[RFI] My power line noise problems, revisited

[Jim Brown]

On Sat, 7 Feb 2004 14:53:43 -0500, WW3S wrote:

> expert referred to them as MOV arrestors. 

MOV's are, indeed, a potential cause of RFI. Here's how. 

When it's working properly, an MOV (metal oxide varistor) has the
charactistic of conducting above some threshold voltage but being an
open circuit below that voltage. When used as surge suppressors, they
are usually connected between one side of a line and "ground." When the
surge or noise spike comes along, they conduct and try to short the
pulse to "ground." 

Sounds like a good idea, right?  Well, it does protect the equipment
(until the MOV fails), but not without a price. That path to ground has
some impedance by virtue of its length, and a voltage drop will appear
across that impedance by Ohm's law.  

Also, the path may be long enough to act as an antenna. Let's say the
MOV is at the top of the pole and the "ground" return is to the metal
pole (or a wire running down a wood pole), and the transmission line
being protected is 30 feet above ground. The ground path is going to be
at least 30-40 feet long, which is likely to be a pretty effective
radiator through much of the HF spectrum!  

Oh, you say, those MOV's only conduct on high voltage spikes. Well,
what if there are bypass capacitors across them, of if there is
self-capacitance or resistive leakage?  They're conducting lower
voltage noise spikes through that ground lead too!  

And note that I prefaced this with, "When it's working properly, an MOV
.  .  ."  All MOV's eventually fail when they have been subjected to
some finite number of joules.  The failure modes vary, and some can be
spectacularly destructive.  

Jim Brown K9YC