How do UPS systems work?

A UPS system can supply emergency power to equipment when the main system fails, which can prevent businesses from a loss of data or serious disruption. The way they work differs from generators or emergency power supplies as they can provide almost instant protection.

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Different UPS systems

As the number of power outages in the UK rises, with 640 reports in 2015 lasting for 50 minutes on average, it’s important to choose the right UPS for your requirements.

There is a range of different types of systems, such as the Eaton UPS models, and these are typically off-line, on-line and line-interactive.

Off-line UPS systems

These types of UPS provide a basic power backup, with both battery backup and surge protection. The system starts up when the voltage coming in falls below a set level and the connected equipment is automatically switched to the DC-AC inverter output. This switch can take up to 28 milliseconds, which will depend on how long the UPS takes to detect the voltage loss.

On-line UPS systems

The on-line UPS systems, such as some of the Eaton UPS available at, can be used in situations where it’s necessary to isolate the electricity and when equipment is sensitive to fluctuations in power. These can be more expensive than the other options and would typically have been employed for very large requirements, but technological advances now mean they can be used as a consumer device.

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The increased cost is due to the AC-DC battery charger/rectifier having a significantly larger current and it is capable of running continuously. These systems are permanently connected to the inverter, which means the power is kept unchanged, as there are no transfer switches.

Line-interactive UPS systems

This type of system can accommodate continuous power surges and brownouts without draining the reserve battery, as different power taps can be automatically selected on the autotransformer. This might cause a minor power disruption as the switch takes place.

This has become a popular option for UPS systems as it makes use of components that are already fitted, and with additional taps, they can be designed to manage a range of different input voltages, but this is obviously a more complex and expensive process. When used in low-voltage environments they will use more current than is generally needed, so a higher current circuit may be required.

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