Most of us know that we need a UPS (Uninterruptible Power Supply) to keep our critical servers, and other electronic equipment, going during a power outage and shut them down “cleanly” if the mains supply is down for an extended period of time but why should we pay the (sometimes considerable) extra to get an On Line model rather than a cheaper line-interactive one?

When should you use an On Line UPS?

Always.

When MUST you use an On Line UPS?
• Sites with generator power or generator back-up
• Equipment with Active PFC power supplies
• Servers

Why?
• Generates true Sine Wave Output
• Eliminates all mains power distortions
• Active PFC devices will generally not run on Stepping Wave power output from low cost Line interactive UPS.
• Tolerant of poor quality input power

Test
• Check your power back-up solution in a controlled test. Don’t wait for a power outage to test it for you!
• Check batteries regularly and replace as necessary.

On Line UPS Features

Remove Noise
A true on line UPS is also known as a double conversion UPS, since it converts mains AC power to DC in the UPS, then converts the DC power back to AC at the output. This means that noise and distortion is removed from the mains input. This is particularly important where the site is located close to heavy equipment or devices such as air conditioners, pumps and electric motors which create significant disruption to mains power.

Generator Support
A second crucial feature of On Line UPS is its ability the to run off generator power. This can of course be crucial where a site’s back-up power is provided by generator. Power from generators is generally noisy and doesn’t conform to a sine waveform meaning most sensitive IT equipment cannot run directly from a generator. A true On Line UPS regenerates the input power from the generator to give a perfect sine wave output making it suitable for downstream equipment. It’s worth noting that Line Interactive UPS not only doesn’t regenerate the power, but the UPS itself can’t run on a generator, meaning the UPS will simply run flat and drop the load when mains power goes off even if a generator is running.

Sine Wave Output for Devices with Active PFC
Thirdly, and possibly most important, the power supplies in much modern equipment incorporate a feature known as Active Power Factor Correction, or Active PFC. Such power supplies will simply not recognise the stepping wave power supplied by a cheap Line Interactive UPS, requiring a Sine Wave as provided by On Line models. As a result equipment with Active PFC when attached to a Line Interactive UPS will simply black out when mains power is lost, rendering the UPS useless. Due to the proliferation of Active PFC power supplies in desktop PC’s, POS systems, printers, monitors and many external power packs this issue has become enormously important. It is worth noting that servers almost exclusively use Active PFC power supplies so an On Line UPS is essential for these. Servers almost exclusively use Active PFC power supplies so an On Line UPS is essential.

When buying a UPS it is easy to be tempted by the low cost of the Line Interactive models and the thought that “I only need a basic solution – this will do”. Unfortunately in many circumstances a Line Interactive UPS simply will not do the basic job of keeping equipment running in the event of a mains power outage. The combination of a stepping wave output which is not recognised by many IT devices, combined with an inability to run on poor quality power input (such as from a generator) means that a Line Interactive UPS is of limited application. Unfortunately many people don’t realise this and many sites currently have a low cost Line Interactive UPS installed, incorrectly thinking that their essential equipment will continue to function if the mains power fails.

OK, you ask, so when can I use a low cost Line Interactive product? Ideally all equipment should be protected by an On Line UPS, but the low cost Line Interactive models can be used for older equipment and devices that don’t have Active PFC power supplies. Note that with a Line Interactive UPS your device is running off mains power and only switches to UPS power, with its resultant Stepping Sine Wave Output, when the mains power fails. With the prevalence of Active PFC in newer devices we do suggest testing the load on your UPS by switching off the mains power in a controlled test rather than waiting for a power black-out. Line Interactive UPS has only very basic power filtering and can’t be used with Active PFC devices.

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