On Wed, 3 Jun 2009 21:05:16 EDT, TexasRF@aol.com 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
>reserve.
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.
73,
Jim Brown K9YC
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