It really is all skin deep when talking about power transmission through conductors.

First off there are two main types of current in our world.  DC  (direct current), which is a constant stream of electrons in one direct and AC (alternating current) where the electrons flow is back and forth. AC is the standard use for power transmission.

I have known that when AC current runs through a conductor that it can create an effect that causes the electron flow to be towards the outer area of the conductor. I have read about this effect on several occasions when it comes to electrical flow in conductors. As for the pure physics involved that I can not help you with. But I can give you a bit of understanding in the lay principles.

With Alternating Current (AC) we have what is known as the “Skin Effect”. This is caused by electromagnetic eddy currents that cause the electrical flow to be greatest towards the surface of the conductor.

The intensity of this effect is the result of the frequency at which the current is being transmitted. The higher the frequency the greater skin effect. At 60 hz (which is what the standard for North America) and with copper as the conductor the skin effect causes electron flow to be concentrated at the outer 8.5 mm’s (or about 1/3 of an inch). The skin effect causes the effective resistance of the conductor to increase with the frequency of the current. At 60,000 Hz (60 kHz), the skin depth of copper conductor is around 0.25 mm or about 0.01 inches. When frequencies are high enough the interior of large conductors do not carry much current.

Where this becomes used in a practical sense is with the high-voltage, high-current overhead power lines. It is common to use aluminum cable with a steel reinforcing core or known as ACSR cable (Aluminium conductor steel-reinforced ). Steel has a higher resistance to current flow than aluminum but it will not matter much because the steel will be below the skin effect depth where essentially almost no current flows. The outer aluminum strands are typically a high-purity 1350 or 1370 aluminium alloy. This aluminum is  because of its excellent conductivity at a low weight and cost.

So next time you look up into the sky at them overhead power cables think about what is going on and remember it may only be skin deep.

“Imagination is more important than knowledge.”

Albert Einstein

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