3Com CoreBuilder 9000 Device View User Manual page 33

The EME may shut down selected interface modules in an attempt to
bring installed module power consumption under the now-reduced
power budget.
In power fault-tolerant mode, power equivalent to one power supply is
held in reserve. This reserve power is not available to installed modules
unless a power supply fails, or if you switch the power mode from power
fault-tolerant mode to power non-fault-tolerant mode.
While the chassis is running in power fault-tolerant mode:
All installed power supplies are functioning and contributing power to
the chassis and modules. No single power supply is a dedicated
standby power supply. Rather, a factory-defined power limit ensures
that power that is equivalent to at least one power supply is available
to replace power lost if a power supply fails.
The amount of power that installed modules require must not be
greater than the number of installed power supplies, minus one (n-1).
When you reserve power that is equivalent to one power supply in
power fault-tolerant mode, the failure of a single power supply has no
impact on installed modules that are already powered on.
If a power supply fails while the chassis is running in fault-tolerant mode:
The EME automatically disables fault-tolerant mode.
Power formerly reserved is made available to modules to prevent them
from powering off.
All modules that had power before the power supply failure continue
to receive power without interruption.
Upon power supply recovery (or replacement), the EME automatically
reenables fault-tolerant mode.
Overheat Power Down Mode
An overheat condition exists when one of the chassis temperature
sensors detects a chassis internal operating temperature that exceeds a
predefined threshold. The allowable ambient temperature operating
range is 0 °C through 50 °C (32 °F through 122 °F). The default threshold
setting is fixed at an upper limit of 60 °C (140 °F) or higher to prevent
module damage.
Managing Power Systems 33