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Whilst cutting up a USB cable for a project I noticed this extra bare wire within the insulation. Now I'm assuming the foil is shielding of some sort, but do I need to connect this bare wire to ground? Or should it be left disconnected?

enter image description here

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  • \$\begingroup\$ I can only assume this is similar to the nylon cores you find inside cables. It's just to strengthen the cable from what I know. \$\endgroup\$
    – Doodle
    Aug 10, 2016 at 14:16
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    \$\begingroup\$ @Hayman The correct answer is down there. Using a copper wire just for that purpose would be pretty wasteful. \$\endgroup\$
    – Eugene Sh.
    Aug 10, 2016 at 14:21

2 Answers 2

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That is the drain wire that helps carry charge off of the foil jacket and carries more current than the foil can.

It is part of the shield/ground of the cable.


As far as how to terminate it, that depends on what function it serves in your system. There are several purposes for that shielding:

  1. Reducing EMI emissions
  2. Reducing EMI susceptibility
  3. Defining cable impedance
  4. Providing a discharge route for ESD

The shield ground should factor into your grounding strategy, especially keeping in mind that there will be large discharges coming down that shield wire in user-pluggable systems. (as opposed to fixed, industrial type installations)

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  • \$\begingroup\$ So given that I have continuity with the drain wire and the other end of the USB cable, there is no requirement to also ground it on the cut end? For context, the cut end is providing power to an Arduino and the USB end will be plugged into a power bank. \$\endgroup\$
    – Calco
    Aug 10, 2016 at 14:31
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    \$\begingroup\$ Grounding systems are a fairly complex topic, and there are several great books about it. If you're just using it for power, it's probably a much less big of a deal though. \$\endgroup\$
    – Daniel
    Aug 10, 2016 at 14:48
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This is a shielding ground wire (or S-GND) which is left bare on purpose, so it makes contact with the foil. It has to be connected (crimped or soldered) to the metal casing of the USB receptacle / plug at the end of the cable. If your device includes a USB connector, S-GND can be connected to the device ground (same as GND), but this connection is optional.

enter image description here

The reason this wire is used in addition to the foil is that you cannot crimp or solder alunimium foil reliably. On the other hand, making the whole shielding out of copper (i.e. copper foil or woven copper shield) would be too expensive.

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    \$\begingroup\$ It's my understanding that the USB specification permits the connection of the USB shield to chassis ground at the host end only, not at the device. \$\endgroup\$ Aug 10, 2016 at 23:46
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    \$\begingroup\$ @JeannePindar My understanding was that both host and slave can connect USB shield to ground if they want ground on both sides to be at the same potential before any actual pins are connected. But I may well be wrong. \$\endgroup\$ Aug 11, 2016 at 8:23
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    \$\begingroup\$ @JeannePindar in practice some fairly expensive and sophisticated kit connects the screen to ground at the device end. \$\endgroup\$
    – Chris H
    Aug 11, 2016 at 11:05
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    \$\begingroup\$ I once accidentally connected shield to +5V instead of ground while trying to charge a phone. The cable almost melted. Every phone I ever disassembled had shield connected to ground. It shows how important the wire under the foil is. Both phone and cable survived, all power went into heating the cable up to almost melting the outer insulation. \$\endgroup\$
    – Agent_L
    Aug 11, 2016 at 14:08
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    \$\begingroup\$ @ChrisH Oh, I misunderstood your meaning of 'kit' as the grounding guts/circuits, not the device itself! \$\endgroup\$
    – Daniel
    Aug 11, 2016 at 19:36

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