History
Emergency power systems were used as early as World War II on naval ships. In combat, a ship may lose the function of its steam engines, which power the steam driven turbines for the generator. In such a case, one or more diesel engine(s) are used to drive back-up generators. Early transfer switches relied on manual operation; two switches would be placed horizontally, in line and the "on" position facing each other. a rod is placed in between. In order to operate the switch one source must be turned off, the rod moved to the other side and the other source turned on.
Read more about this topic: Emergency Power System
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