It may have been my imagination I suppose; after all, I was looking at Juniper EX4500 switches above head level in a rack, and using my fingers to check the airflow. And while I tried to look to verify the truth, data centers aren’t always that well lit and it was difficult to be 100% sure of what I saw.
A Juniper EX4500 switch only requires a single power supply to operate. When power is only connected to one of the two PSUs, the cooling fans run on both.
Answers on a postcard. Or below in the comments 🙂