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I have a standard US home power system - single-phase, 3-wire, 200 A service. As I increase the load (oven, motors, etc.) on the system, the local house grounding system, including the system grounding rods, take on an increasing voltage proportional to the total current draw. Anything grounded in my house has this increased voltage relative to nearby "reference earth ground" which I have established via a ground rod temporarily in my yard away from the normal grounding rod.

The center-tap/neutral line in my panel sees this same voltage. The voltage of both phases and the neutral all change equally. As the neutral increases relative to "earth ground" the 2 line voltages shift equally, e.g. if total current draw from the transformer increases such that neutral N=30 V (relative to "reference earth ground") then V1=150 V and V2=90 V. Any load still sees the correct 120 V or 240 V relative to both N and house ground.

My transformer sits on the ground next to my house. The transformer case sees this same increase in voltage. The dirt near the transformer and my system's grounding rod sees this voltage. I've added a new grounding rod as part of my local grounding system, to no effect.

I've checked and can't find any neutral lines connected to ground lines in any of the circuits. Any single circuit (with all other breakers turned off) can create this condition.

I have thought thru many potential error/fault scenarios and none make sense based on what I'm seeing. For example, if the neutral wire back to the center-tap of the transformer has built up a resistance, that would create a voltage bias on my local N in the panel proportional to the current flowing back to the transformer on that N line. But I see this problem when only using 240 V loads which minimizes current flow on the N returning to the transformer.

Ideas anyone? Thank you!

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    If everything shifts with load, including the transformer itself, that suggests to me that there's a problem with the grounding on the primary side of the transformer. You should call the utility and have them investigate.
    – Dave Tweed
    Commented Jun 4, 2023 at 14:45
  • What made you decide to do this testing? Did you get a mild shock? Have you recently done any digging or drilling in the ground around the pad-mounted transformer? The utility company will come right away if there's a suspected problem in their underground line to the transformer, or the transformer itself.
    – Mark Leavitt
    Commented Jun 4, 2023 at 16:20
  • Where's your neutral-to-earth bond located and is it OK? Also why are you doing these measurements, is there a problem you are solving?
    – Justme
    Commented Jun 4, 2023 at 16:24
  • Thank you all for your prompt response.
    – Luke
    Commented Jun 4, 2023 at 16:58
  • I noticed the problem when working on my AL siding one day.
    – Luke
    Commented Jun 4, 2023 at 17:17

3 Answers 3

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Problem solved. Power company verified it's their service line (which is underground) into my transformer that has a high impedance return wire. Potential difference between the two "ground" points builds up as a function of return current (V=IR). Temporary fix via a copper wire (laid on the ground) tying my transformer to the local 7200V junction box. Will be reinstalling a new service line soon...

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TL;DR DOUBLE-CHECK YOUR PANEL NEUTRAL/GROUND BOUND. THEN CHECK WITH YOUR NEIGHBORS, IF YOU CAN, TO SEE IF THEY ARE HAVING PROBLEMS. THEN (OR BY DEFAULT IF YOU CAN'T CONTACT THE NEIGHBORS) CALL THE UTILITY.

OK. Not 100% certain here. I though LOST NEUTRAL based on 150/90. But you also said that the two hots stay ~ 120V each relative to neutral and 240V relative to each other.

That being said, ground should be the same as neutral.

I see two different possibilities:

Your Grounding System

If your ground and neutral are not bonded correctly, or if your ground rod is not properly connected to your panel then you can get readings like this.

  • Check neutral to ground in your main panel. Should be ~ 0V. If it is not, check for a missing neutral/ground bond screw/wire. It is impossible to have a difference between neutral and ground if they are properly connected together.
  • If your main panel is not a regular main panel but a small meter main or disconnect, where your "functional" main panel (i.e., with dozens of breakers) is truly a subpanel, then if you have a good 0V neutral/ground reading in the true main panel (this is in the panel, not sticking a wire in the soil) then next check the neutral/ground reading in your subpanel. It should read ~ 0V there as well - if not then you have a problem with the ground or neutral (but likely ground, because if it was neutral then you wouldn't have 120V circuits) between main panel and subpanel.
  • Check for continuity between your ground wire (should be clearly visible in the main panel and you should check continuity between that wire (easy if it is a big bare conductor) and the ground bar, neutral bar and panel box. If there is a problem with the connection in the box, fix that.
  • Follow the ground wire all the way out to the ground rod(s). It should follow a route that is not subject to damage, but that is not always the case, and a break in the wire can definitely cause problems.

If all of that is good, then the problem would appear to be outside your system.

Outside Voltage

There may be a problem with someone else's electrical system that is allowing for significant leakage into the physical ground. That will cause problems and can be extremely dangerous. If one of your neighbors has a LOST NEUTRAL but has a functional ground rod then a large amount of their electrical usage (any 120V loads) will try to return to the utility through the physical ground. Since the physical ground is a bad conductor, that will likely not match the nominal neutral transformer center tap, with exactly the results you have described.

If that's the case, one of your neighbors likely has symptoms of:

  • Unequal hot/neutral readings - e.g., 150V hot to neutral on one leg and 90V on the other leg, with the numbers constantly changing.
  • Flickering, dimming, surging, etc. on lights.
  • Intermittent problems with many 120V appliances, possibly including serious damage.

A lost neutral is almost always a utility problem. It can be a panel problem, but from your perspective (as an affected neighbor) it is always a utility problem.

It is also possible that a physically down wire nearby could do this, which would definitely be a utility problem.

Call the utility. Tell them you are reading voltage in the physical ground and think there must be a down wire somewhere. AND THAT YOU ARE WORRIED ABOUT YOUR SAFETY. And you really should be worried about your safety. I'd recommend getting far away until this is resolved.

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    Problem solved. Power company verified it's their service line (which is underground) into my transformer that has a high impedance return wire. Potential difference between the two "ground" posts builds up as a function of return current (V=IR). Temporary fix via a copper wire (laid on the ground) tying my transformer to the local 7200V junction box. Will be reinstalling a new service line soon... Thanks for all the feedback!
    – Luke
    Commented Jun 8, 2023 at 20:37
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If I understand correctly, basically the only symptom that's occurring is that when you measure the voltage between your house's ground rod and the reference ground rod, you see a substantial voltage. Everything else you've described sounds normal considering that your house is connected to its ground rod in the usual way. In particular, I don't think you have a lost neutral, since your loads are still seeing the correct 120 V.

I can only think of two things that would cause this.

The first possible cause is that there's a current through the ground which is producing the voltage because of the ground's resistance. Such a current must necessarily enter the ground at one point and exit the ground at another point. Your house's ground rod could be one of the two points, but I have no idea what the other one could be. (There isn't a single-wire earth return system nearby, is there?) If you have a safe way to measure the current going through the connection to your house's ground rod, that would provide some indication of whether or not this is the case.

The other possible cause is that the two ground rods are actually at the same potential, but your measurement setup is picking up a voltage through inductance. When you measure the voltage between two points using a voltmeter, you're not just measuring the difference in potential between those two points; you're measuring that difference in potential plus the amount of changing magnetic field that's passing through the loop formed by your measurement wires and the thing you're measuring. The bigger that loop is, the more voltage you see.

Basically, if your house has any big loops that current is going through, and your measurement system is also a big loop, then those two loops might be acting as a transformer. If that's the case, I would expect you to see a smaller voltage if your measurement wires are entirely outside, lying directly on the ground, and going in a straight line between the two rods, and a larger voltage if your measurement wires take a long path through your house and high above the ground.

All this is based on my theoretical knowledge of electricity and magnetism; I admit that I have no practical experience with this sort of issue. I may be missing something that would be totally obvious to an actual electrician.

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