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I'm helping a friend with an audio installation, and he asked about the requirements for the microphone cable:

"Wiring Requirements: 2 Conductor shielded cable, 22 gauge with a 24 gauge drain wire.

Wiring Requirements

The requirements imply that a 24 gauge drain wire is a necessity, but if the cable is properly shielded, is the drain wire a necessity? Other than providing some additional strength to the cable and/or an easy way to connect to and solder to the grounded shield, does it serve some additional purpose?

It is intended for an unbalanced electret microphone; I'm interested in other uses if not applicable here.

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  • \$\begingroup\$ I think it's something that's just easier to connect to instead of the shield itself. \$\endgroup\$ – tcrosley Apr 28 '14 at 23:11
  • \$\begingroup\$ The drain wire is relevant primarily for automated connector assembly, or for foil-shielded cable as mentioned by @PeterBenett. For manual DIY assembly, the "drain wire" isn't necessary, I believe. \$\endgroup\$ – Anindo Ghosh Apr 29 '14 at 5:57
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I think I've only seen drain wires in foil-shielded cables (but I could be wrong...). It is not possible to make a secure connection to the foil shield, so the drain wire is the only way to make the shield connection.

A drain wire is not required with a braid shield, as you can unbraid a bit of the shield, and twist the strands to make a "wire" that you can use to make a secure connection.

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A drain wire is normally there to compliment a foil shield. The foil is actually bonded to a very thin film of plastic and stretching of this plastic could cause the foil to break, thus breaking the path to ground. The drain wire ensures that the foil always has a reliable path to ground throughout the length of the cable. In addition, you cant solder foil, so the drain wire allows a reliable bond to ground at the connector if required.

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