VWWall wrote:
bud-- wrote:
VWWall wrote:
wrote:
"New thermally enhanced MOVs help protect a wide variety of low-power
systems against damage caused by over-current, over-temperature and
over-voltage faults, including lightning strikes, electrostatic
discharge (ESD) surges, loss of neutral, incorrect input voltage and
power induction.
I had a microwave oven that had a MOV across the 120V line ahead of
the power switch. The other side of the 120/240 20A circuit supplied
a refrigerator. The loss of the neutral applied a good part of the
240V across the MOV when the refrigerator attempted to start.
The MOV didn't last long! It would probably have been OK on the load
side of the switch.
Using a MOV to protect against loss of neutral (in the article) is
rather futile. Sustained overvoltage will rapidly kill them. Although
if the protected load was across the MOV and a fuse was ahead of both
protection may work. Would be interesting why the MOV was ahead of the
switch.
Good question. In the MW oven case, the switch was a relay controlled
by the timer circuit. It was probably easier to locate the MOV at the
line input.
I have seen cases with a "blown" MOV and the circuit protector tripped.
The MOV, if it tripped the protector, may have saved the following
circuits from the over-voltage condition for a longer period of time. I
haven't tried to calculate the conditions under which this would work.
Normal MOV failure is by high current and overheating (as below). A fuse
may provide protection. Plug-in suppressors likely use the heat as part
of the disconnect. For overvoltage, the disconnect would have to survive
the higher voltage.
I know that refrigerators should be alone on a "home run" circuit,
and neutrals shouldn't be connected with wire nuts, but that wasn't
how it was!
My only complaint with some plug-in protectors is that the MOVs are
often much too small. I've also seen some with only a line-line MOV.
As you know, MOVs lose their capacity each time a "spike" causes them to
conduct. This reduces the remaining capability to handle "surges".
You may already know all of this -
MOVs are damaged by heat from energy dissipated in their clamping
action. The defined end of life of a MOV is when the voltage that
produces a 1mA current decreases 10%. At that point the MOV is still
clamping the voltage across it. Further dissipation continues to lower
the voltage until the MOV conducts at ‘normal’ voltages and goes into
thermal runaway. For surge suppressors, UL required protection
disconnects the MOV when it overheats. It should still be clamping at
that point.
The energy (Joule) rating is for a single event. If the individual hits
are far below the rating, the cumulative energy rating is far above the
single event rating. High ratings give longer life than you might expect.
Service panel and plug-in suppressors do not protect by absorbing
energy. But they absorb energy in the process of protecting.
I would only buy one with fairly high ratings (which are readily
available).
True, but some are marketed as "surge protected" with minimal capacity.
I've replaced the MOVs in several cheap multiple socket strips with
higher rated MOVs from Radio Shack.
UL, as far as I know, requires MOVs to be L-N, L-G, N-G. I thought
that was the standard since the start, which w_ said was 1985.
I think the UL requires only that the MOVs don't start a fire when
exposed to conditions which cause their break-down. They don't rate
their ability to function as "surge protectors".
A Cuttler-Hammer tech note:
http://tinyurl.com/63594d
has some information on UL tests. Suppressors have to remain functional
through an initial set of surges (20 surges - 6kv, 3kA). They can fail
safely after that. (This sounds more like the service panel suppressor
test.)
--
bud--