On 11/25/2010 2:14 PM, David Cutter wrote:
> It is my experience that MOVs should not be placed directly across the
> mains; they should have some limiting impedance, just like zeners do
> in a dc circuit. I always fit them downstream of the mains filter so
> that as they conduct there is a limiting Z for the period of the
> glitch. They are not meant to be sacrificial but they would be if
> placed directly across the mains with essentially extremely low source
> resistance.
MOVs should be installed down stream of fuses or breakers.
I should have said no flammable objects should be placed close to them.
Carl is correct, MOVs don't normally catch fire, but a shorted one can
get very hot, or even explode. I've seen a pair of #6 wires sticking
straight out from where a 50,000 joule MOV "used to be" and I've also
been in the next "cage" over from one when if blew. It takes a lot of
power to explode a 50,000 joule MOV and the results are spectacular.
The idea was to protect a pair of 1200 Amp SCRs (bout the size of a
hockey puck and water cooled) from spikes, which they sometimes did.
<:-)) The fuse they'd take out was pretty spectacular for size too.
All in all even if the SCRs went they were still worth the investment.
73
Roger (K8RI)
>
> David
> G3UNA
>
>
>
>> On 11/25/2010 12:53 PM, Ken Brown wrote:
>> The original post for this thread has not shown up here yet.
>>>> A similar thing happened with garden variety 130 v rated MOV's....
>>>> too many house fires. Modern MOV's for 120/240 v use, will have
>>>> a 3rd lead sticking out [offset to one side]
>>>> and an internal fuse built into em.
>>>>
>>>>
>> A 130V MOV on the 120V AC line would get hot in a hurry 1,414 * 120 =
>> 169.7V so the thing would be conducting a high percentage of the time.
>> You'd want at least a 180V MOV.
>>> I don't understand how this can be a useful device. I thought that
>>> MOVs
>>> were supposed to draw lots of current, instead of letting it got
>>> elsewhere causing damage to more expensive and sensitive electronics.
>> That is correct, but an under sized, under rated MOV could cause a fire
>> by itself.
>>> I
>>> had always thought of MOVs as sort of "sacrificial" devices,
>>> intended to
>>> blow up under the worst power surges,
>> They are.
>>> and mounted inside a metal
>>> cabinet so that when they do blow up nothing else is damaged.
>> Unfortunately many omitted this part.
>>> I would
>>> never mount an MOV where it could set fire to something else.
>>>
>> Many did. <:-)) MOV's are, or used to be, rated by their breakdown
>> voltage. So people were purchasing 130V MOVs for 120 V AC lines
>> forgetting, or not knowing, the peak voltage is 1.414 times the RMS.
>>> DE N6KB
>
>
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