okunewck@gondor.UUCP (Philip E. OKunewick) (03/25/86)
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.
koko@uthub.UUCP (M. Kokodyniak) (03/26/86)
> 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!!!! You will be interested to know that the use of ground cheaters with electrical test equipment connected to hot circuits was encouraged in all of the courses that I took. (This was intended to prevent fried oscilloscopes and test leads, as well as people jumping and knocking over electrical equipment, potentially injuring themselves or others, when things went poof.) However, this was ALWAYS accompanied by a warning about isolating oneself during operation of such equipment. > > 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. Of course you are perfectly right. I suppose I was just careless with my words. The third wire certainly IS a lifesaver where electrical equipment or appliances are found in the same room as wet, damp or uninsulated floors. > > 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. The floors in all of the power labs are specially designed to provide electrical insulation. Thus it is safe to touch a live connection, although ONLY WHILE NOT TOUCHING SOMETHING ELSE AT THE SAME TIME. I always try to avoid touching live connections where possible. But if it necessary, then I always, as a force of habit, put my other hand in my pocket and stand away from all other equipment. > > 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. > An exploding component, however small, CAN be potentially dangerous. Little bits of flying epoxy or molten bits of metal could hit you in the eye. Furthermore, exploding electrolytic capacitors could spray your face with steam and corrosive electrolyte, causing both caustic and thermal burns to the eyes. (I've seen several of these babies go off -- from a distance.) Of course, the solution is simple -- wear safety glasses when there is a danger of small explosions. Alternatively, stand back -- way back! > 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. > I should never have connected the oscilloscope to two mutually 1solated circuits in the first place -- making that connection was the first sign of trouble. None of the oscilloscopes that I worked with had isolated probes. I imagine that underfunding precluded the purchase of such equipment. > 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. I sincerely hope that any "hardware hacks" would have enough sense to talk to someone who is qualified before fiddling around with potentially lethal circuits (like switching power supplies for computer equipment, or the like)! In conclusion I must state that I made a comment that I shouldn't have in my original posting. However, there also is a misunderstanding here: I was working in a LABORATORY environment, which is much more controlled and predictable than a SERVICE environment. The laboratory environment is the ONLY one where I have ever used cheaters. In all other environments (such as my basement workshop), I have used isolation transformers and/or fuses on both sides of the line. Michael Kokodyniak
doug@terak.UUCP (Doug Pardee) (03/27/86)
> 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. Seems that using a 'scope is something people pick up on their own. So not all of the tricks are obvious. The way to 'scope differentials on the more sophisticated 'scopes is to set channel B on "inverted" and the display mode on "add". Then use probe A on the "high" side and probe B on the "low" side. And connect the probe grounds to ground. This isn't a rumor -- why is it here???? :-) -- Doug Pardee -- CalComp -- {elrond,savax,seismo,decvax,ihnp4}!terak!doug