[Amps] MOVs

[Roger]

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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