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I'm trying to understand how telephone wires work.
I've stumbled upon this kind of identification method with colours.
I can't manage to understand the reasons why is used and if it's still common to use it.

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    $\begingroup$ Colors are used to identify wires and often groups of wires. Often colors are shown and stated even though they are not needed as the wires can either way around... Just stops the great unwashed asking... $\endgroup$ – Solar Mike Jul 6 '19 at 12:30
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At the most basic level, wires are color coded to simplify things during installation. And that's all wiring, not just telephone wires.

Think of the challenges involved if you're running a new telephone jack into a room that's a reasonable distance away from the phone block. If the wires weren't color coded, you would have to set up some sort of ringer on each of the lines in order to determine which of the wires to use and where.

On a basic phone outlet (two wire) it's not horribly complicated. On a standard outlet (four wire) it starts to get annoying. With standard telephone wire (8 wire) then it gets really annoying. Color coding simplifies all that.

In case you haven't wired any of those types of connections, this webpage shows the standard color coding for telephone and ethernet wiring.

An additional aspect of color coding is standardized use of the wires. When someone new comes in to perform maintenance on an existing installation, they'll have a reasonable degree of assurance that specific color wires are used for certain tasks. Not following the standard can actually impact the data transfer rate on a cat-5 ethernet cable.

When it comes to higher voltages, there is a safety element as well. Incorrectly wiring a 240V outlet's multiple wires can potentially cause a fire or damage the equipment.

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    $\begingroup$ Reminds me of the problem of 3 red wires at the top and bottom of a skyscraper - the lifts are broken, so how many trips up and down the stairs do you need to label all the wires correctly? You have a simple contunuity meter, tape and marker pen. $\endgroup$ – Solar Mike Jul 6 '19 at 14:06
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    $\begingroup$ Wow. Whenever I work on an "existing installation" I usually find that color coding standards were followed quite rigorously -- there's the standard from 1929, the standard from 1962, the standard that original homeowner always liked, and sometimes the "that's what the hardware store had" standard. Somehow, it's never today's nationally recognized standard, though... $\endgroup$ – TimWescott Jul 6 '19 at 17:07
  • $\begingroup$ my first college room mate's grandfather invented color coding of wires while working for Bell Telephone. $\endgroup$ – niels nielsen Jul 7 '19 at 4:00

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