[sci.electronics] How can I recognize true ground?

spector@brillig.umd.edu (Lee Spector) (07/07/89)

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?

(I am doing this to comply with my computer's manual's warning not to defeat
the ground plug.  Is this really a serious problem?  Would my surge protector
(SL Waber Dataguard DG415S) be making any difference?)

Thanks in advance for your time and wisdom - all comments very welcome.
 - Lee Spector (spector@brillig.umd.edu)

dya@unccvax.UUCP (York David Anthony @ WKTD, Wilmington, NC) (07/07/89)

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

henry@utzoo.uucp (Henry Spencer) (07/07/89)

In article <18425@mimsy.UUCP> spector@brillig.umd.edu.UUCP () 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?

If you are really, seriously concerned about whether it really is grounded,
you can't assume that the screw is grounded, and in fact you can't assume
that the third prong in a three-prong outlet is grounded -- electricians
do get sloppy now and then.

I'm unable to think of a quick no-equipment test to make sure; perhaps one
of the higher gurus can.

>(I am doing this to comply with my computer's manual's warning not to defeat
>the ground plug.  Is this really a serious problem? ...

Long odds that warning was written by lawyers worried about liability
rather than by technical people.  Grounded outlets *are* a good idea,
though, and I'd proceed cautiously.  Certainly tying the wire to the center
screw is worth doing.
-- 
$10 million equals 18 PM       |     Henry Spencer at U of Toronto Zoology
(Pentagon-Minutes). -Tom Neff  | uunet!attcan!utzoo!henry henry@zoo.toronto.edu

spector@brillig.umd.edu (Lee Spector) (07/08/89)

I asked how to tell if a house outlet is properly grounded...

MANY THANKS to all respondants!  I've received about 8 email answers
already!  It turns out that about half of the outlets in my house are
properly grounded...  I found out using one of those "3 LED outlet-testing
gizmos".  Thanks again.  -Lee

barry@hprmokg.HP.COM (Barry Fowler) (07/08/89)

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.


Second method:

Most discount hardware stores have a wall outlet checking device.  It is 
simply plugged into the outled.  There are a couple of color coded LEDs
on the device that will tell you if the outlet is wired properly and
if ground is REALLY there.

'That's my two cents.


  ----------------------------------------------------------------------
  
  Barry M. Fowler                        Hewlett Packard Company

  Networked Computer Manufacturing Operation - Roseville, California

  e-mail:  barry@hprmokg.HP.COM
  Verbal:  Hey Barry!
  R.F.  :  WB6JZL

psfales@cbnewsc.ATT.COM (Peter Fales) (07/08/89)

In article <1989Jul7.155721.19105@utzoo.uucp>, henry@utzoo.uucp (Henry Spencer) writes:
> In article <18425@mimsy.UUCP> spector@brillig.umd.edu.UUCP () 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?
> 
> If you are really, seriously concerned about whether it really is grounded,
> you can't assume that the screw is grounded, and in fact you can't assume
> that the third prong in a three-prong outlet is grounded -- electricians
> do get sloppy now and then.
> 
> I'm unable to think of a quick no-equipment test to make sure; perhaps one
> of the higher gurus can.

If I understand what you are asking, any hardware store should have an
AC line tester.  This is not a "no equipment" test, but it doesn't cost
more than a couple of dollars, and 
is a small device about the size of a three prong
plug containing two green and one red neon bulbs.  When you plug it into
a three prong outlet (or your grounded 3-2 adapter), it will show 
several common wiring errors such as reversed hot/neutral, missing ground,
and a few others.  It is probably not 100%, but it gives you a better
feeling.  I found several outlets in my house with hot/neutral reversed.

i
n
e
w
s

f
i
l
l
e
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-- 
Peter Fales			AT&T, Room 5B-420
				2000 N. Naperville Rd.
UUCP:	...att!ihlpb!psfales	Naperville, IL 60566
Domain: psfales@ihlpb.att.com	work:	(312) 979-8031

roy@phri.UUCP (Roy Smith) (07/08/89)

In article <18425@mimsy.UUCP> spector@brillig.umd.edu.UUCP () writes:
> I've been told that the center screw and casing of the outlet boxes is
> generally grounded, [...] How do I (a vertual electronics know-nothing)
> check to see if this is working in my case?

	I've seen enough electrical systems to know that you shouldn't take
anything for granted.  I wouldn't even assume that a 3-pin outlet is really
grounded (from what I can tell, most electricians don't really understand
that there is a difference between ground and neutral).  Anyway, get a
volt-ohm meter.  Find a piece of copper plumbing (preferably a cold-water
supply line) and measure the voltage between the supposed ground point and
the copper pipe.  It should be essentially zero (you might well have
several volts between the neutral side of the outlet and the ground,
however).  Assuming you read zero volts between the ground point and the
copper pipe, switch the meter to the lowest resistance scale and measure
the resistance between the pipe and the ground.  It should be very low, a
few ohms at most.  If it is, you've got a good ground.  You might have to
clean up a bit of the pipe with sandpaper to make a good connection.

	If you either get any appreciable voltage (I would guess even a
tenth of volt on the ground is reason to be suspicious) or any appreciable
resistance (more than a few ohms) between the ground and the copper pipe,
your supposed ground is *probably* not really grounded.  It is possible
that the pipe is not really grounded -- there might be a piece of plastic
pipe somewhere between it and the earth, or some teflon joint tape might be
getting in the way, but by and large, if you've got solder-joint copper
plumbing, you should be able to count on any point in the cold water supply
being a good ground (but not the hot water supply; you don't usually have
metalic continuity through the hot water heater).  But, by-and-large, if
your "grounded" outlet fails the tests above, I'd bet it was the outlet
that was the problem, not the pipe.
-- 
Roy Smith, Public Health Research Institute
455 First Avenue, New York, NY 10016
{allegra,philabs,cmcl2,rutgers,hombre}!phri!roy -or- roy@alanine.phri.nyu.edu
"The connector is the network"

larry@kitty.UUCP (Larry Lippman) (07/10/89)

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.
<>  UUCP   {allegra|boulder|decvax|rutgers|watmath}!sunybcs!kitty!larry
<>  TEL  716/688-1231 | 716/773-1700  {hplabs|utzoo|uunet}!/      \uniquex!larry
<>  FAX  716/741-9635 | 716/773-2488     "Have you hugged your cat today?" 

dya@unccvax.UUCP (York David Anthony @ WKTD, Wilmington, NC) (07/10/89)

In article <18447@mimsy.UUCP>, spector@brillig.umd.edu (Lee Spector) writes:
> I asked how to tell if a house outlet is properly grounded...
> 
> MANY THANKS to all respondants!  I've received about 8 email answers
> already!  It turns out that about half of the outlets in my house are
> properly grounded...  I found out using one of those "3 LED outlet-testing
> gizmos".  Thanks again.  -Lee

	The three-LED outlet testing gizmo does **NOT** verify the most important
aspect of a proper safety ground, namely, that it can carry the **entire** fault
current between the "hot" leg and the safety ground.

	While the device provides a "no-go" indication of improper wiring (say,
hot and neutral interchanged, ground definitely open, neutral definitely open
or no power) it does not verify the fault current handling capability of the
ground wire.

	I personally would not stake my life (or any equipment worth more than
a nominal sum, say < $ 500) on a tester such as this. How high does the resistance
in the ground leg have to be before the LED fails to light ????

York David Anthony
WKTD Wilmington, NC

john@stiatl.UUCP (John DeArmond) (07/11/89)

In article <3283@kitty.UUCP> larry@kitty.UUCP (Larry Lippman) writes:
>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.

Well, I considered it a bit too trite, especially since my dog knows more
about electricity than most electricians I've met (yep, still got my IBEW
journeyman card here somewhere :-).  When I was an apprentice, the major
thing I was taught was the three rules of electricianing:

1)	The foreman is an a**hole.
2)	Payday is on Friday.
3)	One hand in the pocket at all times.  (to keep current away
	from your chest if you get shocked)

First, since the original poster is an admitted novice, some background.
Power is supplied to you by the utility from a center-tapped 230 volt 
transformer secondary.  This center-tap is connected to earth ground 
at the pole and at several other places.  The end result is that you 
receive power via three wires - 2 are "hot" and one is ground or "neutral"
The 2 hot wires present a voltage difference of about 230 volts and each
hot wire presents about 115 volts to the neutral.

Since you are generally at ground potential by virtue of standing on
the ground or touching a grounded object, the general objective of
safety grounds is to ensure that whatever you touch is always at or
near the same voltage potential as the earth (or neutral lead since
it is connected to ground.)

A safety ground (what the third wire is for on your 3 prong plug) must
a) be at ground potential and b) have enough amperage capacity to conduct
any possible fault currents (currents resulting from a hot lead contacting
a grounded object) while holding the voltage drop to an appropriate level,
and c) be able to sink enough current to activate the short-circuit feature
of your overload protection device (fuse or circuit breaker).  The last 
part is important because if the ground does not have this capacity, the 
protective device may not trip or may trip later.  In either case, the 
tremendous heat generated by the heavy-but-not-quite-enough current could
start a fire.

When the power comes into your distribution box in your house, the 2
hot leads connect to the main breakers (or fuses in older installations)
while the neutral lead connects to a grounding strip.  This strip is 
bonded via a heavy copper lead to earth ground (typically a water pipe
or iron stake in the ground).

The individual branch breakers are connected to the downstream side of
the main breaker.  These breakers distribute the energy to the individual
branch loads (outlets, stove, air conditioner, etc) via branch wires, 
typically 12 to 14 gage wire.  A normal 115 volt branch will have 3 
leads:  A black lead which is the hot wire connected to the branch 
breaker, a white lead, which is the neutral connected to the grounding
block in the breaker box and a ground lead, normally bare or green if 
insulated, which connects TO THE SAME GROUNDING BLOCK in the breaker box.
In other words, AT THE BREAKER BOX, the neutral and ground leads are 
connected together. 

For a normal three prong outlet, the black (hot) wire connects to the
right (smaller) prong, the white (neutral) connects to the left 
(wider) prong and the ground wire connects to the round pin.  This round
pin is normally bonded via the mounting flange to the outlet box.  (outlet
viewed from this prospective:)

	|     |
	|
	   o

Normally, power flows from the hot prong, through the load, and back to the
neutral prong.  The ground pin normally carries no current.  

Now consider what happens when a wire inside your electric drill fails and
contacts the metal frame.  The metal frame is normally connected to the
ground pin.  Line voltage would be applied to the frame.  A properly
functioning ground lead would conduct this current directly back to 
ground.  In other words, a direct short circuit is created.

All protective devices have a time vs overload response curve that is
specified in a standard (ok, so I don't remember which one it is :-).
What this means is that a circuit breaker or fuse will carry a small
overload for a long period of time while tripping faster as the overload
increases.  At some point, defined as the short-circuit point and generally
about 10 times the device's rating, it must trip instantaneously.  Thus, for
a 15 amp branch, the breaker must trip immediately at about 150 amps.

The job of the safety ground is to ensure that this much current can be
handled so that immediate tripping is guaranteed.  A side benefit is that
a high quality ground will conduct enough current that quite often the 
offending wire is vaporized by the arc, removing the fault even before
the breaker can trip.

If the ground were inadequate and capable of, say, only 60 amps under
short circuit conditions, the breaker would handle the load for several
seconds or minutes, meanwhile, your device is smoking and/or you
are being shocked..


Our poster has the problem of only 2 wires - a hot and a neutral.  But no
safety ground.  In these instances, the only real alternative is to run
a safety ground to the outlet.  In some cases, the neutral is connected
to the outlet box/ "ground pin".  This is worse than nothing at all.

So a ground wire must be run - but how?  Well, the ground rules (pun 
intended) are that the ground should have at least the ampacity of the
branch wiring which means it should be of the same AWG as the branch or
at a minimum, one AWG smaller.  Thus, a branch wired with AWG 14 wire 
should have a ground lead of AWG 14 or minimally, AWG 16.  Rule of thumb:
it should handle the full load of the branch with no more than 5 volts
rise as measured between the ground prong and a known good earth ground.
I don't remember if this is in the code but it's a parameter I've always
used.

How to run this wire?  In a modern house, you will probably want to 
snake it along the same path as the branch wire.  This is sometimes 
difficult.  Another alternative is to run the wire OUTSIDE the house,
especially if the house has siding which could hide the wire.  In an 
old apartment building I once owned in PA, I ran a ground bus (awg 8) wire
around the outside of the house up under the overhang of the siding.
At each outlet, I drilled a hole through the wall and siding and ran a 
ground wire out.  The wire was soldered to the ground bus.  The ground
bus was routed back to the grounding block on the breaker panel.  After
sealing the holes and painting the bus, it was almost invisible.

So how to test a ground?  Well, the little outlet testers mentioned in
other postings verify that a connection exists but will not verify 
the ampacity of the ground.  What I have done is build a tester as 
follows:


Three prong plug
                       push button switch  (15 amp)

|     |---------------/ ----------------|
|                                       |
   o                                    LOAD  (hair dryer, Iron, etc)
   |                                    |
   |------------------------------------|
  "a" 

What this circuit does is place  a heavy load - typically 1000 watts or
better - between the hot leg and the safety ground.  Voltage is
then measured between the safety ground pin ("a") and a known good
earth ground such as the grounding strip in the breaker box.
The voltage here should be less than about 5 volts.

*** WARNING - If the ground is defective, the outlet box will become
electrically hot as long as the button is pushed.  Keep this in mind
while testing.

While most electricians don't do this test, I've found enough marginal
grounds to warrent it.  Doing your own electrical wiring is not hard
or unsafe if you follow established standard, are careful and THINK.
I'd suggest that the original poster get a copy of the current National
Electrical Code (and any local codes) and a book on home wiring.

As a person who was almost electrocuted by a defective battery charger
plugged into a 3 prong outlet with no ground wire attached, I can 
personally testify to the value of a good grounding.  Were it not for
me falling down as I was being shocked and thus loosened from the short,
I'd probably be dead.

John

-- 
John De Armond, WD4OQC                     | Manual? ... What manual ?!? 
Sales Technologies, Inc.    Atlanta, GA    | This is Unix, My son, You 
...!gatech!stiatl!john    **I am the NRA** | just GOTTA Know!!! 

rmrin@cbnewsk.ATT.COM (r.m.rickert) (07/11/89)

A not perfect but quick check is to buy a tester at a hardware store 
or Radio Shack (about $3) that looks like a plug and has three leds on it.
The Micronta (Radio Shack) version also has a decal telling what the leds are 
saying.  Plug this into a grounding adapter wired into the socket and see what
the leds are telling you.  If it says you have a problem, you probably do.
-- 

Dick Rickert  AT&T Consumer Products Laboratory

don@zippy.eecs.umich.edu (Don Winsor) (07/12/89)

In article <5651@stiatl.UUCP> john@stiatl.UUCP (John DeArmond) writes:
> ... Our poster has the problem of only 2 wires - a hot and a
> neutral.  But no safety ground.  In these instances, the only
> real alternative is to run a safety ground to the outlet.
> In some cases, the neutral is connected to the outlet
> box/ "ground pin".  This is worse than nothing at all.

I've run into a poor wiring job in an older house belonging to
friends where I found a multitude of sins.  One of them was that
some grounded receptacles were wired with old two conductor cable
and they just had the neutral and ground pins tied together at
the receptacles.  In the interests of safety I suggested they
pull modern NM cable to these receptacles, which didn't seem too
hard to do, since there was easy access down through the wall and
into the (unfinished) basement.  My friend didn't think this was
necessary, and I couldn't come up with a good, convincing argument
that this was a bad idea.  His line of reasoning was that the
ground and neutral wires all went to the same bus bar on the
main breaker panel, so running another conductor offered no
advantage.  Can anyone offer a good, clear, convincing argument
to refute this?  About the best I could think of was if the neutral
wire got broken somewhere between the breaker panel and the
receptacle, and you then plugged a low resistance device with a
grounded case into the outlet (say an electric drill with a three
prong cord in a metal case), there could be a significant leakage
path from hot, through the motor windings, out the drill's neutral
wire, across the neutral-ground tie in the receptacle, and back up
the drill's ground wire to the drill case.  Thus, we now have a
hot drill.  Is this the reasoning?  If not, can someone give a
better explanation as to why "this is worse than nothing at all"?

                Don Winsor
                Electrical Engineering and Computer Science
                University of Michigan, Ann Arbor, Michigan
                don@dip.eecs.umich.edu

rdi@cci632.UUCP (Rick Inzero) (07/12/89)

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

henry@utzoo.uucp (Henry Spencer) (07/12/89)

In article <178@zip.eecs.umich.edu> don@zip.eecs.umich.edu.UUCP (Don Winsor) writes:
>... His line of reasoning was that the
>ground and neutral wires all went to the same bus bar on the
>main breaker panel, so running another conductor offered no
>advantage...

If neutral is carrying a significant current, there will be a significant
voltage drop between the outlet end and the bus-bar end of the wire.  So
neutral can be "hot" by a noticeable amount at the outlet even if it's
grounded at the bus bar.

NEUTRAL IS NOT GROUND!
-- 
$10 million equals 18 PM       |     Henry Spencer at U of Toronto Zoology
(Pentagon-Minutes). -Tom Neff  | uunet!attcan!utzoo!henry henry@zoo.toronto.edu

john@stiatl.UUCP (John DeArmond) (07/13/89)

In article <178@zip.eecs.umich.edu> don@zip.eecs.umich.edu.UUCP (Don Winsor) writes:
>
>I've run into a poor wiring job in an older house belonging to
>friends where I found a multitude of sins.  One of them was that
>some grounded receptacles were wired with old two conductor cable
>and they just had the neutral and ground pins tied together at
>the receptacles.  In the interests of safety I suggested they
>pull modern NM cable to these receptacles, which didn't seem too
>hard to do, since there was easy access down through the wall and
>into the (unfinished) basement.  My friend didn't think this was
>necessary, and I couldn't come up with a good, convincing argument
>that this was a bad idea.  His line of reasoning was that the
>ground and neutral wires all went to the same bus bar on the
>main breaker panel, so running another conductor offered no
>advantage.  Can anyone offer a good, clear, convincing argument
>to refute this?  

YES, I can.  The neutral wire has a finite resistance.  At full load,
even a good connection will drop a few volts.  These few volts would
be imposed on the frame of your "grounded" appliance.  Even if this
voltage is not enough to shock you, it could be enough to damage 
sensitive electronics or cause electrolytic damage if the device becomes
wet.

And, of course, if the neutral wire ever corrodes, you get zapped.

A last consideration is that if your house ever caught on fire and
electrical problems were indicated as a cause, the insurance company
could try to use the code violations as an excuse not to pay.  they
may not be successful but is it worth the chance?

John


-- 
John De Armond, WD4OQC                     | Manual? ... What manual ?!? 
Sales Technologies, Inc.    Atlanta, GA    | This is Unix, My son, You 
...!gatech!stiatl!john    **I am the NRA** | just GOTTA Know!!! 

myers@hpfcdj.HP.COM (Bob Myers) (07/14/89)

>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    

ted@blia.BLI.COM (Ted Marshall) (07/15/89)

In article <29646@cci632.UUCP>, rdi@cci632.UUCP (Rick Inzero) writes:
> 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!!

Actually, a GFCI does not require any ground connection to operate. It
works by comparing the current flow through the hot line with that through
the nutral. If one is passing more current than the other, the button
pops. The ground line is not involved at all other than you assume that
the extra current must be flowing to ground somewhere.

I assume that the test button works by connecting a resistor between hot
after the GFCI circuit and nutral before the circuit.

Finally, be aware that in houses where there is no ground run to the
outlets (i.e. built before the National Electrical Code (or whatever
the name actually is) reqired 3 prong outlets), the Code allows you to
replace two prong outlets with GFCI three prong outlets with the ground
lug unconnected! I originally read this fact in Popular Science
magazine and it was confirmed by a code inspector for the city of San
Jose. Because the GFCI will detect current flowing from the hot line to
a ground (including through your body), this provides the same kind of
protection as a conventional, properly wired 3 prong outlet.

-- 
Ted Marshall          ...!uunet!blia!ted   <or>   ted@blia.bli.com
ShareBase Corp., 14600 Winchester Blvd, Los Gatos, Ca 95030     (408)378-7000
The opinions expressed above are those of the poster and not his employer.

roy@phri.UUCP (Roy Smith) (07/16/89)

In article <11170023@hpfcdj.HP.COM> myers@hpfcdj.HP.COM (Bob Myers) writes:
> 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.)

	This is not to say that a GFI is the be-all and end-all when it comes
to protection.  Imagine the (admittedly rare) case of shorting the hot wire
directly to the neutral.  You'll get a perfectly balanced short (well, from
the point of view of a power engineer, it's an unbalanced phase-to-neutral
fault, but from the point of view of your GFI it's balanced).  Your GFI will
be perfectly happy to keep the circuit open until the wire melt if it didn't
have a conventional breaker to back it up.

	Don't think it doesn't happen.  Imagine a kid, looking remarkably like
I did when I was about 11 years old, putting two screwdrivers into the wall
outlet and laying a third screwdriver across the top.  BOOM!  Hey, how come
all the lights went out?  Sometimes I wonder how I stayed alive long enough to
understand the crazy things I did as a kid.
-- 
Roy Smith, Public Health Research Institute
455 First Avenue, New York, NY 10016
{att,philabs,cmcl2,rutgers,hombre}!phri!roy -or- roy@alanine.phri.nyu.edu
"The connector is the network"

aj-mberg@dasys1.UUCP (Micha Berger) (07/18/89)

spector@brillig.umd.edu.UUCP (?) 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?

Henry Spencer (henry@utzoo.uucp)  writes:
>If you are really, seriously concerned about whether it really is grounded,
>you can't assume that the screw is grounded, and in fact you can't assume
>that the third prong in a three-prong outlet is grounded -- electricians
>do get sloppy now and then.

>I'm unable to think of a quick no-equipment test to make sure; perhaps one
>of the higher gurus can.



If your house isn't of this decade, your plumbing is grounded. This is because
they bury your pipes uninsulated. So, if you're really nervous....
(New houses can have plastic pipes.)
Barring lightning, I wouldn't panic about grounding. Especially if your
outlet strip has a fuse / circuit breaker.
-- 
					Micha Berger

"Always should [the child of] Adam have awe of G-d in secret and in public,
admit the truth, and speak truth in his heart." 

blew@tc.fluke.COM (Bob Lewandowski) (07/20/89)

In article <10269@dasys1.UUCP> aj-mberg@dasys1.UUCP (Micha Berger) writes:
>spector@brillig.umd.edu.UUCP (?) 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?
>
     {{Text deleted}}
>
>Barring lightning, I wouldn't panic about grounding. Especially if your
 ^^^^^^^^^^^^^^^^^  ^ ^^^^^    ^^^^^ ^^^^^ ^^^^^^^^^     
>outlet strip has a fuse / circuit breaker.
>-- 
>					Micha Berger
>
I've watched this discussion go on for a while now and most of the advice
has been good, BUT now to insert my $.02!!

Many if not all PC's and a large portion of commercial grade electronics
have some sort of power line filter to meet FCC and other RFI/EMI specs.
Most of these filters have a pair of capacitors from each side of the line
to "ground". (read chassis)  Modern equipment probably use capacitor values
such that the reactive current to "ground" is less than 0.5ma for 120V
lines. However, I've seen equipment from not that long ago that had caps
as big as .1uF from both lines to ground. When the chassis of such 
equipment is not connected to earth or mains ground, the chassis floats
at about 1/2 the line voltage! I've frequently seen people (who should 
know better!) use "cheater" plugs (3 pin to 2 pin converters, with the 
little ground wire disconnected, or cut off) to "float" some piece of 
equipment, austensibly to eliminate ground loops. This may eliminate the
dc ground loop, but the 60 HZ line current to "ground" is forced to flow
through the signal leads, which usually doesn't help the problem.

There are a lot of items in a home/shop in addition to plumbing and damp
floors, that are directly or indirectly tied to ground, even in older
buildings. A good example is heating equipment, radiators, floor
or wall registers, sheet metal ductwork, etc. It can be a real surprise
to be standing on a floor register in your bare feet and touch a "floating"
appliance. The current may be limited to a few ma. or less, but shocks
are dangerous under any conditions, especially 60 Hz.

A 120V bulb can be rigged up with some care as a test light.  One side
is connected to the ground in question, and the other connected to the
high side of the line (usually a black or red wire). If the bulb glows
brightly the ground is of some value. If it doesn't light or lights
dimly the ground is not good. Compare the brightness when the lamp is 
connected to the ground being tested to when it is plugged directly into
the outlet. Comparative brightness is a good rough measure of voltage
equality, because it varies significantly with small variations in 
voltage. This is *DEFINITELY* only a minimal test of ground integrity.

The best advice is the licensed electrician.

-----Bob

-- 
            Bob Lewandowski
    Domain: blew@tc.fluke.COM
     Voice: (206) 347-6100, Ext. 5368
      UUCP: {microsof,sun}!fluke!blew
  U S nail: John Fluke Mfg. Co. / P.O. Box C9090 / MS 273G / Everett WA  98206

michael@xanadu.COM (Michael McClary) (08/05/89)

In article <178@zip.eecs.umich.edu> don@zip.eecs.umich.edu.UUCP (Don Winsor) writes:
>
>I've run into a poor wiring job in an older house belonging to
>friends where I found a multitude of sins.  One of them was that
>some grounded receptacles were wired with old two conductor cable
>and they just had the neutral and ground pins tied together at
>the receptacles.  In the interests of safety I suggested they
>pull modern NM cable to these receptacles, which didn't seem too
>hard to do, since there was easy access down through the wall and
>into the (unfinished) basement.  My friend didn't think this was
>necessary, and I couldn't come up with a good, convincing argument
>that this was a bad idea.  His line of reasoning was that the
>ground and neutral wires all went to the same bus bar on the
>main breaker panel, so running another conductor offered no
>advantage.  Can anyone offer a good, clear, convincing argument
>to refute this?  About the best I could think of was if the neutral
>wire got broken somewhere between the breaker panel and the
>receptacle, and you then plugged a low resistance device with a
>grounded case into the outlet (say an electric drill with a three
>prong cord in a metal case), there could be a significant leakage
>path from hot, through the motor windings, out the drill's neutral
>wire, across the neutral-ground tie in the receptacle, and back up
>the drill's ground wire to the drill case.  Thus, we now have a
>hot drill.  Is this the reasoning?  If not, can someone give a
>better explanation as to why "this is worse than nothing at all"?

If you tie the protective ground pin to the neutral, the voltage drop
in the neutral wire (perhaps several volts, even if everything is
within spec) appears on the case of everything connected to the circuit.
If one of the things is a motor, you could find forty volts for a moment
when it starts.  You'll also be looking at fifty volts for a moment when
the circuit is shorted, assuming both neutral and hot have the same
size wire and identical connections.  (But if it's miswired, it's a
good sign there are other problems as well.  And if a neutral wire
connection vaporizes before the breaker goes, the breaker won't go,
and the drill stays at 110 indefinitely.)

It's current that kills, and you CAN do somebody in with 1 1/2 volts
if the connection is good enough.  (Like salty-sweat on your hands,
as you hold the drill and steady yourself against a really-grounded
piece of metal.)  20 amps when the case of your equipment contacts
a real ground might not be good for the finish, either.

(Story circulated when I was at UofMich EE school:  Bunch of freshman
EEs at another school had bet going on whether you could kill
somebody with a single dry cell.  Guy betting "no" put hands into
two buckets of salt water, guy betting "yes" applied 1 1/2 between
buckets.  They couldn't get "no" defibrilated.  R.I.P.)

Ask an EKG operator at your local hospital about "microelectricution"
some time.  Or ask the hospital electrician about the lengths they
go to on the grounding.  (Periodic inspections, transparent plugs
so you can see if the wire comes loose...)  The main thing that
enables you to handle a hundred volts is your skin resistance.
A cut, electrode paste, or a needle through the skin, and you've
gone from several megohms to nearly zero.  Sweat is enough when
you're dealing with house wiring.  20 ma up your left arm and
you'd better have the paramedics onsite, because you won't have
time to call them.

al@gtx.com (Alan Filipski) (08/08/89)

In article <fqSWQ#11dxbq=michael@xanadu.COM> michael@xanadu.UUCP (Michael McClary) writes:
->
->(Story circulated when I was at UofMich EE school:  Bunch of freshman
->EEs at another school had bet going on whether you could kill
->somebody with a single dry cell.  Guy betting "no" put hands into
->two buckets of salt water, guy betting "yes" applied 1 1/2 between
->buckets.  They couldn't get "no" defibrilated.  R.I.P.)
->

Yikes.  I frequently test a 9-volt battery by touching the electrodes to
my tongue.  Is this dangerous?


  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 ( Alan Filipski, GTX Corp, 8836 N. 23rd Avenue, Phoenix, Arizona 85021, USA )
 ( {allegra,decvax,hplabs,amdahl,nsc}!sun!sunburn!gtx!al       (602)870-1696 )
  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

myers@hpfcdj.HP.COM (Bob Myers) (08/10/89)

>Yikes.  I frequently test a 9-volt battery by touching the electrodes to
>my tongue.  Is this dangerous?

Not especially.  While you are providing a low-resistance path for the current,
the path does not take the current anywhere near what might be considered
"important parts".  (Such as the heart, etc...)  I'm not sure what the 
long-term effects on your TONGUE will be ("Oh no, we don' hafta cut that off!
Two, three days maybe, it fall off all by itself... :-)), but you're
not going to stop your heart by zapping your tongue.  (Otherwise, we'd see
all the TV ER doctors clamping defibrillator paddles onto their patient's
toungues.  Well, maybe in a Mel Brooks movie...)

Bob "Hmmm. Strange mood today, eh wot?" M.

pcf@galadriel.bt.co.uk (Pete French) (08/11/89)

From article <1119@gtx.com>, by al@gtx.com (Alan Filipski):
> In article <fqSWQ#11dxbq=michael@xanadu.COM> michael@xanadu.UUCP (Michael McClary) writes:
> ->
> ->(Story circulated when I was at UofMich EE school:  Bunch of freshman
> ->EEs at another school had bet going on whether you could kill
> ->somebody with a single dry cell.  Guy betting "no" put hands into
> ->two buckets of salt water, guy betting "yes" applied 1 1/2 between
> ->buckets.  They couldn't get "no" defibrilated.  R.I.P.)
> ->
> 
> Yikes.  I frequently test a 9-volt battery by touching the electrodes to
> my tongue.  Is this dangerous?

No : You are only running the current through the tip of your toungue. If you
put your hands into two buchets and applied the current directly accross your
body then it might be just a little risky though since trhe current would then
be flowing accross your heart.

I heard a similar story aabout a British University at which they were doing
_OFFICIAL_ research into electric shocks. They wanted to find out how much
current a person could *feel* (not to kill him). To do this they put a
volunteer into a tank of water with an electrode on him and earthed the other
end. They were going to start at 'negligible' voltage and gradually increase
it untill he could feel something.

With the first shock they killed him...


-Pete French.

john@stiatl.UUCP (John DeArmond) (08/16/89)

In article <10269@dasys1.UUCP> aj-mberg@dasys1.UUCP (Micha Berger) writes:
>
>If your house isn't of this decade, your plumbing is grounded. This is because
>they bury your pipes uninsulated. So, if you're really nervous....
>(New houses can have plastic pipes.)
>Barring lightning, I wouldn't panic about grounding. Especially if your
>outlet strip has a fuse / circuit breaker.
>-- 

I'm amazed that people make such irresponsible statements in public.  
Perhaps it's so easy to type, they don't bother thinking.

Micha,
Consider things that people do to old houses:

They patch old pipes with plastic pipe bandages.
They remodel and extend piping with plastic.
They feed the house from a well with plastic pipe.
They tie into city water with plastic pipe.
They replace rusted out sections with plastic pipe.

To rely on a water pipe goes beyond foolish into stupid.  Even if you detect
ground with your VOM, the pipe can quite likely not be capable of handling
fault current.  And there is no reason to think piping would not be
rearranged in the future.  Plumbers know pipes carry fluids; they don't give
a thought to its current capacity.

As to not worrying about grounding other than lightening, well..... I'll let
your words speak for themselves.  Perhaps you ought to experience electrocution
sometime.  Might change your emphasis.


-- 
John De Armond, WD4OQC                     | Manual? ... What manual ?!? 
Sales Technologies, Inc.    Atlanta, GA    | This is Unix, My son, You 
...!gatech!stiatl!john    **I am the NRA** | just GOTTA Know!!!