Hope all had a great Thanxgiving! Weather was great! Went on service call to a retail store, customer claims she was "shocked" when plugging in surge strip to a metal handybox plug. Plug tested 120V HOT to NEUTRAL, 120V HOT to GROUND, 60V GROUND to NEUTRAL, 60V GROUND to METAL HANDY BOX. Junction box feeding plug (above ceiling grid) tested exactly same. Everything is EMT & MC w/ metal strip inside. All conduit & boxes are bonded as should be, as far as I could tell. I did not recieve any sort of shock from handy box in question, but turned off until I could return. Any Ideas?
Sounds like a main bonding or an open neutral issue. First ensure that the main bond is intact. If that checks out, if you can attach a solenoid type voltage meter to a point of bad voltage. Shut off the main to verify the problem is on sight. If the voltage is gone, the problem is on the premise. You can isolate the circuit by shutting off panel feeders then shutting off each circuit. Keep us posted
Thanks for all the tips. Will try all. Yes, it is a MWBC. My first thought was problem w/ main bonding. It seems as though I have ran across this before. Since they are open all day & weekends, I will have to go in after hours to start shutting things off & tracking the culprit. Another long day, but at least I have work! Stay tuned.
60v really sounds like what happens when you have an open ground and a PC or other equipment with RF filters on them. They have an LC network on the hot and neutral, tied to ground at the center point ... hence 60v or half of line voltage if the ground floats. Since the box is OK I agree with the IG receptacle idea. It might be as simple as an installer who thinks "Isolated Ground" means isolated FROM ground. Believe me, I have seen it. A wiggy will probably "ground" this so don't be surprised if the problem goes away when you use it to test this "ground to box". If you wiggy from hot to equipment ground you might see 120v on the ground referenced to the box.
If the neutral is being shared between 2 of 3 phases, and the neutral goes open, and if the loads on the two hots are roughly equal, you could see a reading of 60V on the bad neutral to ground. You can visualize this by drawing a Y, 120/208 system, and draw a line between 2 of the hots. The midpoint of that line will be about 60V from true ground (center of the Y). If the loads were resistive, and identical, a hot to the bad neutral would read 104V (half of 208V). A slight imbalance could give you 120V. Computer and similar power supplies are usually rated to accept 100VAC (so they can be used in Japan, a 100V country), and if enough are on the bad circuits, they may behave normally.
Found IT!! gretwell was right on. Open ground. Circuit goes thru 5 j-boxes between service panel & plug. Previous "electrician" used romex between j-box #3 & #4, and did not bond either end to a grounding conductor(EMT or wire). I only checked box #1 & #2, where grounding conductor was present & properly bonded, so assumed rest were fine. Explains why store clerk was shocked by metal handy box. Originally posted I was getting 120V HOT to GROUND, but was getting 60V HOT to GROUND, as well.(Sorry for that misinformation.) Grounded plug to grounding conductor & bonded handy box; now everything test & works fine. Thanks to ALL!