Path: utzoo!utgpu!jarvis.csri.toronto.edu!rutgers!cs.utexas.edu!uunet!peregrine!ccicpg!cci632!rdi
From: rdi@cci632.UUCP (Rick Inzero)
Newsgroups: sci.electronics
Subject: Re: How can I recognize true ground?
Message-ID: <29646@cci632.UUCP>
Date: 11 Jul 89 21:26:26 GMT
References: <18425@mimsy.UUCP> <10040007@hprmokg.HP.COM>
Reply-To: rdi@ccird3.UUCP (Rick Inzero)
Distribution: na
Organization: CCI, Communications Systems Division, Rochester, NY
Lines: 53

In article <10040007@hprmokg.HP.COM> barry@hprmokg.HP.COM (Barry Fowler) writes:
>I can think of two simple methods that will allow you to tell if 
>"the center screw is ground".
>
>First, turn the circuit breaker off that controls that outlet.  You can 
>simply plug a light in the socket and verify that it goes off when the
>proper circuit breaker is shut off.
>
>Remove the wall plug face plate (remove the center screw).
>
>Examine the inside of the wall box.  If it is grounded, it will have a bare
>copper wire connected to a screw that is attached to the wall box.

This isn't the greatest method in the world--  What if the OTHER END
of the bare copper wire isn't grounded??  This actually happened to
me in my bathroom- the one end I *looked at* was firmly connected, but the 
*other end* of the wire, 6' away through the wall was just dangling there. 
The previous owner of the house had added this outlet, daisy chained to 
another in the adjacent room, but forgot to connect the ground wire 
at the tapped box!

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.)

Since we're talking about electrical safety, realize that in some
house wiring, the hot wire is not *necessarily* the wire that is switched 
by a wall switch!  I got zapped while fixing a ceiling light that I had
switched off at the wall.  After the zapping, I got out the meter and 
found a full 115 volts between the hot wire and the "grounded" metal 
light fixture, although the bulb was not lit!! In my house, built in 
1920, they switched the *neutral* wire from the wall switch!!  EEyowww!  
I don't even want to change the light bulb in that light now without 
killing the breaker!!  That was pretty unexpected to me, but I guess it's 
pretty common since it saves much $$ as far as running wire goes, plus
offers some other advantages (it makes it really easy to wire up ceiling 
fans where the fan gets power all the time- turn it on by pull chain, 
and the light in the fan is controlled by the wall switch).

---
Rick Inzero					rochester!cci632!rdi
Computer Consoles Inc. (CCI)			uunet!ccicpg!cci632!rdi
Rochester, NY					uunet!rlgvax!cci632!rdi

								
"Every time I hear that weasel tune, something pops inside of me, 
then everything goes black."  
				- Curley Howard in "Punch Drunks", 1934