IBM DS3500 Installation, User's, And Maintenance Manual page 138

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The power supplies are customer replaceable units (CRUs) and do not require preventive maintenance.
Use only the supported power supplies for the storage subsystem.
Each power supply has a built-in sensor that detects the following conditions:
v Over-voltage
v Over-current
v Overheated power supply
If any of these conditions occurs, one or both power supplies will shut down. If the power remains off
(does not automatically restart), make sure that the environment is optimal (no overheating has occurred,
all electrical receptacles are working, and so on). For more information, see "Restoring power after an
unexpected shutdown" on page 4-17.
If both power supplies fail or if the power supplies cannot maintain an internal temperature below 65°C
(149°F), the power supplies will automatically shut down (an overtemperature condition). If this occurs,
you must cool the storage subsystem and restart it. See "Restoring power after an unexpected shutdown"
on page 4-17.
Attention: The fans in the power supplies draw in fresh air and force out hot air. The power supplies
are hot-swappable and redundant; however, if the fans in one power supply fail, you must replace the
entire failed power supply within 72 hours to maintain redundancy and optimum cooling. Do not
remove the failed power supply until you have the replacement power supply. When you do remove the
failed power supply, be sure to install the second power supply within 10 minutes to prevent any
overheating due to the interruption to the air flow that cools the storage subsystem.
Do not run the storage subsystem without adequate ventilation and cooling, because it might cause
damage to the internal components and circuitry.
To replace a power supply, complete the following steps.
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System Storage DS3500 and EXP3500: Installation, User's, and Maintenance Guide