What is the drain wire in shielded cables used for?

• A. It carries the main current.
• B. It provides mechanical support.
• C. It is tied to ground to drain EMI noise.
• D. It stores energy.

Answer: (C)

The drain wire serves to ground the shield of a shielded cable. It’s a separate conductor bonded to the shield (foil or braid) so you have a low-impedance path to earth or chassis ground. Any EMI or interference that is captured by the shield is safely drained away through this wire, helping keep the signal lines from picking up noise. It’s not used to carry the main current, nor for mechanical support or energy storage. Grounding the shield via the drain wire is a key step in maintaining signal integrity.