AC voltage

When an AC source is stated as being a particular voltage (eg 240V) that is usually an RMS value ( 1 / sqrt(2) * peak).

So peak voltage of the '240V' AC source is 339V, and the voltage at any given time varies between 339V and -339V.

And don't forget that all voltages are a statement of potential difference between 2 points, in this case the two terminals of the source.

So if we are giving the voltage in terms of the difference between the brown 'Live' wire and the blue 'Neutral' one, at positive peak brown is at +339V with respect to blue. (brown 339V, blue 0V)

1/4 cycle later brown is at 0V with respect to blue. (brown 0V, blue 0V)

And after another 1/4 cycle brown is at -339V with respect to blue. (brown -339V, blue 0V)

The absolute potential difference between the two terminals varies between 0 and 339 volts and its root mean square value is 240V, which is where we started.

Thanks to Grob: Basic Electronics for finally explaining this one in a way I could grasp.

(Incidentally although the difference between the two terminals says nothing about the difference between either and any other point, in a domestic wiring situation you would expect a 240V RMS reading between either terminal and the earth wire).