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Our electric power provider delivers 240 V, 60 Hz split-phase power to residential buildings. I've been checking our three-prong receptacles and I normally get the following measurements:

line-to-line: 240 V

line-to-ground: 120 V

However, there are two receptacles which have a different line-to-ground reading: 60 V instead of the expected 120 V. For both, line-to-line is also 240 V; only the line-to-ground voltage is different.

What possible connection/grounding error could have caused this?

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    \$\begingroup\$ Most likely a missing ground connection. \$\endgroup\$ – skvery Mar 1 '17 at 15:04
  • \$\begingroup\$ Missing ground connection as in the receptacle's ground prong is not connected to ground at all, or that the ground prong is connected to a length of wire, but that wire run is not properly connected to the grounding bus bar at the main panel? Could this be caused by capacitive/inductive coupling? \$\endgroup\$ – Darwin Bautista Mar 1 '17 at 15:15
  • \$\begingroup\$ Follow up: when I measured the line-to-ground voltages of the receptacles which are known to have a disconnected ground, I get a reading similar to the one I get when one probe is floating (not connected to anything). I don't get a stable 60 V. \$\endgroup\$ – Darwin Bautista Mar 1 '17 at 15:17
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    \$\begingroup\$ The power phase is split! So the two lines are 180 degrees apart in phase. So they are 240 to each other and only 120 volts to ground. Wikipedia has an excellent article on split phase power at: en.wikipedia.org/wiki/Split-phase_electric_power \$\endgroup\$ – Peter Camilleri Mar 1 '17 at 15:30
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    \$\begingroup\$ @PeterCamilleri, yes I actually mentioned that in my question. But instead of measuring 120 V line-to-ground, I'm getting 60 V. \$\endgroup\$ – Darwin Bautista Mar 1 '17 at 16:43

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