In shielded wiring, what is the purpose of a drain wire?

• A. Carry main current
• B. Ground the shield to the chassis
• C. Transmit data
• D. Change impedance

Answer: (B)

In shielded wiring, the shield acts as a barrier to block external interference from reaching the signal inside. The drain wire is a conductor bonded to that shield and provides a dedicated grounding path, usually to chassis or earth ground. By tying the shield to a defined ground reference, any EMI that is induced on the shield is safely diverted away, keeping the signal conductors quiet. The drain wire itself isn’t part of the signal path, so it doesn’t carry the main current or data, and it doesn’t set or change the signal impedance. It’s all about giving the shield a low-impedance ground connection so the shielding works effectively.