ATS (Automatic Transfer Switch)

What is ATS?

Transfer switches are critical components of any emergency or standby power system. When the normal (preferred) source of power is lost, a transfer switch quickly and safely shifts the load circuit from the normal source of power to the emergency (alternate) source of power.

The ATS permits critical loads to continue running with minimal or no outage. After the normal source of power has been restored, the retransfer process returns the load circuit to the normal power source.

Transfer switches are available with different operational modes including:

  • Manual
  • Automatic

How do an ATS works?

ATS is mostly a relay logic control unit, but sometimes available as the programmable logic control unit. The typical control diagram of an ATS is as below

The main items that are used in ATS are conductors with electrical & mechanical interlocks. Two coupled contactors with mechanical interlocks doesn’t energize at the same time. If one contactor is energized then automatically other contactor will be de-energized. That means at any moment path is provided for only one source, not the both of them.

The task of the timer T1 is, to wait a given time to observe whether there is any failure again in the main supply ( To avoid continuous switching in case of a back to back failures when generator runs) T2 timer is used to provide a delay to energize CEB side. And T3 timer is used to provide a delay to energize generator contactor (This is in case of a little time failure. To avoid the starting of a generator for a little time failure).