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From: koko@uthub.UUCP (M. Kokodyniak)
Newsgroups: net.rumor
Subject: Re: Computer Horror Stories
Message-ID: <260@uthub.UUCP>
Date: Fri, 21-Mar-86 14:52:05 EST
Article-I.D.: uthub.260
Posted: Fri Mar 21 14:52:05 1986
Date-Received: Fri, 21-Mar-86 17:28:03 EST
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Organization: CSRI, University of Toronto
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>                                  The modem and the computer tried to share
> a common ground, but in reality there was a whopping potential difference
> between them, and when they were hooked up, sure enough, the computer
> generated sparks and bad smells - something computers are not generally
> supposed to do.

	This reminds me of a nasty accident I had in the Power Electronics
Laboratory.  I had a terminal connected to a 6809-based microcomputer board.
The board was in turn connected through an interface, driver circuit
and isolation transformer to an SCR power module.  The module was connected
directly to the 117-volt line, which was protected by a 50-amp breaker.
	In the course of debugging the circuit, I had connected an
oscilloscope -- isolated, of course -- to the circuit.  I connected
one channel, with its ground wire, to some point in the power circuit.
I had other channels of the scope connected to the microcomputer interface.
I understood that the microcomputer ground now became hot, but this
was okay since the microcomputer power supply and terminal were both
isolated -- or so I thought.  Then I turned on the 50-amp breaker switch
to energize the power circuit.  BANG!!!  A large current, enough to pop
the 15-amp breaker supplying the computer and terminal, went from
the power circuit, through one set of scope leads, through the scope,
through another set of scope leads, through the computer ground trace,
through the ground wire in the RS-232 cable and into the terminal.
The goddamn terminal had its RS-232 signal ground strapped to the
earth ground in the 117-volt line.  The current blew a trace on the
computer board.  When it finished off that path, it proceeded to find
the path of next lowest resistance -- the line driver and receiver
chips in the computer board and the terminal.  All four chips, plus
some TTL chips in the terminal, were burned out.  But one of those chips
had a hole blown right through it!  I could see remains of the substrate
through the hole.  Fortunately, the 15-amp breaker tripped before
anything else was damaged.  But the 15-amp breaker was slightly damaged --
it tends to stick a little upon turning on.  (I left my mark in the lab.)
	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.

		Michael Kokodyniak, University of Toronto