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#63928 - 03/27/06 03:39 PM EMI Filter
Webmaster Offline


Registered: 10/07/00
Posts: 3142
Loc: NY, USA
This is mounted below a residential single phase service I'm upgrading. I haven't traced the wires yet. Anyone know what it is?

Dave (Tiger)

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#63929 - 03/27/06 03:52 PM Re: EMI Filter
SolarPowered Offline

Registered: 07/05/04
Posts: 615
Loc: Palo Alto, CA, USA
It's probably a pi network, or maybe a ladder network. Basically, it's a low-pass filter intended either to keep RF that's on the power line out of the load, or to keep the load from putting RF into the power line. Or possibly both. (The filter works in both directions, but RF of concern is probably just coming from one direction.)

Edited to add: "EMI" means "Electromagnetic Interference."

[This message has been edited by SolarPowered (edited 03-27-2006).]

#63930 - 03/27/06 04:58 PM Re: EMI Filter
JoeTestingEngr Offline

Registered: 11/04/05
Posts: 786
Loc: Chicago, Il.
I have seen many of these over the years, but always inside of equipment. Hash filters like these usually appear like transformers mounted sideways on schematics. They often include them with input plug and fusing assemblies.

#63931 - 03/27/06 05:42 PM Re: EMI Filter
Ron Offline

Registered: 03/13/02
Posts: 577
Loc: White Plains, NY
Just a guess, but is there any power line communication devices in the home? Like a "wireless" switch arrangement X-10, or the like?

#63932 - 03/27/06 05:59 PM Re: EMI Filter
NJwirenut Offline

Registered: 09/15/01
Posts: 816
Loc: Bergen County, NJ
Is that thing at least mounted INSIDE of a panel or enclosure of some kind?

Anyway, here's a typical schematic or what's inside one of those things:

[This message has been edited by NJwirenut (edited 03-27-2006).]

#63933 - 03/27/06 08:28 PM Re: EMI Filter
Tiger Offline

Registered: 05/04/05
Posts: 714
Loc: Crystal Lake, IL USA
I shot the picture a little too close. It is inside a box. The picture is with the cover removed & a nipple attaches it to the panel.


PS If I'm following you right this is going to some equiptment, so I guess I'll be tracing the wiring.

#63934 - 03/28/06 04:08 AM Re: EMI Filter
NJwirenut Offline

Registered: 09/15/01
Posts: 816
Loc: Bergen County, NJ
Would most likely be used to feed either sensitive electronics (computer, home theater, etc.), or something that would produce large amounts of RF interference (furnace ignition transformer, ham radio gear, etc.).

#63935 - 03/28/06 06:51 AM Re: EMI Filter
PEdoubleNIZZLE Offline

Registered: 06/16/04
Posts: 176
Loc: McKeesport, PA, USA
NJ Wirenut - I'm not to savvy on EMI filters. Are you saying at the load end, you can connect either something you're protecting, or something you need protection from (something transmitting EMI)? Or are their different filters for sensitive equipment vs. EMI Producing equipment

#63936 - 03/28/06 11:08 AM Re: EMI Filter
NJwirenut Offline

Registered: 09/15/01
Posts: 816
Loc: Bergen County, NJ
The filter will work either way. It is essentially a lowpass filter, which allows the 60 Hz to pass right through, but shunts high frequency noise to ground. The shunt capacitors present a high impedance at powerline frequencies, but look like a short at RF. The series inductors are just the opposite, having very low impedance at 60 Hz, but effectively "choking" high frequencies from passing through. There are different types of filters made, which are each optimized for use either at a noise source or at the sensitive equipment, but generally this type of packaged filter works pretty well in either application in my experience.

If used to prevent a noisy device from junking up the powerline, it would be best to locate the filter as close as possible to the noise source, so as to reduce the length of "dirty" feeder cable which will radiate the noise like an antenna.

[This message has been edited by NJwirenut (edited 03-28-2006).]

#63937 - 03/28/06 11:55 AM Re: EMI Filter
hbiss Offline

Registered: 12/16/03
Posts: 893
Loc: Hawthorne, NY USA
Regardless of what it is and what it's supposed to do it doesn't belong there. Obviously a DIY or maybe an EE installation. It's "home brew" and not listed so not allowed. Any inspector worth his salt will red tag it.


[This message has been edited by hbiss (edited 03-28-2006).]

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