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John Larkin wrote:
> Nobody has yet answered my question: what are typical one-shot versus
> lifetime joule ratings for MOVs?
Your question was answered last month even with numbers. MOVs are
not 'one-shot' devices. A one-shot device is grossly undersized;
ineffective protection. Some plug-in protectors are grossly undersized
so as to be damaged by a first surge. Being grossly undersized sells
more ineffective protectors. Answer to John Larkin's question
reposted:
> MOVs that vaporize or disintegrate were used beyond what the
> manufacturer intended. An unacceptable and catastrophic MOV
> failure is not what joules are measuring. MOVs fail by degrading.
> A degraded MOV has no physical burning, but has a voltage
> change of typically 10%. ...
>
> To determine how long an MOV may last, consult charts provided
> by manufacturers. For example, a 180 joule MOV may shunt 1
> million 20 amps transients, 1,000 300 amp transients, 10 600 amp
> transients, or 1 4000 amp transient. In each case, the MOV has
> no observable damage. ...
>
> As MOV joules increase, joules that it may shunt to earth
> increases exponentially; increased life expectancy. Those joules
> shunted to earth are vastly more than the MOV joule rating
> because the function of an MOV is not to absorb energy. Its
> function is to shunt as much energy as possible to earth.
>
> As MOV degrades, its varistor voltage changes. MOVs degrade;
> must not blow out or open circuit. Some mistake that open circuit
> MOV as normal operation.
One shot operation means an MOV was grossly undersized for that
design. Again, as MOV joules increase, joules that it may shunt to
earth increases exponentially. Answer starts with manufacturer
datasheets such as John Popelish provided:
http://www.littelfuse.com/data/en/Data_Sheets/CA.pdf
. John Larkin would not be making his claims and not be asking this
question had he first read manufacturer datasheets and application
notes. He would then understand why an MOV shunts energy to earth; why
it does not absorb the surge.
A 20,000 amp lightning surge will only contain 200 joules? Of course
not. First MOV in that datasheet will shunt (divert, earth) a 20,000
amp transient - and in the process that entire 20,000 amp transient is
only 200 joules? Of course not. That 20,000 amp transient may
dissipate hundred of thousands of joules into earth while only
dissipating 200 joules in the MOV.
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