[RFI] Putting Toroids on the Service Lines

[Jim Brown]

I would NOT expect conventional power filters to be at all effective 
when the problem is true common mode on that service. When I say "true 
common mode," I'm talking about current that is common to all three 
conductors, including the neutral.  Conventional power filters do 
nothing to address true common mode current.

It's important to realize (and I suspect that most EMC professionals on 
the list do) that what the Power world calls common mode is voltage 
between neutral and ground, and that's what the performance 
specifications of power line filters are describing. But that has no 
meaning on the power system service -- there are earth grounds on the 
neutral on both ends of a service drop, albeit not very good ones, 
mostly intended for lightning protection.

The neutral current on a power system service, especially if that 
service is any form of 3-phase system, typically includes massive 
amounts triplen harmonics of the power frequency, as well as noise 
generated throughout the power system.This is especially true if the 
power system is a "high-leg Delta" system (also called "wild leg"). 
because single phase customers (us) get nearly all the neutral harmonic 
current from the 3-phase users.

The RF component of this current radiates, which is what those toroids 
are intended to suppress.  The problem is that it takes a LOT more 
choking Z than is practical to obtain with a single turn choke (a core 
clamped onto the feeder), and a single turn choke is the only thing that 
will fit. And because the choke is inductive at the frequency of 
interest, we have all of the associated problems of that. :)

Granted that the line filter can kill the differential component of the 
trash, but I'm not convinced that's where the bodies are buried. Of 
course, I'm ready to be convinced otherwise by someone doing some good 
science on it. :)

73, Jim K9YC

  On 1/28/2013 8:53 PM, Cortland Richmond wrote:
> I got hold of a couple of Corcom 3 phase 200A filters and have been 
> thinking about running one single phase.  IIRC that would require 
> derating to 100A, plenty for my service, and as long as the meter 
> doesn't read reactive current...
>
>
> Cortland
> KA5S
>
> On 1/28/2013 1729, Jim Brown wrote:
>> On 1/28/2013 2:12 PM, Aaron Kreider wrote:
>>> How crazy of an idea is it to put toroids on the incoming service AC 
>>> lines? 
>>
>> Having done considerable research on the use of ferrite chokes to 
>> suppress RFI noise, it's one of the LAST things I would bother to do, 
>> except under very unusual circumstances. When I lived in Chicago, the 
>> AC line was an twisted triplet (120-0-120) with bare neutral, about 
>> 80 ft long,  that ran about 20 ft below my wire dipoles, and more or 
>> less in parallel.  I put about six 2-inch long, 1-inch i.d. #31 
>> clamp-ons on that feeder, hoping that it might minimize the common 
>> mode trash coming radiating from that section of the power system.It 
>> have helped by a dB or two on some bands, but I was never convinced 
>> that it did anything useful.
>>
>> 73, Jim K9YC
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