In a line interactive uninterruptible power supply the inverter section is typically energised but not connected to the load. The load is connected to the inverter when the mains power supply fails via a relay switch. There can be a small break in output supply of 2-4milliseconds but this is well within the energy storage capacitance of the switch mode power supply (SMPS) in modern electronic devices like servers, computer, bridges, routers and storage devices.