A reader writes:
After an idle evening reading the comments section (I know) on the blog of the BBC's US correspondent, Mark Mardell, I came across this ... interesting perspective.
258. At 04:12am on 09 Feb 2010, KingLeeRoySandersJr wrote:
I can answer why electrical power in most of the USA is above ground. The reason is simply in the USA power lines are carrying much more voltage and current than in Great Britain for the most part and travel greater distances. Electricity doesn't simply flow through the wire but on the outside of a wire. The circumference of the wire carries the power if it were underground much of it would be lost in the ground.
Now here is something you don't know. Power companies use different transformers under different conditions. Ever plug in a device and the wire gets warm but other times it doesn't? That happens because when there is a great power demand the power companies try to fool the public that there is adequate power by simply supplying the voltage and the device works.
But this is not what they are telling you. The voltage is there but not the current the device demands in it's productive use of wattage to function. It can't obtain it on the gauge of wire it is designed for and the wire gets hot, homes burn down, lives and possession are lost! Simply because inadequate power is produced. Voltage ratings exist but only because current is decreased. This creates the illusion of adequate electrical power.
[...]
I can't identify a single thing in that comment that appears to be true. Am I wrong?
Jonathan
Yes, "KingLeeRoySandersJr" does appear to have a very independent mind. Perhaps he read something about power factor somewhere, and then took further guidance from disembodied voices.
But no, he's not wrong in everything he says. I guess, for instance, that if you were to run un-insulated power lines underground, you probably would lose a lot of power. For analogous reasons, jet fighters without windscreens do not work very well and cars without wheels have disappointing top speeds. Humanity waits patiently for the genius who can unravel these mysteries.
(Fortunately, the extra weight of insulation ceases to be a problem when you no longer have to hang your wires from poles. A lot of people find it surprising that overhead power lines are almost always un-insulated; this often seems to be because they don't know the difference between insulation and shielding. My learned colleagues at Harmonic Energy Products had this problem many years ago, and the confusion also cropped up in connection with this gloriously stupid audiophile power cable.)
The first thing KingLeeRoySandersJr says, about current flowing through "the circumference of the wire", is also not complete nonsense. He's talking, assuming he's got some connection with consensus reality, about the "skin effect", in which the higher the frequency of the AC you're trying to push through a wire, the shallower will be the depth into the wire in which significant current flow occurs. This has to do with eddy currents, which cancel each other out in the middle of the wire but increase current flow on the surface.
Some huge power-transmission lines are DC, which has an infinite skin depth, and some transmission lines for exotic applications - like particle accelerators - run at high frequencies. But changing the frequency of AC is as difficult as changing its voltage is easy, so the vast majority of high-voltage long-distance lines run at the same 50 or 60Hz as the rest of the grid. "Skin depth" - the depth at which current density is one-on-e, or about 37%, of the current density at the surface - at 50Hz is around 9.3mm for pure copper and almost 12mm for pure aluminium, unless the calculations I just did based on Wikipedia's tables of permeability and resistivity are based on subtly vandalised numbers. At 60Hz the depth drops a little, to around 8.5 and 10.9mm, respectively. If you're for some reason shifting 1kHz AC, your skin depth falls to 2.1 and 2.7mm, respectively.
Audiophile nitwits sometimes bang on about skin effect, and pay big bucks for cables with zillions of tiny separately-insulated conductors, maybe woven like Litz wire and maybe just floating around as a cloud, in order to defeat it. The theory is that skin effect increases cable resistance for high frequencies, so you lose treble - or "musicality", or "coherence", or whatever it is they've made up now - if your cables are too fat.
But even if your golden ears have the mystic ability to perceive 40kHz sound, an octave higher than the usual rule-of-thumb 20kHz upper bound for human hearing and higher still than the maybe-14kHz that's the highest most young-ish adults can perceive, skin depth in copper wire will still be around a third of a millimetre at that frequency. This gives plenty of copper to conduct your line-level or speaker-level signals, at all audio frequencies, in just about any cheap cable you care to name, and a resistance difference for 40kHz versus 10Hz of three-fifths of bugger all (a technical term), even if you hook everything up using the now-nearly-proverbial coat-hangers.
(God help me, I just searched for "skin effect" and "digital interconnect" and yes, right there on the first results page are people selling a carbon-fibre RCA cable for digital data that's supposed to be better because, among numerous other brain-hurting explanations, it ain't got no skin effect. It can be yours for a mere $US225!)
Clearly, at normal mains frequencies you need a pretty darn thick conductor before skin effect makes much difference. Big power-transmission cables are pretty darn thick conductors, though, so yes, it affects them. Most aerial power cabling is aluminium (which has higher resistance per unit area than copper, but lower resistance by weight, which is very important for cables strung from towers), but I think it's quite common for those cables to have thin steel wires in the middle to improve their strength. Steel is a pretty terrible power-transmission material, having a skin depth of less than a millimetre at mains frequencies (and yet mild-steel coat-hanger wire keeps passing those blinded audio tests!), but it doesn't matter when skin effect confines most of the current to the outer, aluminium portion of heavy power-transmission cable.
