UFC 3 -520-01
June 10, 2002
13-4.4.5 Very small UPS systems (less than 2 kVA) are sold in an extremely
competitive market. Accordingly, the manufa cturer often has to design the UPS to be
price-competitive rather than highly reliable for a long life. UPS systems are installed
because the user determined that uninterrupted power was a design requirement. Very
small UPS systems must be included in the facility maintenance program so that they
are periodically checked for proper performance and the battery replaced.
Design Criteria .
13-4.5.1 UPS systems are widely available from many companies. Although the
electronics design and the equipment layout vary widely, the UPS performance
requirements are usually very similar for each application. The following summarizes
the key UPS design criteria for any application.
13-4.5.2 The UPS system must provide uninterrupted power. Upon loss or improper
performance of the normal power source, the UPS system has to provide continuous,
uninterrupted power to its loads. Some UPS designs, such as an off-line UPS and
possibly a line interactive UPS, include a momentary break in power as the UPS
transfers between the normal source and the battery. This type of design might not be
a true UPS and should be avoided unless the downstream system performance is
confirmed to still be acceptable with a momentary break in power.
13-4.5.3 The UPS system must provi de power from its backup source for the required
duration. Generally, the stationary battery has to be properly sized and adequately
maintained to meet this requirement.
13-4.5.4 The UPS output frequency has to remain stable regardless of changes to the
input or to the loads. Each UPS system will have a particular operating frequency--50,
60, 400, or higher Hertz. A common specification is for the UPS to maintain the output
frequency within 0.5 percent.
13-4.5.5 The UPS output waveform should be sinusoidal, with a voltage total harmonic
distortion less than 10 percent, with no single harmonic frequency greater than 5
percent of the fundamental frequency. Depending on the UPS design, the output
waveform might vary from almost purely sinusoidal to a squa re wave. This particular
attribute requires evaluation during the UPS selection process. Also, the ability of the
UPS to supply nonlinear loads must be evaluated. Determine with the UPS supplier if
power conditioning or an output load filter is required for the expected harmonic
13-4.5.6 The UPS input voltage range might require adjustment for UPS systems
applied in locations where commercial power is subject to larger than normal variations,
such as outside the continental United States . In such locations, the UPS adjustable
range should be a design consideration.