What is an Offline UPS, how does it work, and what are its advantages and disadvantages?

What is an Offline UPS, how does it work, and what are its advantages and disadvantages?

The Offline UPS, or, Standby UPS as it is popularly known is the most common type of UPS used primarily for Personal Computers.


In normal use, the UPS connects the power line directly through to your computer and its battery charger draws only a slight current to keep the UPS battery topped off. When the power fails, the UPS switches into action – and switch is the key word. A relay inside the Offline UPS switches the wires that go to your computer from the wall outlet (passing through the UPS) to an inverter connected to the battery pack inside, or, outside the UPS. The inverter then supplies power to your computer with the batteries as the energy source. The inverter only starts when the power fails, hence the name “standby”.

The topology of the Offline UPS is shown below:

                              Offline UPS topology

The switching process requires a small but measurable amount of time. All available off-line UPSs switch quickly enough that your computer never notices the lapse. Most off-line UPSs switch in 3 to 10 milliseconds (one cycle in a 50Hz environment is 20 milliseconds).  Even the slowest off-line UPS has a safety margin when it comes to switching time.

The main drawback of the off-line UPS is that it offers very limited protection against power problems. Only a few UPS manufacturers add filters and surge circuitry that provides adequate noise filtration and surge suppression to electrical disturbances from the grid power, but the offline topology in general cannot cope with sustained over-voltages or prolonged sags. The protection against under-voltage problems is limited to switching to battery back-up power – which means that the battery capacity limits the protection period.

The advantages of the offline UPS are its low cost and high efficiency (typically 95% – 98%). It is not a complete power protection solution but given its cost and efficiency, serves the purpose of protecting your computer against a single power problem – the outage.

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