I just cut open a CAT5e cable since I wanted it for a project which required several twisted shielded pair cables. To my surprise there were 9 wires with isolation? I was under the impression that there were 4 twisted pairs, shield and possibly a drain wire (which I believe is not isolated). The last wire is of course not part of a twisted pair, what purpose does this wire fill?

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    \$\begingroup\$ Is the plug still connected to the cable? If so, why don't you check for continuity between the 9th wire and the plug's shielding? \$\endgroup\$ Dec 21, 2021 at 14:20
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    \$\begingroup\$ A photo of the cable with isolation stripped and wires spread would be great. \$\endgroup\$
    – fraxinus
    Dec 21, 2021 at 14:33
  • \$\begingroup\$ Interesting. Maybe they put the drain as a discrete wire ?(why?) \$\endgroup\$ Dec 21, 2021 at 14:54
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    \$\begingroup\$ That's surely the marketing wire: "We have put a spare wire in, in case one breaks, you have one already in place!" \$\endgroup\$
    – Arsenal
    Dec 21, 2021 at 14:58
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    \$\begingroup\$ Does it read anything about the make and model of the CAT5e cable on it? \$\endgroup\$
    – Justme
    Dec 21, 2021 at 15:07

1 Answer 1


I stripped the wire and measured, seems like it is indeed a drain/ground wire. It has continuity to the shield.

stripped wire

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    \$\begingroup\$ Hi, Lukas. It is not clear if you have posted this as an answer or if it is additional information for your question. If it is additional information then it should be added into your question, not posted as an answer. Your photo doesn't make it clear what wire you are talking about. Is it the outer braid or the red "wire"? \$\endgroup\$
    – Transistor
    Jan 24, 2022 at 18:51
  • \$\begingroup\$ He's talking about the red wire \$\endgroup\$ Mar 24, 2023 at 7:04

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