Operational considerations are important key factor in UPS selection. Efficiency and reliability often depend on the type of unit, the load, and the environment. The higher a unit's efficiency, the lower the operating cost. The type of load being protected is an important consideration, too. Microprocessor-based equipment may require a different type of UPS unit than other loads. For example, switching power supplies, found in most computers, react differently to certain types of UPS systems. Today's UPS units should be capable of supplying non-linear type loads without derating or degradation in total harmonic distortion (THD). Load size determines what size UPS is needed. Systems usually are sized in kilovolt-ampere ratings. Before selecting UPS capacity, a profile of critical electrical loads should be computed, listing the power requirements of each piece of equipment to be protected. 

Most UPS manufacturers also recommend allowing for a 25 to 30 percent future expansion factor. Systems generally are available in both single-phase and three-phase models to suit the application. Consideration should be placed on the application of the UPS. Computer-room UPS components are more densely packaged to create a smaller footprint. Many times, isolation transformers are removed from the front end of a computer type UPS rectifier/battery charger. Although this usually is acceptable in most office/computer-room environments, it can be a problem in an industrial facility due to the presence of large equipment that may cause disturbances during starting. Noise emitted by an UPS can be a factor, depending on the surrounding environment. In most cases, UPSs placed in computer centers, control rooms, or office-type environments need to operate as quietly as possible. Noise levels vary according to the type of system. UPS manufacturers' specifications generally include noise-level data.

Separately, questions must be asked about the UPS manufacturer's service capabilities; depending on how critical your computers are for day-to-day and hour-to-hour business, depot repair services and express delivery of replacement UPSs may not be enough. A manufacturer who has 7-day-a-week, around the clock dispatch capabilities, and offers on-site support by factory-trained service specialists, can prove invaluable, particularly in emergency situations. A mature service organization also speaks to a UPS manufacturer's understanding of the need for systems availability and commitment to deliver highest levels of reliability.

- US Army Corp of Engineers
- National Power Corporation
- Tri-State Generation and Transmission Association, Inc.
The cartoon is from Ron Leishman   


2001 U P S c i . c o m