Drain

In wiring, a drain is a dedicated wire that is sometimes employed to work in conjunction with a shield, which is often a very fine braided wire or foil material (though drains are normally only used with foil shields). The drain serves a couple of major purposes. It provides an easy way to connect the shield to other circuit components. The foil is a very difficult material to work with, especially when it comes to soldering it to connections. It also provides a more robust mechanism inside the wire to maintain the electrical continuity of the shield in case the fragile foil breaks apart somewhere along the length of a cable.