From: neilson@osf.org (Peter Neilson)
Subject: Re: ADVICE WANTED:  Changing house outlets from 2 to 3 prongs
Organization: Open Software Foundation
Distribution: usa
Date: Thu, 30 Jul 1992 13:17:15 GMT

In article <NUGE.92Jul29095030@newport.med.ge.com> nuge@newport.med.ge.com (James A. Nugent) writes:
>In article <1992Jul29.083446.9248@osf.org> neilson@osf.org (Peter
>Neilson) writes:
>
>> There are circuit testers, and there are circuit testers.  Only some
>> can test the efficacy of a ground.  The one you have handy is not
>> necessarily the right kind.
>
>	What kinds of ckt testers are available to test the efficacy
>of a ground?
>
I don't own one, and haven't looked into the situation for years, so I
can't tell you, but your electrical supply dealer (not your hardware
store) should know about them.  They can't possibly be cheap.  The
trick is to measure a very low resistance correctly, which (in the
laboratory) is a complicated procedure, requiring a so-called double
bridge with separate voltage and current connections.  Fortunately,
there is another (and preferred) way to be sure you have a good
ground, and that is to follow established, proven methods of
installation.

A jury-rigged method of testing a ground could work this way:  Wire a
100-watt bulb between hot and your supposed ground, and measure the
potential difference (voltage) between neutral and your ground.  If
the ground is good, the current from the bulb should not be able to
float the ground.  This method is necessarily inaccurate and hazardous.
>
>> Old BX cable (without the added ground wire) is notorious for
>> producing poor grounds because it easily pulls loose from the
>> connector (the clamp that holds it to the box).
>
>        Are you talking about the thin 'bonding strip' that you wrap
>around the lip of the clamp at the connections, or a actual ground
>_wire_? I've seen the former, but not the latter.
>
I'm talking about OLD procedures, the way they USED TO DO things,
that'll get you in trouble if you don't realize what they did.  I'm
talking about the clamp itself that you fasten onto the end of the
BX cable with one or two screws, and that attaches to the knockout hole
in the box with a big, flat nut in the box.  The clamp always fastens
poorly to the BX.  The BX corrodes, flaps about, gets shifted when
someone remodels the house, and sooner or later the electrical
connection at the connector (where it clamps to the BX) is tens or
hundreds of ohms, instead of milliohms.  For this reason, modern BX
carries a ground wire.  ==> Do not depend on no-ground-wire BX for a
ground connection. <==

Another reason to be wary of BX as a grounding conductor is that when
it first came out, some folks added it to knob-and-tube installations.
You'll find BX at the box, and think, "Oh, good.  A ground."  But the
other end of the BX ties into K&T somewhere in the attic.  Also, some
folks used the old, tiny K&T boxes with BX.  Those boxes don't have
standard knockouts, can't take connectors (clamps), and cannot provide
a ground.  The electrician would just shove the BX into the hole in
the box, and rely on hope and darkness to hold it in place.  Hey, it
was a good thing, 'cuz you could get twice as many wires into the same
box as you could with K&T!!  K&T wires, wrapped in insulation (loom),
are one per hole.  With BX you could put a pair of wires through one
hole!  I wonder if any wiring code ever actually approved this use of
BX, at one time or another?
>
>--
>Jim Nugent
>
>nuge@med.ge.com
>
>nugentj@med.ge.com


-- 
            >>>  Peter Neilson  --- neilson@osf.org  <<<
        PROTECT YOUR SIGNATURE FILE AGAINST UNNECESSARY TRANSMISSION.
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