Twisted Pairs [RE-wrenches]

[Robert Warren]

Windy,
  I always heard that the principle behind twisted pair is that an 
induced electrical field in one small section of wire closest to the EMF 
field will then get cancelled out by the opposing field it encounters 
from the other wire in its pair. This is kind of like having to separate 
out the two wires of a cirucit to get your Ampprobe clamp meter around 
just one wire to get a current reading. 
  As you know, this is considered standard practice in 100 years of 
telephone practice, but mainly just in terms of weak RFI and EMF fields. 
What we were taught in power electronics was that a signal wire longer 
than 50 feet should be shielded or twisted pair, because that is the 
half-wave wavelength of a broad spectrum of RFI interference.  To a 
certain extent, it will help with weak electrical storms but not ones 
not right overhead. The cable industry provides us with a lot of options 
for cabling, but usually they prefer to shield heavy cables and earth 
the shield. Typically you will see the shield is netting (effectively a 
Faraday cage) and the power conductors are spiralled. But spiralling is 
not twisting, as all conductors are still paralled. It is worthwhile to 
experiment like you were doing, though. Now that you have twisted your 
power cables, install them somewhere and see if it makes any difference. 
Lighting is pretty unpredictable stuff, though, if it likes a tree as a 
good conductor to earth. 
 The thing to remember about lightning is that it doesn't have to touch 
down: a cloud to cloud strike can induce 200,000 volts into straight 
overhead wires like the ones in the utility grid.
  I think if you get a lot of lightning in your area, it is best to have 
a dedicated lighting conductor as well as your normal PV framing ground. 
The problem is, a lot of metal roofs should have had a lightning 
protection system installed when they were first built, but didn't and 
so far they have been lucky. Our problem as installers is that we now 
have put up a grounded metal structure that attracts lighting more than 
before, because before its was a floating ground, and it had a lower 
potential. So we are the last ones on the roof, and we will get blamed 
for lighting. It is a good idea to forewarn your client when you do a 
site survey that circumstances may require the addtional expense of a 
dedicated lighting protection system. 
Robert Warren


Wrenches,

I have always heard that twisted-pair wires pick up much less induced 
current from nearby fields. We all see twisted-pair telephone and 
other audio lines designed to reduce AC hum pickup. It seems to be 
well-accepted that twisting power wires makes a circuit safer from 
lightning-induced surges. R Perez reported in Home Power long ago 
that twisting the lines was more effective than shielding, per his 
experiments.

I don't understand the principle.

I twisted some wires together for a distance of 150 feet and it was 
not easy! I could have divided it into sections and twisted it in 
opposing directions in each section, which would have made it very 
easy, but since I don't understand the principle, I couldn't assure 
that it would be effective.

Can anyone induce some understanding into my twisted mind?

Windy


What will we do if there is another fuel shortage? 
Did you know that there is a pollution free, completely
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Don't replace the engine, replace the fuel.
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robertwarren at mail.com

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