Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!cwjcc!gatech!ncsuvx!mcnc!unccvax!dya
From: dya@unccvax.UUCP (York David Anthony @ WKTD, Wilmington, NC)
Newsgroups: sci.electronics
Subject: Re: How can I recognize true ground?
Message-ID: <1547@unccvax.UUCP>
Date: 7 Jul 89 14:52:42 GMT
References: <18425@mimsy.UUCP>
Organization: Univ. of NC at Charlotte, Charlotte, NC
Lines: 52

In article <18425@mimsy.UUCP>, spector@brillig.umd.edu (Lee Spector) writes:
> Question from a novice: My house has "two prong" outlets.  I've been told
> that the center screw and casing of the outlet boxes is generally grounded,
> so that I will have a correctly grounded outlet if I attach the wire of a
> "3 to 2 adapter" to the center screw.  How do I (a vertual electronics
> know-nothing) check to see if this is working in my case?

	Basically, you must consult a licensed electrician.

	There is no guarantee that the centre screw, or the outlet
box, is in fact grounded appropriately.  Although licensed electricians
are supposed to wire the house while under construction, and city
inspectors are supposed to inspect the thing, there is no guarantee
that it was done correctly. ( I'm not old enough to remember what
was done with two-prong outlets in new construction...Larry Lippman,
are you out there???)

	Assumption in a situation like this can be deadly. I made
a stupid assumption in using the three wire to two wire adapter with
my 'fridge.  The defroster heater shorted to its case approximately
in the middle of its winding, basically placing the entire 'fridge
frame at a lethal potential.  Had the metal sheathed cable been
a continuous metallic path back to earth ground, all would have been
well, the breaker would have tripped.  Evidently, the metal sheath
is not continuous in my home, as my wife rudely found out while
washing dishes (she brushed against some of the brightwork and
received a frightening shock.)

	Now, this wasn't "leakage" current, as the giant power resistor
that was the defrost heater was perfectly capable of delivering enough
current to the frame that the breaker immediately tripped when the fridge
was bonded to a **real** ground.  The situation was dangerous for two
reasons:

	1) The handle provides an excellent opportunity to demonstrate
how you can't let go of an energized conductor beyond a certain potential.

	2) If the metallic sheath had been partially continuous, arcing
could have resulted, with its resulting fire hazard.

	At this point, we had a contractor rip out all the crap wiring,
and install proper wiring with three-prong outlets.

	Please do not attempt to "class I-ize" your outlets by running
a separate wire to ground.  The ground conductor must be capable of 
carrying the full fault current until the protection device for that
branch opens.  While there is no doubt an NEC code for "isolated ground"
outlets (as might be used in a computer installation) 
CONSULT AN ELECTRICIAN.

York David Anthony
DataSpan, Inc