Power Utilization; Three-Phase Versus Single-Phase Power - Compaq 226824-001 - ProLiant - ML750 Basics Manual

For example, the Emerald Book addresses the common modern-day grounding issues and difficult
installation scenarios for grounding sensitive electronic equipment. The IEEE standards books, the
color books, listed in Table 1 can be purchased from IEEE Standards Association. For more
information, see the following URL: http://standards.ieee.org.
CAUTION
It is vital when planning, wiring, installing, and maintaining electronic
equipment to follow all appropriate national (NEMA and IEC) and local
standards as they apply. If a piece of equipment becomes separated from
ground, the resulting power buildup in the chassis (called leakage current)
can cause an electric shock. Standard wiring techniques and a permanent
ground connection will prevent such a hazard.

Power utilization

In North America, commercial power is usually delivered as three-phase 480VAC or 480/277VAC.
In most of the rest of the world, it is delivered as three-phase 200/346 to 230/415VAC. It is
delivered as 575VAC in Canada and as 690VAC in parts of Europe and offshore facilities.
Transformers are added in the electrical system to transform the voltage to 208/120VAC (three
phase) or 240/120VAC (single phase). What is normally referred to as high-line power in United
States industry is actually 208V bi-phase, where load is connected across two phases. In the
Americas and other parts of the world that follow North American commercial wiring practices,
organizations have the choice between low-line power (100—120VAC) and high-line power (200—
240VAC) for their servers. This is an important choice, since high-line service is the most stable,
efficient, and flexible power for server and data operations. High-voltage, three-phase power offers
greater efficiencies than single-phase power.
Like other countries that have converted from 220V or 240V to the (roughly) 230V international
standard, Australians and the British still refer to "two forty volt" service as a synonym for mains
because it lies within the range of tolerance (plus or minus 10 percent). Standards in the Americas
and in Canada specify residential power as 120VAC but allow a range of 114V to 126VAC. Japan
delivers household power at 100V; although various provinces vary the frequency from 50 Hz to 60
Hz in wavelength (therefore appliances sold in Japan generally can switch between the two
frequencies).
Most computer equipment operates on single-phase power. Single-phase loads, such as computer
equipment, are connected to one of the transformer's phase windings and the neutral connection. In
the United States, high-line connections are made across two of the transformer windings with no
neutral. Equipment requiring more power, including data center environment support systems, runs on
three-phase power. Three-phase loads, such as air-conditioning equipment, are connected to all three
transformer windings.
Larger computer systems are moving to higher amperage or three-phase power. Many enterprise-class
machines presently use three-phase power, and almost all data centers are currently wired for three-
phase power.
For a table of voltage and frequency use by country, see Appendix B, "Voltages and frequencies of
individual countries."

Three-phase versus single-phase power

Three-phase power distribution is typically more efficient than single-phase power distribution because
higher power can be delivered using smaller cables and fewer distribution panel connections. Table 2
17