HardwareBanter - View Single Post - Hum from phone wires running next to mains?

**Tom Horne**

Foxtrot wrote:
On Tue 04 Mar 2008 21:22:30, wrote:

In alt.engineering.electrical wrote:
| On Tue, 4 Mar 2008 19:13:07 -0000, "Graham."
| wrote:

|It is quite difficult to induce hum into telephone wiring.
|Use twisted pair cabling rather than the flat ready-made
|extension cables.
|
| Exactly!
| The phone company has millions of miles of cable running right
| below power lines and hundreds literally touching each other in
| the jacket of the cable. That little twist they put in the pairs
| is excellent in isolating them from crosstalk.

That twist is a great little means to ensure induced signals,
whatever they may be, are induced in equal amount on both wires, so
they do not contribute to the actual intended signal that is a
differential between those two wires.

However, a risk exists when two different pairs are present next to
each other and each pair is twisted at the same pitch. The signal
carried by one can end up being induced differentially on the
other. So don't twist those power lines, or if you do, twist them
at a pitch with a ratio to the phone line twist that is not a whole
number.

CAT5 cable is an example. It has 4 different pairs twisting along.
Each of the pairs has a different twist pitch by design (unless
you get some cheap cable not manufactured correctly).

I do not have any technical knowledge of this area.

I would like to ask about a cable which has two or more twisted pairs
in it.

Is there is a greaterlikelihood of hum if I connect a "2 wire" phone
extension by using one wire from a twisted pair and taking the second
wire from a different twisted pair?

That practice is known in the North American communications industry as
a split pair. It is usually the cause of a host of troubles of which
induced noise is only the most common.
--
Tom Horne

"This alternating current stuff is just a fad. It is much too dangerous
for general use." Thomas Alva Edison