This type of UPS has the ultimate protection. The output is sinewave and it will have an automatic bypass switch to protect the UPS in the event of a fault or overload condition. The output is monitored and if a short-circuit is detected the bypass routes the load to the main power until the fault is removed. Voltage regulation is achieved via the AC-DC-AC process. See Figure 1, above.
The Off-line UPS supplies AC power directly to the load by switching ON the transfer switch. In the event of a power failure, the offline UPS supplies the power from the battery backup. An important difference between On-line UPS and Off-line UPS is the requirement of a larger heat sink in an On-line UPS. Since the current drawn by the AC load is continuously flowing through its whole circuit, the temperature of the system increases. Therefore, it needs comparatively larger heat sinks and the components that can withstand high temperature and it can tolerate current flow for very long duration. Due to such a requirement, the cost of Online UPS increases significantly. See Figure 2, at right.
The Standby UPS is most commonly used for Personal Computers. The transfer switch is set to select the filtered AC input as the primary power source and also switches to the battery/inverter as the backup source in the event of a failure in the AC primary source. In the event of a power failure, the transfer switch will open to switch over to the battery/inverter backup power source. The inverter will only start when the power fails. The main benefits are high efficiency, small size, and low cost. See Figure 3, at right.
This type of UPS has intermediate protection. The output may be sinewave, step wave, or square wave and there is no automatic bypass. Voltage regulation is achieved via a built-in Automatic Voltage Regulator (AVR)/Automatic Voltage Stabilizer (AVS). See Figure 4, at right.