they'd always be replaced every season. I've
survived direct hits via the mains that exploded the Power MOV but
stayed on the air. Here we have no lightning.

I am very, very certain my Brickwall Surge Protector, Model PW8R15 has
saved my electronic thingys more than once from lightning strikes.
www.brickwall.com www.pricewheeler.com My Brickwall was Made in
U.S.A.too.It cost me over $200.00, plus shipping and handling
charges.Money well spent, in my opinion.
Guranteed for Life, will not fail.Don't buy that sissy greasy kidstuff,
buy a Brickwall.
cuhulin

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.