PlantFlow 3D › Guides › Automatic transfer switches (ATS) for standby generators
Automatic transfer switches (ATS) for standby generators
How an ATS moves an essential load from the mains to a standby generator and back, and why the two sources must never be connected together.
What an ATS is for
Hospitals, fire pumps, data rooms and many factories have loads that must keep running when the mains fails. An automatic transfer switch watches the mains. When it fails, the ATS starts a standby generator and moves the load to it. When the mains returns, it moves the load back.
The sequence
A typical open-transition sequence is: confirm the mains failure for a short time (to ignore brief dips), send the start signal to the generator, wait until the generator voltage is healthy, open the mains contactor, wait a short dead time, then close the generator contactor. On mains return it waits a return delay to be sure the mains is stable, transfers back the same way, and lets the generator run unloaded for a cool-down time before stopping it.
Never parallel the sources
The mains and the generator are separate sources with different phase angles. Connecting them together would cause a huge current. The mains and generator contactors are therefore interlocked electrically (each coil circuit passes through a normally-closed auxiliary contact of the other) and often mechanically as well. Closed-transition transfer, where both are briefly connected, needs synchronising equipment and the approval of the supply authority.
Testing
An ATS should be tested regularly by simulating a mains failure, checking the start, transfer, return and cool-down times, and confirming that the essential load really runs on the generator.
Try it yourself
Choose “ATS” in the simulator, press “simulate failure” and follow each step of the transfer in the status bar. Then try the troubleshooting challenges.
Open the PlantFlow 3D simulator →Safety: this guide explains principles for learning. Real installations must follow the applicable codes, the equipment manufacturer's instructions and the approval of a competent, qualified person. Isolate, lock off and prove dead before touching any circuit.