Re: Lightning Strike and surge


"david" <none@xxxxxxxxxx> wrote in message news:g72d78$ku0$1@xxxxxxxxxxx
On Sat, 02 Aug 2008 04:27:53 +0000, John Doe rearranged some electrons to
say:

david <none@xxxxxxxxxx> wrote:

On Fri, 01 Aug 2008 18:40:37 +0000, John Doe rearranged some electrons
to say:


shunting electricity is not really what protects the circuit.

Incorrect. The surge energy is dissipated in the MOV.

http://en.wikipedia.org/wiki/Varistor

Don't believe everything you read on Wikipedia.

I don't, but this article happens to be correct. You can search for
yourself if you don't believe it.


Since the circuit input and output voltages are the same thanks to the
MOV becoming a short-circuit, destructive current flow through the
protected device is (ideally) prevented.

Incorrect again. The MOV has a low, but non-zero, resistance once its
threshold voltage has been exceeded.

Doesn't have to be zero, it just has to be low enough to limit voltage
at the circuit inputs, and that's how it protects the circuit.

The MOV clamps the voltage across its terminals at its turn-on threshold,
which could be more than 400 volts, depending on the breakdown voltage of
the particular device. It is not a short circuit.



-------------

Problem is, sometimes it can be difficult to identify a consumed MOV.
Constant replacement is mandatory in most cases to ensure proper
functioning.

Ed Cregger


.

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