The DSE330 is an Automatic Transfer Switch Controller. This module will monitor the voltage and frequency of the incoming S1 AC supply and in the event of failure will issue a start command to S2.
Once S2 is available and producing an output within limits, the DSE330 will control the transfer device and switch the load from S1 to S2 automatically.

Once the S1 supply returns to within limits, the module will command a return to S1 and shut down S2.
Various timing sequences are available to prevent nuisance starting on minor supply breaks.