An uninterruptible power supply (UPS) is a system for providing uninterrupted flow of electric power to some essential load which cannot tolerate even a momentary loss of power. Other loads which, though very important, can ride through brief outages of power, can use other systems for restoring power such as transfer to a secondary utility source or to a standby engine-driven generator set. Some motors, for instance, should not have power restored after an interruption until the residual generated voltage has dropped to a low value. These do not require a UPS. On the other hand, some elements of a control system or a computer may be disrupted completely by a brief loss of power and, consequently, must be protected by an uninterruptible source of power. The consequences of such disruption would be intolerable.
The simplest form of UPS is the floating of a battery across the supply lines for a device operating on direct current. Most of the devices of interest here, however, operate from AC systems and require some more elaborate type of UPS. These are the UPS systems to be discussed briefly here. (A more thorough treatment can be found in the IEEE Orange Book: Recommended Practice for Emergency and Standby Power Systems: STD 446-1974 or later.)
A first step in selecting a UPS system is to identify those loads which are truly essential and which are also intolerant of ANY interruption of power. All other loads, even some highly-desirable ones, should be dismissed from consideration for UPS service because (1) including them would require a larger size for the UPS with consequent increased cost, and because (2) every nonessential device on the system is a potential source of disturbances affecting the purity of the UPS output.
