🚀 go-pugleaf

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!!!