The first commercial applications of carrier current technology included the setting up of long-distance telegraph, telemetry, and telephone communication by electrical companies over their high-voltage distribution lines. This approach had a major advantage over standard telegraph and telephone lines, because radio signals can readily jump over any small gaps in cases when there is a line break. In May 1918, the Imperial Japanese Electro-Technical Laboratory of Tokyo successfully tested "wave telephony" over the Kinogawa Hydro-Electric Company's 144-kilometer (90-mile) long power line."Telephony over Power Lines (Early History)" by Mischa Schwartz, "Presented IEEE History Conference, Newark, New Jersey, August 2007 and annotated since". (ethw.org) In the summer of 1920, a successful test transmission over 19.2 kilometers (12 miles) of high-tension wires was reported from New Jersey,"Interplant Telephonic Communications Established Over High-Tension Lines", Electrical World, July 17, 1920, page 141. and by 1929, 1,000 installations had been made in the United States and Europe. The majority of power line communication installations use transmissions in the longwave band, to avoid interference to and from standard AM stations.
