On Wed, 03 Aug 2005 16:03:52 -0400, Steve London wrote:
>That is true, but every MOV I see in the Mouser catalog is rated in RMS volts,
>so you can indeed choose a 130 or 150 volt MOV for your ~120 VAC power.
You are missing the point. MOV's get fried from an accumulation of spikes that
they dissipate. When they fry, they may short, or they may open, and they can
even catch fire! When MOV's are used in most equipment, there is one across
line, one from line to ground, and one from neutral to ground. Since neutral
must be bonded to ground at the service panel, it's unlikely that the one from
neutral to ground will blow (unless the strike is so close that your internal
wiring gets hit). If the line to ground or line to neutral MOV's develop a
short, it should pop the breaker. But if it opens, you never know.
There is another VERY important problem with using MOV's for suppression --
conduct the surge to whatever equipment ground they are connected to! Since
there is lots of resistance and inductance between that "ground" point and
(which is where the lightning hit is going), Ohm's law says that the current
will raise the voltage of that equipment ground by many KV. Now, what about
other equipment connected to that same equipment? There's many KV between that
equipment and this circuit, and that other equipment can be toast. Not nice.
That's why I like SurgeX series-mode suppression for power circuits.
Jim Brown K9YC
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
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