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John Larkin wrote:
> I don't like MOVs because of their leakage, their very soft conduction
> curve, and their tendency to accumulate damage.
MOVs have other problems such as too much capacitance (see
datasheets). But MOVs have superior life expectancy compared to gas
discharge tubes (GDT). A rather unique 'MOV like' solution that
replaced GDTs was once standard in telephone protection. That solution
has since been obsoleted by a semiconductor solution. Some examples of
semiconductor solutions include Sidactors and products from Sematech:
http://www.semtech.com/pc/downloadDocument.do?id=532
Above solutions are when protector's capacitance could impact signal
integrity. AC electric does not have same restrictions. MOV
technology is a best AC electric protection when considering other
parameters such as life expectancy and costs.
MOVs are typically selected so that only serious and destructive
transients cause an MOV response. Lesser transients should be made
irrelevant by protection inside appliances. A typical transient
frequency would be maybe one transient every eight years. A number
that changes significantly even within a same town due to variations
such as geology.
If frequency of transients is significantly higher (ie central FL or
some parts of WV), then joules are increased so as to improve MOV life
expectancy exponentially. Joules are a ballpark number to estimate MOV
life expectancy.
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