🚀 go-pugleaf

In article <1547@unccvax.UUCP>, dya@unccvax.UUCP (York David Anthony @ WKTD, Wilmington, NC) 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.

	Some readers may think that the above is a trite answer, but it is
really the best advice, which I trust you will understand by the time you
have read this article.
 
> 	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???)

	You rang? :-)

	One of Life's Great Mysteries is whether there is in fact an adequate
"equipment" ground on a given electrical outlet.

	An inexpensive "outlet safety tester" which utilizes three or so
neon lamps to ascertain if an outlet is correctly wired should NOT be
relied upon in situations where health, safety and critical equipment is
involved.  A neon lamp is a pretty crude voltage indicator; typical neon
lamps (like the NE-2) fire at 65 to 70 volts, so a non-illuminated neon
lamp on such an outlet tester can STILL allow 60 volts to be present!

	Think for a minute: one could fool those neon lamp outlet testers
with a piece of 30 AWG wire as an equipment ground, although there will
obviously be little current-carrying capacity in such an arrangement!

	The efficacy an outlet ground can only be ascertained through
careful wiring inspection and/or through the use of a special ohmmeter
intended for that purpose called a "ground resistance meter" (manufactured
by such companies as Associated Research, Biddle, etc.).  The real test
involves measurement of resistance between the electrical outlet equipment
ground and earth ground - NOT just measuring potential difference between
the outlet wires in an energized state.

	While I have identified the correct technique for outlet equipment
ground testing using resistance meaurement, I would urge caution on the
part of anyone attempting this with other than a ground resistance meter.
Many VOM's have ohmmeter circuits whose accuracy may be adversely affected
by the presence of AC potential differences between the equipment ground
and earth ground (say, a water pipe) arising when measurements are taken on
an system with energized loads.  If someone is determined to try this method
using a conventional VOM, at least try to open the main disconnect to
preclude any chance of such error.

	Just because a duplex electrical outlet has a grounding screw does
NOT mean it is grounded.  Consider the following techniques which have been
used in residential wiring, along with some corresponding caveats:

1.	Before 1930, most one and two-family homes were wired using a
	technique known as "knob and tube", in which case there were just
	two rubber/cloth-insulated wires run to each outlet (neutral and
	hot).  The outlet box, while metal, was NOT grounded - so neither
	was the outlet or grounding screw!  While many houses built during
	that time period have since been rewired for 240 volt service, there
	is no guarantee that every single branch circuit was in fact rewired
	to provide an equipment ground.  Do not assume anything in an older
	home!

2.	"Better" one and two-family homes built before 1930 used rigid metal
	conduit or metallic-sheathed cable (BX), and most other one and
	two-family homes built between 1930 and 1950 used BX.  Metal outlet
	boxes and outlets used in this construction are grounded, PROVIDED
	that the BX sheath was properly teminated and that BX and conduit
	connections have not become loose or corroded.

3.	Romex did not see general use until after 1950.  Early Romex used
	an impregnated cloth sheath, while Romex manufactured after about
	1960 or so used an extruded plastic sheath (as today).  Many one
	and two-family homes built between 1950 and 1960 used only two-wire
	Romex with NO EQUIPMENT GROUND WIRE.  This means that a home built
	during the 1950's may NOT have grounded outet boxes, even though
	they are metal (plastic outlet boxes did not become common until
	will into the 1960's).

4.	Three-wire grounded outlets became mandatory for NEW construction
	sometime in the mid-1960's.  At this time, any Romex had to carry
	an equipment ground wire in order to satisfy this requirement and
	ground the outlet.

5.	Then there came aluminum wire, which pervaded the residence wiring
	market from the mid 1960's to the early 1970's.  The equipment
	ground wire in alunimun Romex has no insulating sheath to give it
	mechanical protection, and hence it is very fragile.  It is not
	uncommon to see an outlet grounded using aluminum Romex to have NO
	ground due to a broken or loose aluminum ground wire!


	There are three morals to my article:

1.	Do not assume anything with respect to outlet grounding!

2.	Do not place implicit trust in a neon lamp "outlet safety tester"!

3.	Have a competent person inspect your electrical wiring where the
	issue of equipment grounding is important!

<>  Larry Lippman @ Recognition Research Corp. - Uniquex Corp. - Viatran Corp.
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