[TowerTalk] Sparky Pays a Visit

[Jim Lux]

At 09:24 AM 8/3/2005, Jim Brown wrote:
>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!

Which is why there has to be a fuse in series with the power lead going to 
the MOV.

>  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.

MOVs also "wear out".  Every transient they suck up degrades them a bit. (I 
learned this when trying to suppress spikes on a variable frequency motor 
drive...)

Things like vacuum sparkgaps (e.g. those from Victoreen, also used for 
transient suppression, and similar in cost to the MOVs) don't have a 
wearout mode.  Neither do gas tube type devices (essentially rugged neon 
bulbs).



>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.