Importance

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Abstract

An automatic transfer switch having two sets of phase switches, one set for connection between a normal source of power and a load, and the other set for connection between an emergency power source and the load. The transfer switch includes two pairs of neutral switches, one pair for connection in parallel between the normal source neutral and the load neutral, and the other pair for connection in parallel between the emergency source neutral and the load neutral. The phase switches can operate to provide an 'open transition' transfer, wherein the phase switches connecting the load to one source open before closing of the phase switches which connect the load to the other source, or a 'closed transition' transfer, wherein the phase switches connecting the load to one source open after closing of the phase switches which connect the load to the other source. Prior to transfer operation, the pair of neutral switches corresponding to the closed set of phase switches are closed and the pair of neutral switches corresponding to the open set of phase switches are open. During transfer of the load, whether by open or closed transition transfer, one neutral switch of each pair are closed before the open set of phase switches close and those neutral switches remain closed until after the closed set of phase switches opens. Thereafter, the pair of neutral switches which had originally been open are both closed, and the pair of neutral switches which had originally been closed are both opened.

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First Claim