Had a problem at a school a couple of years ago. We had an aprentice go to all the recetacles with the plug-in tester. All rec's showed wiring OK. Most of it was in block work with a 4" sq. box above drop ceiling for termination. There was 120 for power and 277 for lighting. One of the lighting whips was mistakenly tied into the receptacle j.b.. Tester read circuit OK. This tester doesn't tell you voltage. Later on a carpenter plugged in his drill and without even touching the trigger it jumped, spun, smoked and flew out of his hand. I don't know if anything like this ever happened to anyone, but it was a bad mistake, specially in a school.
Thank your lucky stars that they didn't plug one of their new PCs into it. Luciano, who had been a helper for more than 20 yrs, did the same thing with some 277 travellers. Fortunately, there was no V between them. In Spanish: Luz=light ya =still no = no Total equation = Still no lights...S
As for the plug-in testers, the only people I see use them are home inspectors by us. Unfortunately, the readings they get with them cost some poor homeowner a service call by me. The results they get just don't seem to be correct very often.
If an electrician can't figure out how to use real testers, they should do some other kind of work. And it doesn't look to professional walking around with one of those either.
I will use one to try and cypher what a home inspector was getting as far as eroneous readings.
If a receptacle had a ground wire from a box tied to the ground screw, and also to the neutral screw, would not the readings be correct on the tester? And no neutral in the box to begin with.
Then comes the voltage issue, real testers will show it is more than 120V.
Now, you folks bring up the issue of lower and higher voltage in incorrect locations. I always make sure that the two shall never meet. 208/120 in it's own runs, 480/277 in it's own runs. Indicating the higher voltage on j-boxes in some locations as a safety precaution for future reference is sometimes nice too.
I think where the problem is as first mentioned, it was a whip that tied into a rec. j.b.. There was a lighting j.b close and the two whips were crossed. All the conduits were separated, the J.B.s were all marked for circuits and voltages. But all the M/C looks the same. Is there M/C available where the outer jacket is colored to signify voltage? This has only happened once that I've seen or heard about.