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
---
This email is free from viruses and malware because avast! Antivirus protection
is active.
http://www.avast.com
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|