The typical sequence of operation for an Automatic Transfer Switch (ATS) is as follows:
- The ATS permanently measures the electrical parameters such as voltage and frequency, of both the primary and the back-up power supplies.
- If the connected primary power source is not available like utility power the ATS will switch the load circuit to the alternate power source in this case the generator if it is available.
- The ATS expects a time (tS) to elapse before it begins the transfer cycle.
- The ATS gives the signal to the circuit breaker that is connected to the prime source to break the connection.
- The ATS remains idle for another set delay (tCE or tF).
- The ATS instructs the circuit breaker connected to the alternate source to close thereby taking the load on to the alternate source.
- When the primary source is restored, the ATS will delay a fixed time (tBS) before starting the retransfer sequence.
- The ATS instructs the circuit breaker for the source connected to the alternate source to open.
- The ATS waits for another set delay (tCN or tF) to pass and similar to the call release delay, it ranges between 50- 200 ms.
- The ATS turns on the contact of the circuit breaker associated with the primary source to open; thereby switching back the load to the primary source.
Delays in seconds and the quantity of steps may change depending on the ATS model and its usage settings. Depending on the particular application, the ATS can be set for in-phase transfer, for time-delay neutral transfer, or for in-phase and time-delay neutral transfer sequences.