Editing Direct current (section)

===Telecommunication===
[[Telephone exchange]] communication equipment uses standard −48&nbsp;V DC power supply. The negative polarity is achieved by [[ground (electricity)|grounding]] the positive terminal of power supply system and the [[battery (electricity)|battery]] bank. This is done to prevent [[electrolysis]] depositions. Telephone installations have a battery system to ensure power is maintained for subscriber lines during power interruptions.

Other devices may be powered from the telecommunications DC system using a [[DC-DC converter]] to provide any convenient voltage.

Many [[telephone]]s connect to a [[twisted pair]] of wires, and use a [[bias tee]] to internally separate the AC component of the voltage between the two wires (the audio signal) from the DC component of the voltage between the two wires (used to power the phone).