A transfer switch allows safe switching between your mains supply (primary source) and your standby Generator supply (secondary source) whilst ensuring both sources cannot be connected simultaneously.
It does so by continually monitoring the incoming mains supply. If the voltage of the mains supply fails, or is significantly reduced to create a brownout condition, it will immediately disconnect the mains supply and command the Generator to start.
Once the generator is producing a stable supply it will transfer the load to the Generator.
When a stable mains supply is restored it will carry out the reverse process, transferring the load back to the mains and shutting down the generator.