This Guide addresses the application of transformers in the presence of oscillatory switching transients. These oscillatory transients are typically produced by the interaction of the switching device, transformer, load, and system. This Guide defines operating conditions that may produce switching voltages damaging to the transformer insulation system. It discusses the electrical characteristics of the system source, switching device, transformer, and load and the nature of their transient interaction. It outlines several mitigation methods. Two examples are included. This Guide recognizes that many devices and/or system operations can produce oscillatory transient waveforms. The focus of this Guide is on the interaction between a transformer, the system, and a switching device as a result of several reports of transformer internal winding failures. This Guide focuses only on mechanical switching devices and does not cover semiconductor switching devices. Transformers that are inductively loaded (reactors, or stalled or accelerating motors) are not covered by this document but are addressed by several other referenced publications.