Questions and Answers About EMF Electric and Magnetic Fields Associated with the Use of Electrical Power.
January 1995.

Electric Power Background

ELECTRIC POWER FACILITIES

There are two basic types of power lines: transmission lines and distribution lines. Transmission lines are high- voltage power lines. The high voltage allows electric power to be carried efficiently over long distances from electrical generation facilities to substations near urban areas. In the United States, most transmission lines use alternating current (AC) and operate at voltages between 50 and 765 kV (1kV or kilovolt = 1000 V).

Utilities use lower-voltage distribution lines to bring power from substations to businesses and homes. Distribution lines operate at voltages below 50 kV. For residential customers, these levels are further reduced to 120/240 V once the power reaches its destination.

Electrical substations serve many functions in controlling and transferring power on an electrical system. Several different types of equipment may be present, depending on the functions of the particular substation. For example, transformers change the high voltages used by transmission lines to the lower voltages used by distribution lines. Circuit breakers are used to turn lines on and off. The figure below shows how these facilities fit together in an electric Power system.