[RFI] What is a MOV

[Jim Brown]

On 1/11/2019 8:22 AM, Ken Winterling wrote:
> The Metal Oxide Varistor or*MOV*  is a voltage dependent, nonlinear
> device that provides excellent transient voltage suppression.

Delete the word "excellent" from this sentence. They are, indeed, 
ubiquitous, but they can also create more problems than they solve. They 
should NEVER by used on branch circuits -- inside equipment or in outlet 
strips when there is any interconnection between equipment plugged into 
different outlets, or which is grounded at points in addition to the 
green wire. This is because MOVs conduct any power line spike onto the 
green wire, raising the potential of the "ground" in the equipment. The 
increased potential will be different at each piece of equipment 
(different induced current from the strike, different length of green 
wire), and the potential difference fries the interconnected equipment.

We learned this the hard way in the world of pro audio, where equipment 
for large installed sound systems is spread out over a wide area. And 
I've heard many reports of computers linked by wired Ethernet getting 
fried by a nearby strike.

The ONLY good place for an MOV is at the power entry panel (a so-called 
"whole house protector) where it is bonded directly to the ground system.

The only good (and safe) surge suppressors for branch circuits are 
"series mode" types, which store the energy from a strike in a large 
inductor, then discharge it slowly after the strike. The pro audio world 
uses products made by Surge-X. Brick Wall is another mfr.

73, Jim K9YC