Cummins Onan

Understanding Transfer Switches


Transfer switches are an essential part of a standby power system. The utility (normal power source) is backed up by a generator (emergency power source). Should the utility fail, the transfer switch automatically switches the electrical load from the utility to the generator. When utility power returns, the transfer switch automatically switches the electrical load back to the utility.

Automatic transfer switches, capable of automatic operation without operator intervention, perform the basic function of transferring the load to the available power source. A controller monitors each source for allowable voltage and frequency range.

The transfer switch performs the following functions:

  1. Senses the interruption of utility power
  2. Sends a start signal to the generators
  3. Senses generator power is available
  4. Transfers the load to the generator
  5. Senses the return of utility power
  6. Retransfers the load to the utility
  7. Sends a stop signal to the generators

Transfer switch RSS residential (PDF)
Service entrance transfer switch RSS residential (PDF)