Hi Dave,
And now for 3rd year EE circuit theory.
That's a lot of questions. Jon, let me take a bit of the load.
OK. Start with one wire coming from a 3? 60 Hz AC source. It doesn't matter what the source is (generator or transformer), nor does it matter what the source configuration is (delta or wye). But, to start out, imagine the source as the secondaries of the PoCo stepdown wye connected transformers. 4 wire, 120/208 Volt, grounded neutral. Ground is the zero voltage reference.
If, like a bird on an overhead power line, I touch just that one wire with my voltage OR timing measurement device, and nothing else, I can't tell that anything is happening with that wire.
If I had load, I could measure the magnetic field created by the load current. . .but for this discussion, no ammeters allowed, just voltmeters and clocks. Only voltage and time.
Now, if I use my voltmeter to measure between the one wire and ground, I can measure the 120 volt sinusoidal wave, and with the aid of my clock I can time it to measure the frequency as 60 sinusoidal cycles per second, or 60 Hz.
If I measure each wire, in turn, to ground, I can tell they are 120 volt 60 Hz. That's all. I can't tell, by timing of only one voltage measurement at a time, any thing about the timing in relationship to the other two voltages on the other two hot wires.
However, if I keep track of the time between each measurement, as well as the time I spend doing the measurement, I can establish a history time line, placing a zero for the beginning of time (the time reference), and I can measure, in time, the differences between the time of one voltage peak and the next voltage peak.
Look at that last paragraph, again. I just introduced a THIRD measurement point for time, the "beginning of time" reference, that is, the "REFERENCE". This "beginning of time" is comfortably thought of as "before anything else", but for AC circuit theory, it doesn't have to be. The beginning of time measurement in AC circuit theory only has to happen at A time, at a singular time that is the same instant in time for all measurements taken. Given that the PoCo is always running, and I start work at 8 AM today. . .I'll start my "reference" time at, say, 8:15.
I can now tell (using the zero time reference) that the voltage on one hot wire to ground hits a peak one third of a cycle different than the voltage of another hot wire measured to ground.
And, consider that last paragraph. I am now using TWO hot wires and GROUND as I'm considering the voltage. Three points.
Let's go back to measuring the voltage between just two points, the two hot wires. Ground is not included, so the only voltage I can measure is the difference between the two hot wires. Well, I know that it's 208 V, 'cause that's what the bill from the PoCo tells me. . .but that is actually a third reference. . .let's restrict ourselves to just two reference points, that is, the two points touched by the voltmeter probes, the two hot wires. 208 Volts.
When I start the clock, I can tell that the 208 Volts is also 60 sinusoidal cycles per second, or 60 Hz.
I have to reference back to a beginning of time (the time reference) that includes the line to ground voltage measurements before I can tell ANYTHING about the temporal relationship of the voltage line to line to the voltage line to ground.
When I do establish a common time reference for both the line to line and line to ground voltage, I find that the line to line voltage is NOT IN PHASE with the line to ground but, rather, since it is the DIFFERENCE between the two line to ground voltages on two different hot wires, is shifted 30 degrees.
OK. . .I gotta break for meat space concerns.
