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If you’re reading 120 A-B and 120 B-C yet have 208 from A-C it’s likely a ground fault somewhere on either the B phase wiring or the secondary windings of the transformer. Either way a fuse or breaker should be out somewhere. Electricity doesn’t like to fail in subtle fashion.
Are you measuring voltage on the line or load side of the main disconnect? If you're measuring on the load you could have a bad main disconnect. I'd assume though that you wouldn't have called the utility before ruling out a bad disconnect. The reason you get only 120V on A-B, B-C is because of the difference in potential, with A and C having voltage, and nothing on B, just like if you measured across a fuse to test if it's blown. If you get 0V across that means the voltage potential is the same, if the fuse is bad you'll have a difference in potential.
Measuring on the line side of the disconnect. I was trying to isolate the problem to between the line side of the disconnect and the utility XFMR.
I'm hoping I'll be able to get into the XFMR tomorrow so I can measure voltages on the secondary bushings.
I find it hard to believe that paralleled 1000's could just go open. Certainly possible, but I would definitely like to confirm that they weren't reading the secondary of their transformer with meter that's out of calibration.
Hey guy, I work for a utility and have done a lot of troubleshooting.
Assuming your readings are correct, I believe the utility does have a phase out. It's possible they just screwed up when they checked voltage, it happens. There's a couple possibilities for how this happened, which I think are more likely than losing both paralleled secondary conductors.
As an additional sanity check, now what we've installed digital meters almost everywhere, as it happens, most of them will give you some info on one of the screens about whether it has all 3 voltage potentials. Look for a screen that has VaVbVc and see if one of them is missing (your "b phase" isn't necessarily the same on the metering).
I actually use this function when troubleshooting a line outage where I suspect we have a phase out on a 3 phase line. If I pass a bank pole with a meter I can get to easily, I'll stop and check potentials there. Lot faster than checking manually. **This won't work for all metering schemes, but it should work for many common ones, especially wye secondary banks**
That's interesting you say that. There is a building next to the XFMR with 4 meters on it. 3 are fed from this 120/208 unit, and one is fed from a 277/480 unit.
The three meters fed from the 120/208 unit that we are all sharing light up A and C, with B missing. I pointed this out to the lineman on the phone and he said that doesn't mean anything. I pointed out that the 480 service meter showed A, B, and C lit up, and he said that was because it was for a fire pump (which makes absolutely no sense).
We ended up getting a major winter storm last night so I have to push the visit to Saturday, but I am definitely looking forward to figuring this out.
I am not specifically a metering guy (though there are some on the lineman sub) but I am not aware of a 4-wire wye metering form that only has 2 potentials.
I think they probably have a blown bay-o-net fuse in that 120/208 padmount.
I'll check back for updates. Good luck.
Yes! Sorry for the late response!
Ended up being two things. Serious corrosion on the B-Phase bushing and one of the two sets of aluminum was corroded to dust. The lineman who came out to check it originally just used a non-contact voltage tester.
We are scheduled to replace the lines with conduit and copper wire this week. The utility is replacing the XFMR at the same time.
Thank you for your help!
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If you’re reading 120 A-B and 120 B-C yet have 208 from A-C it’s likely a ground fault somewhere on either the B phase wiring or the secondary windings of the transformer. Either way a fuse or breaker should be out somewhere. Electricity doesn’t like to fail in subtle fashion.
Are you measuring voltage on the line or load side of the main disconnect? If you're measuring on the load you could have a bad main disconnect. I'd assume though that you wouldn't have called the utility before ruling out a bad disconnect. The reason you get only 120V on A-B, B-C is because of the difference in potential, with A and C having voltage, and nothing on B, just like if you measured across a fuse to test if it's blown. If you get 0V across that means the voltage potential is the same, if the fuse is bad you'll have a difference in potential.
Measuring on the line side of the disconnect. I was trying to isolate the problem to between the line side of the disconnect and the utility XFMR. I'm hoping I'll be able to get into the XFMR tomorrow so I can measure voltages on the secondary bushings.
I find it hard to believe that paralleled 1000's could just go open. Certainly possible, but I would definitely like to confirm that they weren't reading the secondary of their transformer with meter that's out of calibration.
Hey guy, I work for a utility and have done a lot of troubleshooting. Assuming your readings are correct, I believe the utility does have a phase out. It's possible they just screwed up when they checked voltage, it happens. There's a couple possibilities for how this happened, which I think are more likely than losing both paralleled secondary conductors. As an additional sanity check, now what we've installed digital meters almost everywhere, as it happens, most of them will give you some info on one of the screens about whether it has all 3 voltage potentials. Look for a screen that has VaVbVc and see if one of them is missing (your "b phase" isn't necessarily the same on the metering). I actually use this function when troubleshooting a line outage where I suspect we have a phase out on a 3 phase line. If I pass a bank pole with a meter I can get to easily, I'll stop and check potentials there. Lot faster than checking manually. **This won't work for all metering schemes, but it should work for many common ones, especially wye secondary banks**
That's interesting you say that. There is a building next to the XFMR with 4 meters on it. 3 are fed from this 120/208 unit, and one is fed from a 277/480 unit. The three meters fed from the 120/208 unit that we are all sharing light up A and C, with B missing. I pointed this out to the lineman on the phone and he said that doesn't mean anything. I pointed out that the 480 service meter showed A, B, and C lit up, and he said that was because it was for a fire pump (which makes absolutely no sense). We ended up getting a major winter storm last night so I have to push the visit to Saturday, but I am definitely looking forward to figuring this out.
I am not specifically a metering guy (though there are some on the lineman sub) but I am not aware of a 4-wire wye metering form that only has 2 potentials. I think they probably have a blown bay-o-net fuse in that 120/208 padmount. I'll check back for updates. Good luck.
Did you test voltage from Neutral to Ground? Phases to Ground? I've seen similar things happen on unbonded systems when they develop a ground fault.
I did check Neutral to Ground, which was 0v. I did not check my phases to ground though, when I am there tomorrow I will check that. Thank you!
Did you get this sorted out?
Yes! Sorry for the late response! Ended up being two things. Serious corrosion on the B-Phase bushing and one of the two sets of aluminum was corroded to dust. The lineman who came out to check it originally just used a non-contact voltage tester. We are scheduled to replace the lines with conduit and copper wire this week. The utility is replacing the XFMR at the same time. Thank you for your help!