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From: okunewck@gondor.UUCP (Philip E. OKunewick)
Newsgroups: net.rumor
Subject: Re: Safety - Third wire in A.C.
Message-ID: <2051@gondor.UUCP>
Date: Tue, 25-Mar-86 06:56:44 EST
Article-I.D.: gondor.2051
Posted: Tue Mar 25 06:56:44 1986
Date-Received: Wed, 26-Mar-86 07:27:45 EST
References: <14700001@hplabsb.UUCP> <476@mmm.UUCP> <637@mmm.UUCP> <260@uthub.UUCP>
Reply-To: okunewck@gondor.UUCP (Philip E. OKunewick)
Distribution: net
Organization: Pennsylvania State Univ.
Lines: 52

In article <260@uthub.UUCP> koko@uthub.UUCP (M. Kokodyniak) writes:

>       [Snap... crackle... pop...]

>       All of this goes to prove that that third wire in the line
>cord does not always promote safety.  In this case, it created a hazard.
>From now on, I will always use a ground cheater for terminals
>when working in that lab.
>

        WRONG!!!!

        NEVER USE A CHEATER WHEN TROUBLESHOOTING!!!!

        The story showed that it DOES promote safety.  The idea
behind the third wire is that it guarantees a potential of 0 volts
between the object you are troubleshooting and the rest of the world.

        If some higher potential touches the ground, it is SUPPOSED
to make things pop (hopefully the breaker, maybe something else.)  In
other words, the third ground prevents YOU from being electricuted.
It doesn't really matter that a bunch of parts went pop pop pop -
you're still alive.  Had you left the ground wire on the scope
connected to a high potential, touching the scope may have been
lethal.  This, of course, depends on how high the potential is and
what else you touch in the shop.

        As far as safety when things are popping - I've popped many
components myself, and they rarely are dangerous when they're smoking
and sparking.  It looks scary, but in reality it's not very
dangerous.  In the seven years I've been servicing computers, I've
never seen a fire started by burning components, nor have I ever seen
something blow up in a dangerous manner.  (I've heard about cap's
going, but you need to put a cap across AC Power to make it pop.)
I've been zapped a number of times though, and checked myself for a
heartbeat afterwards more than once.

        The scope you are using doesn't have two different isolated
probe grounds - most scopes don't.  This is inconvenient - it makes
it a little harder to check relative voltages.  If you're going to do
something about it though, isolate the probe ground in the scope.
As any service tech should know, jury-rigging and mickey-mousing don't
cut it!

                                        ---Phil OKunewck
                                           Senior Service Technician
                                           Miniserv, Inc.

P.S.  There are a lot of inexperienced hardware hacks that follow the
        net.  Don't go posting advice to violate electrical codes.
        They're written with human safety in mind, and I personally
        would not want to be responsible for somebody getting hurt.