[TowerTalk] (no subject)

[Roger (K8RI) on TT]

On 7/29/2014 2:21 PM, Patrick Greenlee wrote:
> Something to remember on MOV's:

MOVs "normally" fail shorted, but not always.  As Patrick says, each hit 
uses some of the MOVs capacity and also lowers the voltage at which it 
conducts. That one hit may lower it a little or a lot.  A major hit can 
vaporize the MOV.  Those are easy to spot, but a hit just strong enough 
to blow one of the lower rated MOVs open may not show at all.

Blowing one of the larger MOVs open is spectacular and leaves no doubt.  
It's likely good, shorted, or blown apart.

NOTE most current MOVs are rated for RMS, but most of the early ones 
were rated at the break down voltage, so if you come across some MOVs of 
unknown history, it's not safe to assume that the ratings printed on the 
device are RMS.  I have several dozen (If I can find them) that are 
rated at the break down voltage.

NORMALLY an old MOV will be shorted, or good, but there is that tiny % 
that have failed open.


73

Roger (K8RI)

>
> Every time they conduct, i.e. "eat a transient" they lose a little 
> capacity.  They are being chipped away at and eventually their 
> capacity is reduced below sufficiency for a specific purpose. This is 
> true with power strips, service entrance sized protective units, and 
> all other applications.  MOV's need to be replaced over time and the 
> difficult part is knowing when.
>
> Some electric utility companies offer MOV entrance panel protection 
> for a monthly fee and they change them on a schedule. Unfortunately 
> most power strip and other users are blissfully ignorant of the fact 
> that their old strips and protective devices with MOV's are probably 
> offering no protection if well aged and have actually done any 
> protecting.
>
> Patrick NJ5G


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