[RFI] Elimination of Treadmill RFI on 160 meters

[Jim Brown]

On Fri,1/9/2015 9:04 PM, Christopher Brown wrote:
> I could be wrong, but based on the picture you appear to have neutral
> and safety ground on the equipment side connected to the neutral
> terminal of the filter.
>
>
> This can present a serious hazard, as the green wire bonds to the
> chassis of the treadmill...  Should there be a neutral fault anywhere
> upstream of that filter and the chassis of the treadmill would go hot
> with full line voltage.
>
> For exactly this reason, NEC allows only ONE neutral to safety ground
> bond AT the primary disconnect.  Otherwise, failure of a neutral
> conductor puts voltage on the chassis and creates a shock hazard.

Actually, there are several reasons why NEC requires ONE, AND ONLY ONE, 
bond between neutral and equipment ground (the green wire) in any 
system. That bond is required to be at the point where the system is 
established, which in most premises, is the service entrance or the 
first panel connected to it. One of the most important (to us) that load 
current flows on the neutral, the green wire, and all the rest of the 
grounds in the premises. This raises the voltage on the chassis, 
increasing the likelyhood of shock. It also causes the magnetic field 
created by load current to spread out over then entire area of the 
return path, which can induce hum and buzz into audio systems, and to 
the audio components of radio systems.

Also, as Chris noted, for Delta filter to be effective, it's enclosure 
(and the green wire) must have a very short bond to the equipment 
enclosure, and the phase and neutral conductors must also be quite short.

Another point. What the power industry calls "common mode" is NOT what 
we call common mode. They are talking about the voltage between neutral 
and the green wire. We are talking about current that is flowing in the 
sam direction on three conductors -- that's what causes the cable to act 
as a random wire antenna. The ferrite choke is the solution for that, 
because it adds a high resistive impedance in series with the common 
mode circuit. The Delta filter (and others like it) ONLY acts on the 
differential voltage between the three conductors. That cable with the 
choke on it looks reasonably well built, and should work well for the 
range of 2-20 MHz. Another with fewer turns would help with higher 
frequencies, and should be closer to the noise source.

73, Jim K9YC