Mains phase indicator

The three voltages of a threephase supply, L1, L2 and L3 (or R, G and B) are 120° out of phase with one another. When, for instance, the positive half-wave of L1 (pin 1) begins, the instantaneous value of L2 (pin 2) is still negative. The positive half-wave of L2 starts 120° later and cuts the waveform of L1 at a level of about half the peak voltage at 150°. At 180° L1 becomes negative; at 270°, L2.