The double auxiliary power supply module automatically selects the most suitable source between two single-phase AC voltage power lines. The selection criterion is based on the presence of voltage within predetermined minimum and maximum limits. Its typical application is to provide auxiliary power to the switches and the control devices within an emergency transfer panel. It can then be used in combination with automatic network switches. The switching between the two lines is carried out with time and thresholds defined and repeatable, ensuring safety to the system. The two lines are separated with electrical safety interlocks. a self-diagnosis under the supervision of a microcontroller is also executed. In case of absence of both the supply lines and / or operating fault self-diagnosis, an additional alarm relay signals to the external devices alarm state.