The Various Modes of High Voltage Propagation

Common Mode :  Common-mode overvoltages appear between live conductors and earth: phase-to-earth or neutral-to-earth (see Fig. J7). They are dangerous especially for appliances whose frame is connected to earth due to risks of dielectric breakdown.

                                                          Fig. J7: Common Mode

Differential Mode :Differential-mode overvoltages appear between live conductors:

phase-to-phase or phase-to-neutral (see Fig. J8). They are especially dangerous for electronic equipment, sensitive hardware such as computer systems, etc.

                                                                  Fig. J8: Differential Mode