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Re: [TowerTalk] MOVs in Stackmatch

To: "" <>
Subject: Re: [TowerTalk] MOVs in Stackmatch
From: "Jim Brown" <>
Date: Thu, 04 Jun 2009 08:54:59 -0700
List-post: <">>
On Wed, 3 Jun 2009 21:05:16 EDT, wrote:

>I was under the impression that MOV devices are "damaged" somewhat during  
>each protection event and eventually enough "damage" accumulates and they 
>have  to be replaced.
>Maybe someone with more knowledge will comment on that.
>If this is the case, it seems that a big hit might use up a lot of the  

That is correct. Think of an MOV as having capacity of a limited number of 
joules, and each discharge event uses up some of that capacity.

Another VERY critical note about MOVs is that they shunt the lightning current 
to some point or circuit. That circuit has some finite impedance to earth, 
which is usually where current is trying to go, and the resulting IZ drop will 
raise the potential at the cathode of the MOV to the value of that IZ drop. 
This point is usually the ground reference for the protected equipment. But 
here's the gotcha -- it is common for that point to be connected to the ground 
reference other equipment, that may or may not have MOVs on it, and that will 
almost certainly have a DIFFERENT induced lightning voltage, a DIFFERENT 
impedance to earth, and a DIFFERENT IZ drop. That DIFFERENCE in potential can 
establish enough current between those devices to cause destructive failure of 
either or both!  

Bottom line -- MOVs must be used VERY carefully -- that is, we must be VERY 
careful about where their discharge current goes, and what might also be on the 
other end of interconnect wiring. 


Jim Brown K9YC


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