DIYbanter - View Single Post - Electrical Interference on Cable Internet???

BillGill

wrote:
On Wed, 21 Feb 2007 16:56:27 GMT, "Edwin Pawlowski"
wrote:

wrote in message
That is certainly a good theroretical design goal but reality is the
power company and the telco/cableco share the same poles and
underground ducts so their wires do run parallel for hundreds of
miles.
If you are getting ingress on coax or twisted pair, you have another
problem. I bet you could see it using TDR in a second.
On poles, they are often a few feet a part. What is the range or field that
is going to matter? I would imagine the difference between a couple of feet
and touching makes a difference in signal, magnetic field or whatever.

It will still add up and on a power pole in a neighborhood you usually
have a single 13kv unbalanced primary. In your house they use balanced
cables with a hot and neutral or 2 balanced hots.

When I was doing communication wiring we set up a torture chamber in
the office and did everything you "can't" do like looping LAN cables
around florecent fixtures and running along next to the building
feeders from the transformer. We found that if everything was
terminated properly it had zero effect. It was only when we had other
problems that it screwed up the data.
This was using test programs that collected LAN statistics with
unusually high loading.
Again, the TDR flagged these problems immediately. Coax and twisted
pair are very good at eliminating outside interferance
That means that good design rules don't apply because
you know that they don't really mean anything. And of
course everybody lives in an ideal world where all
circuits are properly terminated and there is never any
need to follow the design rules.

Power and communications lines do not share conduit in
long distance runs. They may be "near" each other, but
they are separated not only by space but by earth which
makes a pretty good shield.

On the pole out in back of my house the telephone and
cable lines are something like 6 to 10 feet from the 220
lines and about another 20 feet from the 7 KV lines.
With the square cube drop in level that makes quite a
difference in the interference level between say 6
inches and 10 feet.

And power lines are not balanced. One side is grounded,
both the neutral and the ground are grounded at the
entry point to the house. The closest you can come
to a balanced power line is a 220 line. And that
may not be completely balanced, because the device
connected to it may have a 120 circuit in it that will
cause an unbalanced flow in the 2 legs.

The best thing to do, even under ideal conditions is to
always follow conservative design rules. That will help
to take care of the cases where things don't meet the
ideal that you say will take care of all of our problems.

Bill Gill