[TowerTalk] Sparky Pays a Visit

[Jim Brown]

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 the 
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 -- they 
conduct the surge to whatever equipment ground they are connected to! Since 
there is lots of resistance and inductance between that "ground" point and earth 
(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. 
www.surgex.com.

Jim Brown K9YC