To support IT equipment as it was designed to operate, internal power supply units (PSUs) must have incoming power that meets certain basic requirements. Designed to perform under local electric utility conditions around the world, modern PSUs are more robust than ever. They operate normally over a wide range of input voltages and frequencies, and have internal energy stores to ride through brief power interruptions.

In spite of their robust design, PSUs need protection from power quality problems generated by the electric utility or arising within the facility. To operate properly, IT equipment needs a consistent source of conditioned power. Providing that consistent, conditioned power is the job of an uninterruptible power system (UPS).
“When selecting a UPS, it helps to look at the issue from the point of view of the ‘end-user’— that is, the PSU inside the IT equipment,” explains Gary Jameson, country manager at Eaton. “What does the power supply do, and what does it need to do its job?”
A piece of IT equipment may have two, three or even six power supplies. High redundancy is common in the latest generation of blade servers and other high-density servers. With PSU redundancy, the device keeps running even if one or more power supplies fail. If those power supplies are fed by different power distribution paths, the device keeps running even if an input power source fails.
“To serve global markets, most equipment manufacturers use universal PSUs that can support the various input voltages and frequencies found in different countries. Just swap out the power cord and voila, you have a piece of equipment ready for sale and use in any country,” says Jameson. “That means the PSU in your IT equipment is likely to support the low, 100V AC utility voltage used in Japan as well as the high, 240V AC utility voltage used on most other continents, including Europe, Asia and the Americas. PSU manufacturers also ensure their designs can accommodate the voltage supplied by industry-standard power supply cords.”
The UPS must therefore be able to supply voltage within the specified range required by the PSU, for all voltage variations found in the AC power sources (utility mains or generator). “For example, for higher watt rated power supplies requiring an input voltage of 200V–240V, the UPS must deliver power within the 180V–264V range,” says Jameson.
In sizing a UPS, the power rating (kW) is actually more important than the kVA rating (apparent power) due to the high power factor of the IT equipment. “When assessing output power and battery backup time, make sure to use the real power (kW) rating of the UPS,” says Jameson. “If the kW is not apparent in the UPS specifications, it can be calculated by multiplying the UPS kVA rating times the output power factor rating of the UPS.”
The job of a UPS is to provide a consistent source of conditioned power, backed up by battery or generator sources—and sufficient power to address power factor issues and inrush current. That means that the UPS must ensure no interruption in its output that lasts longer than the hold-time of the IT equipment’s PSU. In addition, the UPS must be designed to ensure the voltage to the PSU is not in the prohibited range, since voltage in that range could damage the IT equipment.  Finally, the UPS must be designed to handle high-speed impulses, such as lightning strikes or surge currents of longer duration, even though most PSUs are designed to handle some level of surge current without damage.