Drain wires are used in cables in conjunction with a metallic shield to ensure effective grounding. The drain wire serves to complete an electrical circuit from the shield and carry unwanted electrical noise to the ground away from the circuit. The drain wire is connected to the ground and is in contact throughout its length with the metallic side of the shielding tape. It would be difficult to connect the tape with an earth terminal without the use of a drain wire.
The drain wires are usually tinned copper conductors. The tin coating helps prevent dissimilar metal reactions between the copper conductor and an aluminum screen. Instrumentation cables requiring electromagnetic protection or for use in intrinsically safe circuits are examples of cables with drain wire in their construction.
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