Auxiliary Power Supply Modules supply the auxiliary power supply for the SINAMICS S120 Cabinet Modules. Units connected to this auxiliary power supply system include the fans of the SINAMICS S120 devices installed in the Cabinet Modules. In addition, the auxiliary power supply system supplies the electronic modules with an external voltage of 24 V DC. This is required when the DC link is not charged, for instance, in order to maintain PROFIBUS/PROFINET communication.