fr8mech From United States of America, joined Sep 2005, 4265 posts, RR: 12 Reply 2, posted (4 months 2 weeks 19 hours ago) and read 1177 times:
Hmmm, I never saw the connection before. I guess it's because the L RECIRC fan is always running.
A quick reading of the AMM:
Under normal conditions, 28v dc from the dim control circuit breaker energizes a power control relay which sends 28v dc to the dim control relays. The dim control relays are energized by the DIM position of the control switch providing a ground.
When air conditioning airflow sensors or rack overheat sensors detect insufficient airflow for equipment cooling, or power is removed from the recirculation fan, a ground is removed from the power control relay by the air conditioning trim valve controller. The power control relay then removes 28v dc from the dim control relays and disables the automatic dimming function. This ensures all warning and caution annunciations are visible even if cooling is lost.