Underground secondary AC networks, a brief history

The first low-voltage AC network system is reported to have been installed in Memphis, Tennessee, c. 1907. The network transformers were supplied by primary feeders through distribution cutouts and were connected to a solid grid of low voltage cables that were protected with fuses. In 1921, improvements were made to the basic system in Seattle, Washington by Puget Sound Power & Light Co. This involved connecting the secondary terminals of the network transformers to the solid cable grid through network protectors. These protectors would trip automatically upon reverse power flow and were reset manually. In 1922, the first AC network system, in which network protectors were automatically tripped and closed by relays, was placed in service in New York City by the United Electric Light and Power Company. The cable grid was a three-phase/four- wire system which operated at a nominal voltage of 208Y/120V. By 1925, this type of system became an accepted method of supplying combined power and lighting load and there were six networks with a total load of 27.5MVA (over 100 transformers) in operation. By 1952, 82 companies operated 414 networks using this system. In 1974, 315 US companies had installed the low-voltage network system. Today's 208Y/120V network grid systems are very similar in configuration and basic operation to the first systems.