This describes the amount of time it takes from the initial loss of AC power to the time the UPS takes over the load. The importance of time for the computer translates into the computer locking up or freezing and thereby losing any unsaved information. Instantaneous transfer time means that the computer would not even notice that its power source had switched from one input to another. A characteristic of a UPS in which the output is completely electrically disconnected from the input.
Power is coupled from input to output by magnetic fields in a transformer.
Because there is no connection from input to output, transients cannot cross the magnetics thereby eliminating any transients in the AC output. The neutral to ground bond provides protection from transients on the neutral line which could feedback into sensitive electronic equipment. Magnetic fields are formed when unwanted noise voltages give rise to noise currents. Such noise may adversely affect electronic equipment and cause intermittent data problems. EMI protection is provided by noise filters placed on the AC power line.