The DC power supply in EX4300 switches is a hot-insertable and hot-removable field-replaceable unit. (FRU): You can install it without powering off the swtich or disrupting the switching function. Each power supply has its own fan and is cooled by its own internal cooling system. Each power supply has label AIR OUT (AFO) or AIR IN (AFI) on the faceplate of the power supply that indicates the direction of airflow in the power supply.