The 3500/15 AC and DC power supplies are half-height modules and must be installed in the designated plug-in on the left side of the rack. The 3500 rack can contain any combination of one or two AC DC power supplies, either power supply can power the entire rack, when the two power supplies are installed in a rack, one in the lower slot acts as the main power supply, the other in the upper slot acts as the backup power supply, if installed, the second power supply is a backup of the main power supply. As long as a second power supply is installed, removing or inserting either power module will not allow the new rack to operate. 3500/15 AC and DC power supplies accept a wide range of input voltages and convert them to voltages acceptable to other 3500 modules