"Most high availability data centers use a power system providing dual power paths all the way to the critical loads, and most enterprise class IT equipment offers redundant power supplies and power cords to maintain the dual power paths all the way to the IT equipment internal power bus," states the paper's introduction. "In this way the equipment can continue to operate with a failure at any point in either power path."
See also: White paper compares AC vs. DC power distribution for data centers
"However, equipment with a single power supply [i.e. single-corded data center gear] introduces a weakness into an otherwise highly available data center. Transfer switches are often used to enhance single-corded equipment availability by providing the benefits of redundant utility paths. If not understood, this practice can lead to downtime that would have otherwise been avoided."
The white paper goes on to describe three fundamental approaches to powering single-corded equipment in a dual path data center environment.
Read the white paper here.