I almost had a serious electrical accident last week when I was trying to use an oscilloscope to test out a circuit a student had built.
I had the scope plugged into a floor outlet, and the circuit board powered through a USB cable plugged into a PC in the lab. I was having trouble getting the scope to display a signal with just the probe tip exploring points on the circuit board, so I tried connecting the probe ground alligator clip to the circuit board ground (even though I thought that should be unnecessary), and got a large arc and melted wire and charred parts of the circuit board and some sensors.
After some head scratching and a few more sparks I finally verified with a voltmeter that the metal shield of the USB port on the PC was connected to 120VAC relative to the third prong on the outlet that I had the scope plugged into.
So I am convinced that an electrician made some sort of wiring error when connecting the power to the table in the lab. Instead of being plugged into a wall outlet, the power to the lab table comes from a conduit that goes into the wall opening that serves two lab tables' power needs. A sticker on the conduit shows a wiring diagram indicating that the two connections are supposed to share the neutral wire, but one is connected to "hot1" - a red wire, and the other to "hot2" - a black wire. It also shows a green shared ground wire.
I apologize for the long setup to my actual question, but I am trying to understand what the wiring error could be to cause this situation. I am certain that the USB shield was at 120V relative to ground. Shouldn't the shield be connected to the computer case and therefore be grounded? How could it be at 120V potential then? If the electrician accidentally reversed hot and neutral, shouldn't plugging in the computer have blown a circuit breaker instead of making the shield be at 120V?
I am glad I did not get electrocuted. Now that the scare has worn off, my curiosity about what actual wiring mistake was made is rising. I hope my explanation is clear enough for someone to identify what is wrong. (By the way, I reported all of this to many people at school and an electrician will be brought in to correct things right away.)
Added: I hope I made it clear that I did indeed refer the actual work of correcting this hazard to an electrician the school will bring in. But I am very interested in understanding the error as well, so that I may simply increase my knowledge. I hope someone out there can help with that.
EDIT: WRONG--SEE BELOW "Today I got to school after the electrician had already corrected whatever the error was." I wish I had had the chance to ask what was the matter. I hope I can get more info from the people at school in the next few days or so.
I did use an ohmmeter to convince myself that the USB shield of the computer is indeed tied to the third prong of its power plug, so I am convinced that the entire metal chassis of the computer was hot last week.
I heard second hand that our maintenance people had used some sort of outlet tester on the circuit in the past and that it actually passed the test(!) How is that possible with such a serious fault as the ground being missing and what should have been grounded connected instead to 120VAC? Is there some sort of strange faulty connection scheme that a simple outlet tester would miss?
Since it seems that the job of rewiring the circuit didn't take very long (I got to school at 11 am and everything was fixed) I assume it wasn't something that took a lot of hunting to find. But I am still mystified. I'll add any additional info as I learn more...
IMPORTANT UPDATE: Yesterday I proved myself to be an idiot. What I thought indicated repair work was just evidence of our head maintenance man doing some more checking and turning the power off. I was simply measuring no more 120V on the computer case because the power was turned off. Bonehead.
Today the electrician is upstairs working the problem and finding some very strange stuff. Right now it's looking like the computers are correctly wired to the power, but the floor outlet has some issues between ground and neutral. He showed me how he gets a nice clean tone from his tester across ground and neutral on the computer supplies, but a sort of warbly tone between ground and neutral on the floor and wall outlets.
Also, our maintenance man was finding weird voltages like 36V and 23V on some of the computer power outlets during his experimentation yesterday. Thus the electrician visit today. All simple theories for this seem to be out the window. So it is getting more interesting by the minute ...
THE LATEST -- The instructor in the closest office told me that the guys were cheering success late this afternoon, but I wasn't there to get the answer. Dang. Maybe tomorrow. Whatever the issue was it had the electrician making phone calls for help and our school personnel doing a lot of head scratching all day.
MORE: Today I hear that they tracked down the culprit as the "modular whip in the furniture." Our head maintenance man says (and I saw) that the outlets to the PCs are held in trays in the lab tables, and each strip is sort of daisy chained together. I gather somewhere in the chain there were both open ground and open neutrals. He says he has lost all confidence in the power connection materials that came in the lab furniture.
I still don't understand how an outlet tester would not have detected these faults. Maybe it was just that the tester was only used originally when the strips were indeed healthy and then the fault occurred some time later. Any ideas on what would lead to these sorts of faults developing?
I found something here that fits a lot of the symptoms we discovered. He calls it "reverse polarity bootleg ground." Do you think this could be it? But how could this be a failure in an outlet strip? Wouldn't it still have to be in the building wiring?
ADDED: I just had a very nice phone conversation with Mike Sokol, the author of the above article. He says my accident sounds exactly like his RPBG scenario, and he also made me understand that the "warble" from the electrician's tone continuity tester is likely evidence of a good ground and neutral connection, while the "clean" tone is probably a sign that the ground and neutral lines are bootstrapped straight to each other, a code violation. I think the "warbly" sound is showing there is a voltage drop between ground and neutral, as there ought to be with a load connected. The electrician thought the exact opposite. Hmmm...
MORE RECENTLY: The people at school seem satisfied that the problem has been corrected. However, I'm told that the fix was simply that they unplugged something and plugged it back in(!). I checked things with a non-contact voltage tester and the computer case is now grounded. But I don't understand how simply reconnecting something could fix that. Maybe I did not understand the complete story, but I am still nervous that this situation could reoccur. I'm not satisfied quite yet ...