Electronic – How is line voltage measured

Voltage is like pressure in a hydraulic or pneumatic system. If you have a small diameter pipe (or a high resistance wire), you will need more pressure (voltage) to get the same flow (current). Also, with more pressure (voltage) each unit of steam (charge) does more work.

By unit charge we mean some fixed charge, usually it is 1 coulomb, but for relative calculations you can take any fixed charge (say, 1 electron or 4200 electrons).

A bad neutral connection can cause overvoltage in one side of the line (and undervoltage in the other) if the loads on each side of the line are unbalanced.

^{simulate this circuit – Schematic created using CircuitLab}

If R1 = R2 then no current flows in the neutral, and it does not matter if there is a good connection or not.

However, in the case shown, if the neutral is broken, then R1 sees 192V (VM1) and R2 sees 48V (VM2). The total voltage from red to black wire (VM4) is still 240V.

If the neutral is intermittent or has a high resistance connection, then the situation will be somewhere in between.

Since neutral is about at ground potential at the panel, if you see 20V at the far end (to earth), that indicates the neutral line is not making a good connection. Note that the 90V and 140V you measured add up 230V (close enough to 240), and that if you add 20V to the 90 and subtract it from the 140, you get about 120V each.