Juniper E120 - RELEASE 11.1.X HARDWARE GUIDE 3-25-2010 Hardware Manual page 31

Chapter 1: E120 and E320 Overview
The system monitors the temperature of each module. If the temperature of a module
exceeds the maximum limit, the system immediately goes into thermal protection
mode and the LMs and SFM modules are powered off. The system controllers remain
active and respond on all management interfaces. All other modules remain in a
power-off condition. The failure of any two components (fan or converter), or the
absence of the fan tray, causes the chassis to enter thermal protection mode to
prevent hardware damage. For information about troubleshooting high operating
temperatures, see "Troubleshooting" on page 79.
The E320 fan tray has two primary converters that power four fans each. If a primary
converter fails, a third redundant converter takes over. The E120 fan tray has dual
converters that load share for redundancy. If one converter fails, the other redundant
converter takes over. The system software reports an alarm if any of the fans or
converters fail.
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Redundancy Features