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By James Piper, P.E.
Power & Communication Article Use Policy
Each type of UPS — passive standby, line interactive, and double conversion — offers its own features and level of protection. Managers must consider the needs of the application carefully when evaluating product options because not all UPS are suitable for all applications.
Passive-standby systems. These systems provide the lowest level of protection and are the least expensive of the three. These off-line systems monitor incoming power and switch to a battery source when an interruption occurs. This transfer takes place in milliseconds and is acceptable for some computer-based applications. But the loss of power during the transfer can disrupt the operation of some types of sensitive electronic equipment. These UPS also do not filter power-line noise or voltage spikes or sags. Because of these limitations, their use is limited largely to desktops or similar systems not performing critical tasks.
Line-interactive systems. These systems insert a transformer or an inductor between the power source and the connected equipment, and a bank of batteries helps condition and filter incoming power. The systems offer more protection than their passive-standby counterparts but do not completely isolate the protected equipment from irregularities in the incoming power. The systems offer adequate protection for many facility applications but not enough protection for mission-critical operations, such as data centers.
Double-conversion systems. These systems are true online UPS. They eliminate the momentary loss of power found in the other two types of UPS in the transfer from incoming power to battery-supplied power by using a bank of batteries connected to the direct-current part of the system.
These UPS fully isolate protected equipment from the power source, thereby eliminating most power disturbances. They provide run times that range from 15 minutes to as long as the generator's fuel lasts, depending on the battery bank's size and whether the system includes a standby generator.
Managers specifying a UPS to protect facilities and systems need to be certain it is sized properly for the load it is designated to protect. They at least must be sure to size the UPS so it can provide 150-200 percent of the connected load. This spare capacity protects the UPS from additional power loads while the equipment is starting, and it allows room for growth.
Managers also need to properly size the batteries in the UPS to provide the desired runtime in the event of a power loss. For some applications, the UPS only needs to provide power long enough to allow an orderly shutdown of connected equipment.
But in other applications, the batteries will need enough capacity to provide power for the duration of common power interruptions. The required battery capacity will depend on the nature of the functions performed by the protected load.
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