I’ve noticed the the wall jack for my telephone has four wires but that a few of my telephone cords only have the middle two wires. I’ve tested out wires that have all four wires and those that only have two and nothing seems to change in line quality or noise or anything. Is there a purpose to those other two wires and are there any benefits to having them?
Most telephone wire contains two pairs. Only two are used to connect your phone, typically the red and green. The others are there if you wish to connect anything else.
the 2nd pair (yellow black) is now used for a 2nd line.
The black and yellow wires were originally designed to carry power for lights or phones which needed a separate power supply. Since not may phones need them or use them, they are often used to carry a second line or for other purposes like an intercom or computer network. I am not sure though if this is approved or just stuff people do.
Also I seem to remember the yellow wire was needed for the phone to ring. So was the ringing current sent over another wire at some time? Also would the black be ground (again some time ago). There are still lots of ground start lines out there even today.
If any voltage is present on the 2nd (black/yellow) pair, it is inserted at the home, not at the central office switch. Occasionaly I encounter a wall wart coming out of a phone jack and going to the nearest electrical outlet. That’s what these are for. Needless to say, these must be disconnected prior to adding a second phone line to the existing wiring.
The standard RJ11 jack will also support a third pair though this is rarely seen. If you come across phone wire with a blue/white pair (not an alternating blue/white jacket, but two seperate wires), that is the third pair. Occasionally, a fourth brown/slate pair is used but a larger jack (i.e. RJ45) is required to support this.
Sometimes, when dealing with very old phone wiring, a three-wire system is encountered. This has been out of current use for several decades and the CO equipment that may have supported it has been pulled out of every switching center in the civilized world. That wiring which is still in use has usually been adapted to the current 2/4/6 wire residential standards.
Another thing to consider is that telephone wiring in residences is subject to all manner of strange conditions due to the ignorance of many people who installed it. In businesses, most installations are done by qualified techs. However, when homes are built, phone wiring is often not fully considered and is usually installed by the electrician, who probably doesn’t really know what he is doing. Many homeowners DIY a lot of the time, too, compounding the mess. Once they get a working setup, they figure it’s right, whether any recognizable color code was followed or not.
This all assumes that you are in the US. Outside the US, different standards may apply. In Germany, no standard at all applies ;).
Well here in Saskatchewan they use the second pair of wires for ADSL service, I have a splitter in my place that contains a filter to cut the crosstalk and allow you to have phone service on the same line.
Keith
Two-line phone and answering machines have two jacks… one “Line 1/2” and one “Line 2”. If you have both lines running to the same wall jack, you can just connect a 4-wire phone cord from the “Line 1/2” jack to the wall. Otherwise you have to use one cord for line 1 and another for line 2.
You can also get splitters that will break your two-line wall jack into separate connectors for line 1 and line 2.