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I have always been curious about the 5th wire(aka shield wire) in USB cables. What is its purpose? Does it anyhow affect charging speed for mobiles or data transfer? I do know that it carries the signal GND as well, but why add another wire for it inside the cable.

Does it have anything to do with external field interferences?

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    \$\begingroup\$ The shield reduces radiation coming from the cable. Without a shield, it would be much more difficult to pass radiated emissions testing (FCC testing). \$\endgroup\$ – mkeith Oct 5 '18 at 4:41
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A shield is used to prevent radio waves from coupling into signal or power wires within a cable.

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  • \$\begingroup\$ If, for some reason, my shield wire were to get less dense, would it reduce my charging speed? \$\endgroup\$ – Utkarsh Verma Oct 5 '18 at 4:24
  • \$\begingroup\$ That depends on how much (if any) charging current the shield is carrying. If the shield also functions as GND return, then reduction in shield density would also reduce charging speed. \$\endgroup\$ – Stefan Wyss Oct 5 '18 at 4:27
  • \$\begingroup\$ But shouldn't the GND wire alone suffice for the charging process? \$\endgroup\$ – Utkarsh Verma Oct 5 '18 at 4:28
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    \$\begingroup\$ Yes it should. But if some charger manufacturer decides to use a very thin GND wire for his design, then a portion of the current may also flow through the shield. \$\endgroup\$ – Stefan Wyss Oct 5 '18 at 4:35
  • \$\begingroup\$ To amplify what Stefan said, the shield may carry DC current at times, but its primary purpose has nothing to do with DC. It is primarily a shield for RF. \$\endgroup\$ – mkeith Oct 6 '18 at 4:51

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