Line power is often considered 120 VAC from an AC wall jack. But it could also be considered the 240 VAC three-phase power used for heavy-duty appliances or even a DC voltage source for powering any number of devices. In telecommunications, there is also remote line power.
Remote line power (RLP) is a technique for powering remote devices from a central source using copper cable, typically twisted pair. It can also use existing twisted pair lines used for phone and DSL services. The RLP power source provides 48 VDC converted to a higher voltage for distribution over the copper cables. The cable runs can be as long as 6 to 7 kilometers.
The higher voltage conversion is accomplished with a DC-DC up-converter, usually to 190 VDC, followed by a DC-DC down-converter at the device. The device voltage is often 48 VDC but could also be 12 VDC. The higher remote line voltage reduces the required current and lowers the I2R losses while keeping the voltage low enough to meet safety requirements.
There are three key benefits of using remote line power. It eliminates batteries and simplifies maintenance. It reduces expenditures on remote power equipment. And it minimizes dependency on local electrical utilities.