How does A UPS provide A and B power supplies to network servers?

A and B power supply resilience can be provided by adding an automatic transfer switch (ATS) to the critical power circuit. An ATS is sometimes referred to as an STS and operates by automatically selecting between two power sources whether these are the outputs from two UPS systems or two mains power supplies or a combination of these two. When one power source fails, the other is automatically brought into circuit to power the server loads. Manual selection is also possible. An ATS is a relay-based device. An STS uses static electronics transfer switch technology.