In the field of underground wiring, by the time that the polyphase AC system was conceived in 1886, several major steps had been taken towards making possible a practical underground power transmission system. One was Borel's invention of an efficient lead press in 1879, and another was MacCracken's concept of helically applied paper insulating tapes in 1884. Finally, in 1890, Vincent de Ferranti produced his famous 10,000-volt concentric cable, which was installed in London, England.

Continuous progress in the development of underground power transmission has taken place since that time. The first 3-conductor 25 kV cables were installed in Montreal in 1902, and they operated successfully for over 50 years. Then, in 1917, Emanueli conceived a design for hollow-core oil-filled cables, a breakthrough which permitted eventual transmission of power at high and extra-high voltages.

Gradually, the voltage levels of underground power cables increased, to 66 kV in 1926 in Montreal and Philadelphia, 220 kV in 1936 in Paris, and in recent years, reaching 525 kV in Canada and 535 kV in the United States. Current technology permits underground transmission cable systems as high as 1,100 kV.