>
> If an MOV has accumulated enough charge such that it is
> close to failure, and it fails open during an event, you
> are unprotected.
That is not how a Metal Oxide Varistor (MOV) works. You *never* want one to
open unless the rating is too low. They work by *shorting* transients.
MOVs have two ratings. The first is of course the voltage be it listed in
RMS or P to P. The second it their ability to absorbe and disapate energy.
This rating is in Jouls and *usually* the higher the better.
> And you have no way to know if you're
> in a condition which could go that way with the next shot.
>From what I've seen with Pholyphasors the same is true. They eventually
short to ground, which you can find. They also seem to be polarity
sensitive which would appear they provide less protection from the more
rare, or positive lightning.
>
> Therefore, an MOV solution seems to me to be no solution.
>
They do work as the wide acceptance in the industry would indicate. How
well they work and continue to work is up to question.
I've worked with them from the little ones that look like a disk ceramic cap
to very large 50,000 to 60,000 Joul capacity units used in 440 AC power
cicuits where they were used to supress switching transients. I've also
seen a big black scorch mark with two #6 leads sticking straight out
indicating the path the MOV took on the way out when it tried to swallow
more than it could handle. It is spectacular and noisy. I've had the
dubious honor of seeing and hearing such from no more than 4 or 5 feet away.
> Unless you could pass a trickle current through the device,
> to monitor its resistance, and look for changes as an indicator
> of status. This isn't done, because MOV's are seen as a cheap
> solution...and adding monitor circuitry is costly.
>
MOVs do work and they are inexpensive, but they are no substitute for a good
ground system even when they are new. They are a back up device
Roger Halstead (K8RI and ARRL 40 year Life Member)
N833R - World's oldest Debonair CD-2
www.rogerhalstead.com
> n2ea
> jimjarvis@ieee.org
>
> _______________________________________________
>
> See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless
> Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with
> any questions and ask for Sherman, W2FLA.
>
> _______________________________________________
> TowerTalk mailing list
> TowerTalk@contesting.com
> http://lists.contesting.com/mailman/listinfo/towertalk
>
_______________________________________________
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|