Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!rutgers!ucsd!ucbvax!hplabs!hpfcdc!hpfcdj!myers
From: myers@hpfcdj.HP.COM (Bob Myers)
Newsgroups: sci.electronics
Subject: Re: Re: How can I recognize true ground?
Message-ID: <11170023@hpfcdj.HP.COM>
Date: 13 Jul 89 18:44:09 GMT
References: <1989Jul7.155721.19105@utzoo.uucp>
Organization: Hewlett Packard -- Fort Collins, CO
Lines: 23

>A curious thing (before I discovered this ungrounded wire) is that when I 
>replaced the existing ordinary outlet with a new ground fault interrupt 
>outlet, it thought that there was a valid ground!!  It functioned correctly 
>for about a year and a half (using the "test" button) ALTHOUGH NO ACTUAL, 
>TRUE GROUND WAS PRESENT!!  The 6' length of dangling wire must have been 
>enough of a "ground plane" to fool the circuitry.  I'm lucky we never 
>needed that outlet to actually perform it's life-saving task!  
>(I've fixed it now, so don't worry.)

I believe that a GFI operates by comparing the currents in the "line" and
"neutral" wires, and trips if they are not the same (which would indicate that
the current is going elsewhere, such as the safety ground wire or through the
user of the appliance).  Thus, the GFI will NOT trip simply due to a missing
safety ground connection.  But it WILL provide protection even in the absence
of the ground!  (That's the beauty of a GFI over the panel breakers, which
can't tell whether the current returns via neutral or ground (or someplace
else), and will happily permit current up to their trip point to pass through
your body.)


Bob Myers            | "Writing is not necessarily something to be ashamed of -
myers%hpfcla@hplabs. |  but do it in private, and wash your hands afterwards."
hp.com               |                   - Lazarus Long/Robert A. Heinlein