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UPS Designs - the differences

Off-line UPS

This type of UPS may also be referred to as a back-up system or a standby UPS. Under normal circumstances, the mains supply is fed through a filter and output to the load via a mechanical switch. A battery charger maintains the state of charge of the battery, and there is a detection circuit, which constantly monitors the state of the mains.

When the mains goes outside tolerance, for example during a power failure, the inverter is switched on and draws power from the battery. It takes a short while for the unit to detect the mains failure, turn on the inverter and change the power path via the mechanical switch. This results in a short break in the supply to the load, and this interval is called the transfer time. This is likely to be less than 4 milliseconds, and is imperceptible to the majority of computers or similar equipment.

Line Interactive UPS

These UPS offer enhanced power protection over the basic Off-line designs because they provide additional line conditioning. They can also cope with a wider range of input voltages without resorting to battery.

On-line UPS

This technology is generally regarded as superior to off-line and line interactive designs, and one which is essential where mains power might be subject to fluctuation or interference or for use with a diesel generator.

In double conversion technology, all incoming mains power is converted into DC power by the rectifier /charger. The DC is converted back into clean AC by the inverter and fed to the load, which is fully supported under normal conditions.

When a mains failure occurs, the inverter draws DC power from the battery instead of the rectifier. No form of detection or switching is required, so a double conversion UPS provides a no-break transfer to battery power on mains failure. The voltage regulation and conditioning which is offered is superior to that of either Off-line or Line Interactive UPS.

Some recent on-line designs also carry out a continuous mains monitoring process and can automatically switch to a pass-through economy mode where incoming power has remained of stable high quality for a given period of time.

Parallel Redundant UPS

Often called N+1 redundancy, this technology uses a series of UPS operating in parallel. Typically, there will be one more module in the configuration than is necessary to support the load, enabling a faulty module (battery or UPS) to be hot-swapped out without interrupting the supply to the load.

Originally only available on very large UPS this technique has recently become available at the lower power ratings and the price differential is now very acceptable. The individual modules are almost always of on-line design.

This approach is essential for mission critical applications, or in demanding environments such as industrial process control where downtime for any reason is a costly and unwelcome interruption.

In an IT environment this design is complementary to the RAID array and hot-standby fileservers and like those technologies is directed to removing single point-of-failure weaknesses.