Sun Microsystems Enterprise 250 Owner's Manual page 61

The monitoring subsystem is also designed to detect fan failures. The system
includes three fans, part of a single assembly called the fan tray assembly. Any fan
failure causes the monitoring subsystem to generate an error message and light the
general fault LED on the status and control panel.
The power subsystem is monitored in a similar fashion. The monitoring subsystem
periodically polls the power supply status registers for a power supply OK status,
indicating the status of each supply's +2.5V, +3.3V, +5V, +12V, and -12V DC outputs.
If a power supply problem is detected, an error message is displayed on the console
(if one is attached) and logged in the /var/adm/messages file. The power supply
LED on the status and control panel is also lit. The LEDs located on the power
supply itself will indicate the type of fault, and if two power supplies are installed,
will indicate which supply is the source of the fault.
For more information about error messages generated by the environmental
monitoring subsystem, see "Environmental Failures" on page 261. For additional
details about the status and control panel LEDs, see "About the Status and Control
Panel" on page 8.
N+1 Power Supply Redundancy
The system can accommodate one or two power supplies. All system configurations
can operate with only one power supply installed. A second supply can be used to
provide N+1 redundancy, allowing the system to continue operating should one of
the power supplies fail.
For more information about power supplies, redundancy, and configuration rules,
see "About Power Supplies" on page 57.
Hot-Swappable Power Supplies
Power supplies in a redundant configuration feature a "hot-swap" capability. You
can remove and replace a faulty power supply without turning off the system power
or even shutting down the operating system. The power supplies are easily accessed
from the rear of the system, without the need to remove system covers.
Automatic System Recovery (ASR)
The system provides for automatic system recovery (ASR) from the following types
of hardware component failures:
CPU modules
Chapter 4 Hardware Configuration 43