Three-phase four-wire AC power supply, divided into A, B, C three-phase and neutral. If two wires are drawn from the power supply, when they are all drawn from phase A, the two wires are in the same phase (the same is true for phase B and phase C); when one of them is drawn from phase A, and the other is drawn from phase C The voltage between the two lines is out of phase.
It is very convenient to use a test pen to determine whether the power supply is in phase or out of phase: the tester stands on the insulating board and holds a test pen in each hand. At the same time, it is out of phase, and if it is not bright, it is in phase.
