An interesting story has reached me from Los Angeles.
There are many points where the dense net of overhead power lines crosses the dense net of freeways. At times it happens that a cable snaps and falls to the ground, and it has now twice happened that the drivers whose cars were in contact with a high-voltage line were severely burned and had to be hospitalized. The City of Los Angeles therefore decided to prevent such cases by constructing nets under the lines where they cross the freeways at a cost of $9 billion.
My information comes from one who does not want his name published because he is one of the bidders for designing the nets, and when questioned, he confirmed $9 billion, not $9 million. A single source is not usually enough for a story in this newsletter, and I therefore do not guarantee the truth of the above. However, the interesting points about to be discussed do not depend on the exact truth of the story. One point it technical, the other
¾well, let's call it sociological.Until fairly recently the story could not have been true, for the driver inside a car (as distinct from one who leans out of the window to touch a live line) could no more have been electrocuted than a bird perching on a high-voltage transmission line. The reason is that what kills and injures is not voltage (pump pressure in the water analogy), but the current that the voltage forces through the body of the victim
¾if it has no easy bypass.For water flowing through a pipe, it is obvious that the current through the pipe is proportional to the applied pressure, and how much current you get for a certain pressure depends on how large the pipe's cross section is, i.e., on how much resistance it offers. Pressure and current are thus proportional to each other and hence pressure equals resistance (the constant of proportionality) times current. Substitute voltage for pressure, said Georg Simon Ohm in 1827, formulating what was to become Ohm's Law, a fundamental law of electrical engineering. He was promptly refused appointment at German universities, because anybody who thought it was that simple was obviously a crank and simpleton unworthy of an academic position; and to this day the benighted attitude "if it's simple it can't be true" flourishes on.
The human body is a conductor whose resistance lies mainly in the contacts and therefore varies widely, but 1,000 ohms is a commonly used average. The value of current that can endanger life is sometimes taken as low as 30 mA (milliamperes) for AC (alternating current) and 100 mA for DC. From this it would follow that household power at 115 V could kill, and indeed it can
¾but only if the contacts are good, the circuit lasting and the current from one contact to the other (usually ground) goes through the neighborhood of the heart. Even so, it shows that the human psyche does not fear dangers that are commonplace and well understood; it fears mainly the unknown. Hence many who use electric hair blowers in a moist bathroom after driving home on the freeway can be frightened out of their wits by a little plutonium. (There were 854 accidental deaths by electrocution in 1986, or about 4 per million/year.)However, 3,000 V will kill quickly if the source is "hard," i.e., maintains its voltage when a current is drawn from it. But of course only if the current goes through the body. The usual way to avoid that is to provide a path of much lesser resistance, most often to ground. 1,000 ohms resistance of the human body is at least one million times more than an alternative way to ground via a metallic conductor, so the body gets only one millionth the amount, which it cannot even feel. The grounded casing of an appliance (sometimes grounded via a third pin in the plug) is meant to provide that easier path from the "live" wire to ground.
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Vol. 17, No. 5
Newsletter: Access to Energy Newsletter Archive Volume: Issues Issue/No.: Vol. 17, No. 5 Date: December 01, 2004 03:27 PM Title: The sorry remainders
Copyright © 2004 - Access to Energy Newsletter Archive
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