bud-- wrote:
On 10/6/2012 11:28 AM, gregz wrote:
The devices in the protector do not last once hit. They either reduce
capability or completely fail. One shot, blow up. MOV. Surge
protectors or anything else, are not designed to deal with a direct
lightning strike.
That is all complete nonsense.
Please refrain from confusing people with silliness:
"A varistor remains non-conductive as a shunt-mode device during normal
operation when the voltage across it remains well below its "clamping
voltage", so varistors are typically used to suppress line voltage surges.
However, a varistor may not be able to successfully limit a very large surge
from an event such as a lightning strike where the energy involved is many
orders of magnitude greater than it can handle.
"Follow-through current as a result of a strike may generate excessive
current that completely destroys the varistor. Lesser surges still degrade
it, however.
"Degradation is defined by manufacturer's life-expectancy charts that relate
current, time and number of transient pulses. The main parameter affecting
varistor life expectancy is its energy (Joule) rating. As the energy rating
increases, its life expectancy typically increases exponentially, the number
of transient pulses that it can accommodate increases and the "clamping
voltage" it provides during each transient decreases. The probability of
catastrophic failure can be reduced by increasing the rating, either by
using a single varistor of higher rating or by connecting more devices in
parallel. A varistor is typically deemed to be fully degraded when its
"clamping voltage" has changed by 10%. In this condition it is not visibly
damaged and it remains functional (no catastrophic failure)."
http://en.wikipedia.org/wiki/Varistor
What's important is that a MOV may be completely degraded without showing
outward signs. Of course if the MOV is black and partially melted, you have
a clue... but baring that, you just don't know.
And can't really know inasmuch as there's no way to non-destructively test
the thing.