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RE: [TowerTalk] GFI Circuits

To:, (W2RU - Bud Hippisley),
Subject: RE: [TowerTalk] GFI Circuits
From: Jim Lux <>
Date: Fri, 21 Jan 2005 15:43:28 -0800
List-post: <>
At 03:12 PM 1/21/2005, wrote:
I think there is more here than meets the eye.A gfi breaker is not rf sensitive,its meerly a current transformer that monitors the current relation between the 120volt lead and the return on the nuetral leg.Only an inbalance in this flow would cause such a failure.
There's a fairly sensitive amplifier to measure the differential current (using a transformer with both Line and Neutral running through the core, along with a sense winding). That amplifier or the detection circuitry, might be RF sensitive.

In fact, the Leviton GFCI brochure ( mentions "increased noise immmunity: Higher resistance to electrical noise (EMI/RFI) to help reduce nuisance tripping" as required by UL starting 1 Jan 2003.

That brochure has some nifty looking devices that might be useful.. GFCIs with pilot lights, for instance.

There's also some description of why GFCIs for cord sets (the field day sort of application) are different than for normal receptacles. It has to do with a live hot leg with an open neutral on the line side. The tripping mechanism won't have any power to operate.

Here's a typical chip used in these things:

The latter requires all of 5 microamps to fire... It's easy to see how RFI could get in there and trip it. Don't forget that these are a cost sensitive device: the mfr's not going to put any parts or filtering in there unless absolutely needed (viz. the recent UL rule change).


See: for "Self Supporting Towers", "Wireless Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions and ask for Sherman, W2FLA.

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