Working on a piece of equipment lately I had a voltage reading which stumps me. It’s working now so this is just a curiosity but I haven’t been able to puzzle it out.
An operator was cleaning around the line and accidentally dropped something through a hole in a cabinet. I repaired the hole and replaced a whole pile of fuses. Now I was left with a reading at the disconnect of 345-ground on two phases (it’s a wye-connected 600V supply) and the strange-to-me reading of 790-ground on the other phase! Measuring from that phase to any other phase gave me over 1000V which is the maximum my meter will read safely. There is a 1:1 isolation transformer in the cabinet and no other transformers so I can rule that out. As it was I went back to the main panel upstream from this one and found a blown fuse there. After replacing the fuse all readings were 595/345 V as usual.
Any theories? I’ve certainly never had a blown fuse give me a higher voltage reading in any way shape or form prior to this.
Check your neutral connection for continuity, in addition, check to see if you have a switched neutral on the local isolator.
When the neutral goes awol you get some unusual voltages between phases and to earth - if something fell into the isolator it could be holding connections apart - seen it before.
You’re right. Bad neutrals make for strange readings.
I had to go fetch the schematic after reading your reply since there is no neutral fed in with the 3-Φ but there is control power fed separately which is grounded and the isolation transformer is also grounded on x2. I had dismissed the transformer but hadn’t given any thought to that neutral. Maybe the neutral there from the control side isn’t well-grounded or something. I may wait for some downtime and look further at that.
Also keep in mind that modern digital DMMs can give… strange readings when troubleshooting power stuff. This is due to the very high input impedance of the meter, and thus its ability to pick up on “residual” voltages. When troubleshooting AC power stuff, I stick a 10 KΩ resistor in parallel with the meter leads. This forces voltages w/ high source impedance to be 0 V.
That’s a good idea, thanks! I’ve found more than a few loose terminations by using a light bulb with a couple of leads soldered to it. I get made fun of all the time for that but it’s saved me a lot of troubleshooting time.
It’s primarily an issue when there’s a solid state switch or controller somewhere in the system, such as a SCR, triac, FET, or SS relay (which includes light dimmers). Can also be an issue with neon bulbs and photodetectors. None of those things are truly “off” when they’re turned off. So when the device or system is “off,” a modern DMM will *still *detect a voltage, but that’s because there’s almost zero current. The voltage will “go away” when you decrease the meter impedance a little.
For safety’s sake measure each leg of the 3 phase supply to a known good ground. If you still have a voltage higher to ground than a normal leg, say greater than 500 V, you could have a potential ground fault. This would happen when some component of a 3 phase circuit shorts to ground. Since no other phase is connected, ground is placed at the voltage of this one phase. What makes this a potentially hazardous condition is say the short to ground is through a metal case (as with a motor thermal) and someone by accident touches the case and another leg, they will be exposed to full (600) voltage. Might be a good idea to call your utility company and ask to speak to an electrical engineer. If one is not available ask for a safety officer.
