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Why do cat5 cables sometimes have the fluffy fiber bit in them?

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Hi. I understand that you're new here, which is awesome :-) We're always glad to have new people. Just so you know, the more people that see a question, the more people that can answer (and vote up) the right one. As you found out, sometimes answers either aren't right, or aren't right completely. It's therefore in your best interest to leave a question open for a day or two, just to make sure you get enough feedback. – Matt Simmons Mar 31 '10 at 13:56
up vote 49 down vote accepted

Actually it's for pulling the outer shielding away from the inner wires. When you're punching down the cable you pull the fiber string down from the top of the cable and it makes a nice split in the outer shielding that allows you to pull the outer shielding down to cut it off without damaging the inner wires. Here's a video that shows the process: at 1:23

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+1, but I believe that it's also used to provide strength when making runs, so the pulling stress isn't entirely borne by the wires and sheathing. – Matt Simmons Mar 31 '10 at 14:01
It's not, it's strictly intended as a ripcord. – joeqwerty Mar 31 '10 at 14:07
Interesting :-) I've been misinformed. Thanks! – Matt Simmons Mar 31 '10 at 14:19
Glad to help... – joeqwerty Mar 31 '10 at 14:26
It's surprisingly easy. I was mildly surprised the first time I tried it, but it works like a charm. Give it a try. – joeqwerty Mar 31 '10 at 15:24

In addition to Joe's answer; It adds some strength to the cable for pulling and helps prevent damage to the copper wires. For example, when you are pulling long runs.

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