The 3 Types of UPS Systems: Standby vs. Line Interactive vs. Online UPSs

Detectable power outages account for only a small percentage of power disturbances. Utility power is also plagued with surges, sags, electrical noise, harmonics, load fluctuations, and other interference. Even under normal utility operations — with no storm or lightning in sight — enterprise IT systems can be bombarded daily by conditions that damage critical components.

That’s why it’s important to select an uninterruptible power supply (UPS) that offers more than just standby power. Here’s a look at the three types of UPS systems and how well they protect against power disturbances.

What Is a Standby (Offline) UPS?

When IT equipment is plugged into an offline UPS, it receives power directly from the input source rather than the UPS’s inverter output. The inverter, which converts the battery’s DC power to AC, is “offline.” The battery charger is connected to the input source so that it can keep the battery fully charged and ready to go when needed. When the input power fails, the UPS automatically connects the IT equipment load to the inverter’s output. This can take up to 10 milliseconds.

Furthermore, the offline UPS only has basic surge suppression and electromagnetic line noise filtering capabilities. IT equipment connected to an offline UPS is subjected to many of the power disturbances of the electric grid.

What Is a Line-Interactive UPS?

A line-interactive UPS has a smarter design in which the DC-to-AC power inverter is “inline” with the UPS’s output. When the input power source is functioning normally, the inverter is in AC-to-DC mode to charge the battery. If the input power source fails, the UPS’s transfer switch opens, and the inverter reverses so that the battery can power the IT equipment. The load transfer time for a line-interactive UPS is less than 4 milliseconds.

Because the inverter remains online, line-interactive UPSs minimize switching transients. They also provide under- and over-voltage protection as well as surge suppression and electromagnetic line noise filtering.

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What Is a Double-Conversion (Online) UPS?

As the name implies, a double-conversion UPS converts power twice: A rectifier converts AC power from the primary source to DC power, which is stabilized and fed through an inverter that converts it back to AC power. If the main power source fails, the UPS automatically eliminates the rectifier to switch to battery power. Because the inverter is already engaged, transfer to the battery is virtually instantaneous.

The double-conversion process also minimizes the common power disturbances of the main supply, including harmonics and waveform distortions. As a result, online UPSs provide maximum protection for highly sensitive IT equipment.

Line-Interactive vs. Online UPSs

Generally, standby UPSs are inadequate for mission-critical data center applications due to potential disruptions during battery switchover and limited protection against power disturbances. Line-interactive UPSs and online UPSs are better choices, offering a comparable set of safeguards:

  • Surge/Noise Protection. Both line-interactive and online UPSs provide surge suppression and electromagnetic line noise filtering.
  • Voltage Regulation. Line-interactive UPSs detect when voltage deviates from preset thresholds and uses automatic voltage regulation to correct it. The double-conversion process of an online UPS regulates voltage somewhat more precisely.
  • Pure Sine Wave Output. The power supplies of some IT equipment require pure sine wave output, which is the optimal waveform of electricity. All online and many line-interactive UPSs offer this capability.

When choosing between line-interactive and online uninterruptible power supply, it often comes down to price. Line-interactive UPSs cost up to 40 percent less than online UPS systems. However, if you can’t afford even one millisecond of downtime, an online UPS may be right for you. Check out our UPS buying guide for more information on how to select the perfect UPS for your environment.

Enconnex Uninterruptible Power Supplies

Enconnex offers a complete line of rack mount UPSs. Available in power capacities ranging from 800 VA to 10 kVA, in online and line-interactive battery topologies, and lithium (LiFePO4) and lead-acid battery compositions. Our UPSs provide clean and reliable backup power for critical IT equipment and feature best-in-class features such as load shedding, auto-restart, and pure sine wave outputs. Customers can bundle Enconnex UPSs with our line of EdgeRack micro data center cabinets for a complete, prefabricated solution. Contact our team to learn more about how we can optimize your environment.

Posted by Robert Faulkner on October 3, 2022

Robert Faulkner is the Vice President of Engineering and Operations at Enconnex. He comes from a strong background in product management with over 20 years in the IT industry. He currently holds an MSME and CDCD certification. He earned his MS degree in Mechanical Engineering at University of Nevada, Reno.

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