Automatic transfer switches continually monitor utility power. Any anomalies such as voltage sags, brownouts, spikes, or surges will cause the internal circuitry to command a generator to start and will then transfer to the generator when additional switch circuitry determines the generator has the proper voltage and frequency. When utility power returns or no anomalies have occurred for a set time, the transfer switch will then transfer back to utility power and command the generator to turn off, after another specified amount of "cool down" time with no load on the generator.
Open transition breaks from the emergency source and then connects to the normal source power (break before make). Closed transition connects in parallel to both emergency and normal source before disconnecting the emergency source (make before break), this ensures no power interruption. Delayed transition waits for a specified period after breaking from an emergency source before connecting with the normal source.
A standard transfer switch has no bypass associated with it, these are typically used in non-life safety applications where in case maintenance is required the power can be disconnected. One-way bypasses are typically used in applications where interruptions are unacceptable. The one-way bypass allows you to bypass the transfer switch for maintenace purposes without having any down time. Two-way bypass transfer switches are used in life safety applications where down time is not an option ex) hospitals. A two-way bypass allows the transfer switch to be bypassed on both the normal and emergency side without affecting interruption to any power source.
The loads and use of the transfer switch will determine what options are necessary for proper operation. Options such as elevator signalling, fire alarm signaling, motor disconnect, etc. should always be considered for safety.