[TowerTalk] MOV's

[Ian White G/GM3SEK]

 >>> If an MOV has accumulated enough charge such that it is close to
 >> >failure, and it fails open during an event, you are unprotected.
 >>
 >>That is not how a Metal Oxide Varistor (MOV) works.  You *never*
 >>want one to open unless the rating is too low.  They work by
 >>*shorting* transients.

Not really - they work very much like bipolar zener diodes, by clamping 
the transient voltage. The clamping action isn't very strong, so a large 
spike may still rise to 2-3x the normal operating voltage... but that's 
a heck of a lot better than no clamping at all.

In contrast, a gas discharge tube comes much closer to "shorting" a 
transient by striking a low-resistance arc. The disadvantage of GDTs is 
that the voltage may have to rise much higher before the arc is sure to 
strike.


>When a MOV is hit with a voltage (not spike) much higher than the rating, it
>usually pops like a firecracker.  At this point it is open.

Either a steady over-voltage or a very large transient can literally 
blow an MOV apart. Of course it's open-circuit *after* the event. 
However, it may not be a total failure, because all the energy that went 
into destroying the MOV was diverted away from the equipment it was 
protecting.

The "series-mode" protection that Jim K9YC cited is interesting... but 
in what ways is it different from a mains filter?  An ordinary low-pass 
L-pi mains filter with inductive input will reflect and reduce any 
incoming transients, up to the breakdown currents and voltages of the 
individual components.

The SurgeX information was interesting, but somewhat overbalanced 
against MOVs. For serious protection against mains surges, I would use a 
mains filter *and* an MOV connected at the output (equipment) side - 
with correct ground wiring practises of course. The filter is the first 
line of defence, and if the MOV is called upon as backup, it dumps much 
less current into the equipment ground. Also the filter helps protect 
the MOV from being "worn out" by routine minor transients, so the MOV is 
much more likely to work correctly when it's really needed.

Another obvious way to protect MOVs is to install them *after* the mains 
switch, so they are not being subjected to transients when the equipment 
is not in use.



-- 
73 from Ian G/GM3SEK