Path: utzoo!utgpu!watmath!att!ucbvax!hplabs!hpfcdc!hpfcdj!myers
From: myers@hpfcdj.HP.COM (Bob Myers)
Newsgroups: sci.electronics
Subject: Re: Re: Touching a "hot" connector
Message-ID: <17660006@hpfcdj.HP.COM>
Date: 4 Aug 89 18:09:11 GMT
References: <248@sopwith.UUCP>
Organization: Hewlett Packard -- Fort Collins, CO
Lines: 46


>	It doesn't matter wether it's 120 or 220, They're both
>	lethal.  It's not the volts that kill, it's the amps.


Y'know, that last is said often enough that people are starting to repeat
it as Holy Writ without actually understanding it.  Yes, it is the amount
of current passing through the body (particularly the heart itself) which
determines whether or not you'll just get a tingle, or wind up as a customer
of the local coroner.  However, the Gospel According to Saint Ohm says that
voltage and current are related, and so we should all keep in mind that a
high voltage source is more likely to kill than a low voltage source.
120 and 220 ARE both *potentially* (no pun intended) lethal, but you're more 
likely to get into trouble with 220 - as it can force a deadly current through 
a higher-impedance load, meaning that you don't have to be in *quite* as good 
contact as you would need to check out for good on 120.  I = E/R, so any time 
you set up a situation where you can get 100 mA through the heart, well, 
please smile and leave a happy-looking corpse.

The rules to follow - keep one hand in your pocket at all times, and make SURE
the damn thing is unplugged before you work on it!  Related anecdote: In my
more naive younger days, I was working on a Gazillion-amp +5V supply with
a friend.  The filter caps were the size of Foster's beer cans, and so I wanted
to take the precaution of making sure that they were discharged.  I asked
my able associate if he'd unplugged the supply, and, getting an affirmative
response, proceeded to whack the caps with a Craftsman screwdriver, size
Enormous.  ZZZZZZZOTTTTTTT!!!!  "Well, gee, that one was pretty well charged
up!  Better whack it again!"  ZZZZOTTTTTTT!!!!  About this time, I asked
my partner in crime if he was SURE he'd unplugged the supply.  By way of
response, he simply held up the end of the cord so that I could see for myself
that yes, the damn thing WAS unplugged, stop bothering me.  Well, I continued
to whack away at the capacitors, drawing some really nifty-looking arcs and
pitting the living hell out of the screwdriver shaft in the process.  (Yes,
you too can learn arc-welding in the comfort of your own lab!)  Finally, I
decided to grab the voltmeter we were using to see for myself if any
charge was remaining.  The needle stayed at zero.  Actually, BELOW zero.
Right on the peg, in fact.  My gaze ran along the back of the bench, while
continuing to disintegrate my screwdriver.

Rob had actually unplugged the voltmeter.



Bob Myers            | "One man's "magic" is another man's engineering.  
myers%hpfcla@hplabs. |     "Supernatural" is a null word." 
hp.com               |                   - Lazarus Long/Robert A. Heinlein