240 AC Stick Some Of These Near Your Electrical Box

Disconnect that phase to neutral paths at the neutral and watch the no-current blue flash?

Up -and- down amplitudes of the sine wave are now filled.

But that isn't the point of meters. Meters are to verify and to alert.

And with twin pathways of 120vac the only way to do it...

Phase to neutral voltage and Phase to neutral current

On each phase.

With 240vac phase to phase no neutral the second set of meters is not necessary.

Diagnosing a center tap transformer is similar