Subject:  Re Stray Electrical Field Issue (Gi)(Tegenfeldt).
Date:     Fri, 08 Sep 2000 104900 -0500
From:     Roy Beavers 
To:       guru 
--------------------------------------------------

......Response from EMF-L........

Very good to hear from Clas Tegenfeldt!!!......guru......

-------- Original Message --------
Subject: Re: Stray Electrical Field Issue (Gi).
Date: Fri, 08 Sep 2000 16:20:53 +0200
From: Clas Tegenfeldt 
To: roy@emfguru.com

At 18:31 2000-09-07 -0500, you wrote:
>........Request for info and advice from EMF-L........
>-------- Original Message --------
>Subject: Stray Electrical Field Issue
>Date: Thu, 7 Sep 2000 14:46:30 EDT
>From: NannyGi@aol.com
>To: roy@emfguru.com
...
>We live in a stand alone house in Evanston, Illinois.  Our house has stray 
>electrical field/radio frequency type interference (RFI ) coming into our 
>house from the outside electrical service drop.  The interference could also 
>be described as light dimmer-like interference.
>
>If we shut off our main, the RFI still comes in.  If we shut off the neutral 
>AND shut off the main, the RFI stops.
>
>Once the stray RFI is in our house, it runs all over our conduit  throughout 
>the entire home.  The RFI is usually "on" during most of the day and goes off 
>late at night.  However, it does not follow any set schedule or pattern.
...
>Problem - what do we use as an RF filter to keep out light dimmer-like 
>interference from coming into our electrical service drop?

This is a common problem with conducted RFI, it travels along neutral and 
protective earth (PE) and thus is spread onto every metallic structure of
buildings
with electric contact to (direct or indirect) to PE. Ferrite cores can only
to a 
small amount lessen this kind of interference. Capacitors can in some cases be
used to "short circuit" such noise, but in most cases you gain nothing or 
just worsen the problem by moving the interference to other places. The problem
with capacitors is that they are often the CAUSE of this kind of interference! 
Decoupling capacitors are often used to suppress noise by coupling to the PE, 
thereby moving noise onto PE (and neutral). The real problem is that most
technicians and EMC rules refer to measurements relative to "ground". If you 
have a noisy device and measures it EMC like, you put it onto a ground plane
or inside a metallic room and measure the emissions relative to that ground.
The PE is connected to that ground reference. If you put capacitors there
the noise is coupled onto ground, but since you measure relative to the same
ground you will NOT detect that noise. In real life, much of the noise is 
travelling along PE/neutral and can be a problem.

Best thing to avoid this is by using a full isolation transformer where you 
create your own ground reference on the secondary side. Thus neutral and
PE is local and have no direct connection to the distribution net. This may
require permits from authorities and the power company. There are gas 
discharge devices that can be used to short circuit the local and power grid
PE when overvoltage occurs (such as lightning strokes). 

Good luck!



Clas Tegenfeldt

BEMI
Tornevalla Gamla Skola
S-590 62 LINGHEM, SWEDEN

tegen@bemi.se, http://www.bemi.se
Telephone +46 (0)13-74 000, telefax +46 (0)13-13 47 00


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Reprinted with permission of Roy Beavers, http://www.emfguru.com