Compaq AlphaServer DS20E Reference Manual page 33

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A power backplane integrates the supplies for power distribution, monitoring,
and control. The power supplies can be accessed and removed through the front
of the enclosure. See Chapter 5 for instructions on adding or replacing a power
supply.
The following voltages are provided: +3.3, +5.0, +12.0, –12.0, +5.0 Aux (+5.0 Aux
always powered). Two internal fans cool the power supply. The fans are
temperature controlled and speed up as the power supply temperature
increases.
N+1 Power Supply Configuration
Two power supplies must be installed and working for the system to operate.
The system shuts down if the number of working power supplies ever falls
below two. In a three-power-supply configuration, a power supply may be
removed for servicing without interrupting system operation. An I/O interrupt
is generated whenever the number of power supplies in operation changes.
System Overview 1-17