Drain Wire

A shielded cable will have a wire running the length of the cable between the shield and the insulated wires. This is the drain wire. Normal practice is to have the shield connected to a specific pin at one end.

For example, an RS-232 DB25 has pin 1 connected to chassis. The connector shells are not electrically connected to anywhere. So connecting the drain to pin 1 will put the whole cable screen at the potential of pin 1. No current flows but the cable screening is effective. So we would cut the drain at one end, pin drain by itself to pin 1 at the other end etc.