Automatic transfer switch: A transfer switch that can transfer the electrical load from one power source to another without manual intercession.
Bypass switch: A transfer switch for an uninterruptible power supply (UPS) that is operated automatically to transfer DC battery power to the DC-to-AC converter to sustain the AC load. The switch can be operated manually for maintenance or repair of the conversion circuitry of the UPS.
Emergency power system: A separate source of electric power that can pick up all or part of an electrical load automatically following an outage of normal power. It can take over loads so rapidly that critical lights and equipment will continue to function, assuring the safety of personnel and preventing property damage or loss.
Manual transfer switch: A switch that must be operated manually to transfer the electrical load from one power source to another.
Normal AC power (also called commercial power): Power supplied by a public utility. In some situations, normal power from an alternate commercial source can be switched to provide emergency or standby power as an option to the use of an engine–generator set.
Standby power system: A source of DC battery power that is switched on automatically following an outage of the normal AC line power. It maintains loads such as lamps and other equipment that function with DC power until normal AC power is restored or until the batteries are discharged to the level that they can no longer power the load.
Uninterruptible power supply (UPS): A power supply that normally supplies the load with conditioned AC power from the AC power line when it is present. If an outage of normal AC line power occurs, rechargeable storage batteries supply DC to the inverter to sustain AC power to the load.
This continues until normal power is restored or the batteries are discharged to the level that they can no longer power the load. A transfer switch can bypass the internal UPS circuitry to allow the load to be powered from the normal AC line for maintenance or repair of the UPS.