Re: MOV's

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: David Speck <dave@xxxxxxxxxxxxxxxx>

Dave,
My Dad used to cut the metal oxide composites for MOV prototypes back in 
the days when GE made MOVs.  His comments were that GE's MOVs were sold as 
consumable commodities, and that their clamping voltage tended to increase 
with repeated hits.  After a period of time, they were to be replaced, as 
their clamping ability would diminish to the point where they would no 
longer be an effective protectant.

One corollary is that buying used MOV stacks can be risky, as they may have 
passed their useful life in their original application -- not that I've 
seen many of them available at hamfests any more.  as a side note, some of 
them operate pretty warm -- I have a big GE unit protecting my whole house, 
the diameter of a 5# coffee can, and half as high.  It runs warm, but not 
hot, to the touch, but the guy I bought it from said that was expected 
behavior.  He used to test the units for GE.

Unfortunately, the degradation of the MOVs was very non-linear, and even GE 
had a hard time quantifying how much they degraded.  That's why you don't 
see a lot of "MOV Testers" comparable to cap or diode testers.
The risetime and current profile of the transient pulse affect how the 
device is degraded.
To be honest, the tiny little MOVs in the usual $5.00 power strip are just 
too small to provide anything more than a placebo to the owner.
Truly effective surge suppression devices, like the military surplus 
suppressors that I bought at a surplus sale 2 decades ago have inductors to 
limit the rise time of transients, gas discharge tube transient voltage 
suppressors to carry really big currents, and beefy MOVs the size of 4 
stacked $0.50 pieces, and fuses on all the power lines, with a pilot to 
indicate if the fuses have been blown.
OTOH, because of the power levels involved, no consumer MOV or surge 
suppressor is going to stand up to a nearby lightning strike, or even the 
more common occurrence of a fault path from a high voltage to a low voltage 
circuit, as when a car hits a power pole, and cause a 14.4KV line to 
contact a 220 volt line.  The latter happened at my office a few years back 
and took out $30K of equipment, even though it was all protected by good 
quality surge suppressors and UPSs to boot.
Be aware that any application of a MOV across a power circuit must include 
a circuit breaker or fuse in series with each MOV.  After some transients, 
MOVs may fail as a dead short, and you need an interrupter of some sort to 
protect your house wiring.  I experienced this failure mode a few years 
back, when I was working on my house wiring.  The manufacturer of my home 
security system placed some token MOVs across the power supply inputs, but 
no fuses.  I was wiring in another part of the house, and caused an 
accidental short on another circuit.  Some sort of transient propagated 
back to the security system, and caused both MOVs to fail shorted.  There 
was no outward damage to their cases, but it took me half a day to figure 
out where the hard short was in the circuit.
I've toyed with the idea of building a MOV tester using a Blumlein (sp?) 
pulse generator to make a repeatable, fast, high voltage,  low current 
pulse, but have never gotten around to doing it.  It would be interesting 
to see how much the MOVs in a Terry filter degrade with typical TC 
operation.  However, as I mentioned, MOV testing is subject to a sort of 
uncertainty principle -- each test changes the properties of the MOV, 
though I think a few low energy hits would not degrade them too much.

Dave Speck

> One group maintains MOV's last pretty much forever and replacing things 
> like power strips with MOV's is not necessary.
>
>     Another group maintains that MOV's break down at [whatever] voltage 
> above 120 VAC and the MOV shorts the current across it. This group 
> maintains that whenever this happens, a portion of the MOV's dies, and 
> when all the surface area of the MOV is gone, the thing is useless and 
> must be replaced.
>
>     My own experience is that I've seen them die after fun things like 
> lightning strikes.