Modern power supply units (PSUs) are more robust than ever, and designed to perform under local electric utility conditions around the world. They operate normally over a wide range of input voltages and frequencies. They have internal energy stores to ride through brief power interruptions. Most ave input power factor correction circuitry and operate at a power factor close to unity, and they handle a broad range of transient power disturbances, as defined by industry standards.
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 that meets industry specifications. Providing that consistent, conditioned power is the job of an uninterruptible power system (UPS).