The State of the Art of Telecommunications Power Plants in Japan

Various power supply systems for NTT gigantic communication network which involves the world second largest number of telephone sets are described. As for the standard telecommunications power supply in Japan, -48 volts DC is used for switching systems, while -21 volts DC for carrier and radio communication systems and 200 volts AC for data communication systems. However, they are gradually forced to change their supply patterns into lower voltages by the recent transistorization of the communication systems.