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Re: [PLUG] testing outlets (computers.)
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> So how would one go about thoroughly testing an outlet, or getting it
> tested ?
As has been mentioned, pickup an outlet tester at Home Deopt or Radio
Shack.... something like $5-10.
They test for current leakages between hot, neutral and ground.
There should never be a current flow between the hot line and ground, or
between neutral and ground in a 3-wire outlet. They light up pretty red
lights for those (RTFM).
What to look for?
Basically, two problems are very common in home wiring, especially in older
homes.
The first problem is that virtually zero home wiring prior to about 1985
was "3-wire" wired. The National Electrical Code simply did not specify
3-wire circuits (aka Isolated Ground or IG) . (The NEC is only an advisory
code anyway unless specifically included in Local Code specs. Your Local
Code may not require this even today.) Today, GFI circuits cannot operate
without a "real ground" and GFI is required in all "wet" locations in new
construction - bathrooms, kitchens, basements, outside areas, etc. by the NEC.
Prior to that NEC change, "neutral" was equated with ground.
[The testers show you this.]
That caused the 2nd problem with older wiring -- the neutral wire
frequently is "hot." The reason is usually that somebody screwed up
someplace and via something like a 3-way switch or some do-it-yourself
project, managed to get the wires flipped and the result is that hot side
of the circuit fed into the neutral.
Another problem which frequently occurs is a "floating ground."
It is usually derived from the First problem. When 3-wire outlets were
installed into 2-wire homes -- with the "ground" connection tied to the
outlet box, which may or may not have had a "conduit ground" at one
time. "Conduit grounds" are notoriously flaky simply because they depend
upon a solid mechanical connection via the conduit all the way back to the
ground, and screws get loose. So assuming that you really did get a good
ground when the outlet was installed, over time many things as simple as
normal vibrations and corrosion can cause that ground to "float."
The testers will also show you this kind of problem, sometimes. If the
ground is truly floating (ie not grounded) then it will show up. However,
if the ground is intermittent - like when somebody walks across a floor, or
when the stereo is cranked up high, it probably won't.
How to fix them - the right way...
First, understand - neutral is not ground, and vice-versa.
You must have a 3-wire (white, black and green) cable "home-run" to the
power distribution panel (aka Service Entrance) to guarantee that you have
a real ground at the outlet. And your ground buss must have a REAL ground
rod, that is, NOT a "water-pipe ground" and NOT connected to the Neutral.
Why 3-wire? (aka Isolated ground). Because the normal white, black, and
un-insulated wire when installed correctly does NOT necessarily give you an
"isolated ground", but rather simply ties the bare copper wire to the
distribution panel Neutral (the white wire). Which in turn is usually tied
to a Water-Pipe ground. Depending upon many things, a water pipe ground is
fine, but it is not an Isolated Ground either. The bare copper wire likely
also touches conduit, and other similar things which are NOT grounds and
which can provide "unexpected" current paths.
[A "real" water pipe ground must be connected on the Mains side of the water
meter. The ground cannot be successful THROUGH a water meter. The NEC
specifies this also.]
The difference between copper-wire and Isolated Ground? --
With an Isolated Ground you ALWAYS have a ground. period, guaranteed.
With "normal" bare-copper-wire grounding, you might have a ground. And if
you do, you share it with other "stuff" on the AC power line.
If you have a piece of AC/DC equipment (much less common today than it was
10 years ago), there is a high probability that the (usually) radio or TV
would back-feed current into the neutral -- especially if powered off
(yes, counter-intuitive). You can get similar problems today from the fact
that any remote-controlled device (especially TVs) are never completely
powered off anyway, and somebody used one of those 3-2 prong outlet
adapters, or if one of your "polarized" outlets is wired incorrectly.
Similarly, grounding via neutral leaves your ground susceptible to hash
from everything from the microwave to the washer... all those rotating
motors with their brushes and commutators that you have around the house.
And whatever you do - don't use the same ground that any kind of lightning
arrestors use -- like on the roof of your house or on your telephone line!
Older homes have telephone entrance equipment which contains a "spark gap"
lightning arrestor. It's made of porcelain and big resistors, is usually
about 4 inches square and looks like it was designed by Alexander Graham
himself... it was! Those things SHOULD be connected to their own grounding
rod, but frequently are connected to the good-ol' water-pipe ground... So
guess what happens when there is a lightning strike in your area...
[big, possibly fatal, power surge via what you thought was a ground.]
Anybody with Ball-and-tube wiring, wiring prior to about 1940, or 2-wire
stuff installed up through at least the 1960s, is in deep dodo when it
comes to consumer electronics, let alone computers. [Anything less than
100Amp service probably falls into this category.]
If your home has 100Amp service you are probably a 2-wire+solid-copper-ground
wired house. If you don't have 100Amp service, 99% guaranteed that you fall
in with the ball-and-tube group -- deep dodo. 200Amp service should have
a complete IG capability "in the box," but it may not be extend beyond the
Service Entrance unless everything was re-wired when the service was
upgraded. Contemporary 100/200Amp distribution panels have isolated
Neutral and Ground buss termination strips. They should not be cross
connected and each should be either only white wires (neutral) or green and
copper wires (ground).
Apartments -- assume you are in deep shit before you even start to look for
solutions, unless your building is less than 10years old. (Structural
renovations count but not painting, etc. because once significant
renovations on a commercial property are begun, they have to bring the
whole property up to current code.)
Condos are homes, not apartments -- even in high-rises.
And all of this gets to be great fun if you also play with X-10 Home
automation!
...next week we'll talk about bridging phases and load balancing...
--
www.tru64unix.compaq.com
www.tru64.org
comp.unix.tru64
T.T.F.N.
William H. Magill Senior Systems Administrator
Information Services and Computing (ISC) University of Pennsylvania
Internet: magill@isc.upenn.edu magill@acm.org
http://www.isc-net.upenn.edu/~magill/
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