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From: dya@unccvax.UUCP (York David Anthony @ WKTD, Wilmington, NC)
Newsgroups: misc.consumers.house,sci.electronics
Subject: Re: Grounding old electrics
Keywords: Electrical, Ground, House, Hints
Message-ID: <1619@unccvax.UUCP>
Date: 24 Aug 89 13:37:37 GMT
References: <2171@netcom.UUCP> <5057@teklds.CAE.TEK.COM> <26924@amdcad.AMD.COM> <12073@ulysses.homer.nj.att.com>
Distribution: misc
Organization: Univ. of NC at Charlotte, Charlotte, NC
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In article <12073@ulysses.homer.nj.att.com>, smb@ulysses.homer.nj.att.com (Steven M. Bellovin) writes:

> As I recall the NEC, the ground wire does not have to be the same size;
> it won't be carrying the current for that long...  However, you're
> correct for 12 and 14 gauge wires; it's only for the larger wires
> that the smaller size will do.

	If you value your life, you will make that ground conductor capable
of carrying the full fault current so that the breaker trips first.

> [there is an easy way to ]
> check it!  Using a voltmeter, verify that there is ~120 volts between
> the hot (black) wire and the outlet box.  If there isn't, that outlet
> is not groundable without running a new line to the breaker box.

	No, no, no, no, no, no, no, no, no, NO!

	Verifying that there is 120 volts between the hot wire and the
outlet box with an ordinary voltmeter (especially the very high impedance
meters in the trade today) proves nothing more than the  resistance of
the ground path is not a significant fraction of the instrument's
input impedance.  Even for a 2000 ohm/volt instrument (which is very
low impedance) a ground path resistance of 20 ohms would not be noticed
as a voltage drop.

	Trusting your voltmeter to make a critical decision such as this
is not, as my father would say, "erring on the side of prudence." There
are two or three good ways to verify the ground circuit performance, but
the general public should consult either an appropriate PE or licensed
electrician.

	I've torn out enough armoured cable in radio stations to know
that the ground path is not reliable.  It is almost always high enough
in resistance to completely screw up "single point bonding" in high
RF fields (where the studios  and transmitter are co-located).

	If you are in any doubt as to ground performance, consult an
electrician.  You are safest if you either install dedicated ground
wiring, or replace the branch circuit in question in its entirety.
The latter is what we are doing in our home after my wife's encounter
with an alleged "armoured cable" ground path and a very much live
refrigerator frame.

York David Anthony
WKTD Wilmington, NC

P.S.  I didn't know that the cable in some parts of our house was armoured
and some was Romex (it had two additions made since original construction).
If you are buying an older home, two or three hours of an electrician's
time to assess the condition of the electrical plant is money very well
spent. Rewiring costs $$$$$$$$$$$$$$.