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Re: [TowerTalk] MOV's

Subject: Re: [TowerTalk] MOV's
From: Roger D Johnson <>
Date: Wed, 03 Aug 2005 20:40:28 -0400
List-post: <>
Jim Jarvis wrote:
> I did some consulting work for a manufacturer of surge
> protection equipment, several years ago.
> We evaluated MOV's and their failure mechanisms, using
> Keytek surge generators (whom I represented at the time).
> Whether we WANT them to fail shorted or not, they fail
> either shorted OR open, depending upon the total number
> of Joules they have absorbed, the direction of the currents
> which flowed, and what the latest peak does to them.  They
> do it without asking us permission.
> My conclusion at the end of the project is that an uncontrolled
> variable in the MOV production makes it impossible to predict in
> advance whether they are predisposed to fail in one mode or another.
> Simply exciting them differently did NOT produce different results.
> Yes, MOV's operationally clip peaks.  They just don't do it forever,
> and they have no way of telling you that they just died, unless you
> get lucky and they short and blow a fuse.  That's maybe half the time.

> The MOV's in my "whole house" surge suppressor are made with a fuse in
series. They have 3 leads and are connected to some sort of indicator
circuit which monitors their status. Mine are made by Littlefuse. More
info here:

73, Roger

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