Tom Székely, P.E., LEED AP

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December 15, 2006

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‘Tis the Season - Mythbusters and Christmas Tree Lights, or, Hello Rodney Dangerfield, we’ve finally made it. 


One of the lamentations I have repeatedly heard from my fellow nerds has had to do with how we Engineers “don’t get no respect” and how the lay public has no idea of what it is that we do.  If that were ever really true, it’s fast becoming a thing of the past, as evidenced by the TV show referred to in the title of this piece.  The Discovery Channel show I refer to is but one of several which have been popping up on cable channels in recent years, including Modern Marvels, Engineering an Empire, and the like.  Of course, die-hard colleagues of mine complain that there is no engineering equivalent to prime time pure entertainment shows such as Boston Legal or CSI, to name just two.


OK, but to look on the bright (I mean really bright) side, I just saw a Mythbusters episode where the two madmen who star in the show strung a gazillion watts of lights on an oven-baked Christmas tree to see if the heat of the lights would set the tree afire. The service they performed was in showing that while that didn’t occur, overloading an extension cord softened the insulation to the point of failure, allowing the wires within to touch each other and short-circuit. 


The drama was in the fact that a short circuit being pushed along by the 120 volts of house current is explosive in nature, and generates enough heat to set insulation (a petroleum product) afire.  (Those of you who are regular readers of this publication know I’m taking literary license in the phrase “ . . . 120 volts of house current . . .”, but stay with me.)


Those of you who have had occasion to operate electric trains (whether they were/are what’s referred to as fine scale or tinplate) may recall derailments accompanied by a profusion of sparks as the wheels of the rolling stock short-circuited the rails.  While there was usually no apparent permanent effect, on rare occasions one might discover some soot or very tiny deposits of what had been molten metal.  This with only ten to twenty percent of line voltage (that is 12 or 24 volts rather than 120 volts) pushing the short circuit current along.


To get back to exploding extension cords.


As I’ve alluded to before, the flow of an electric current in a wire or cable is analogous to the flow of water in a pipe, with the insulation serving the same function as the pipe.  That is, if the pipe develops a hole or crack, water leaks out. If a conductor’s insulation is similarly compromised, electricity leaks out.  If it finds it’s way back to the source via the adjacent conductor which has become similarly compromised, it’s called a short circuit because it thus never gets to where it was intended to be sent, in this case, the Christmas tree lights. 


If only the one conductor’s insulation has become compromised, and some (not a whole lot) finds it’s way back through the source’s connection to ground by going through you because of your connection to ground (that is, something like touching a pipe with your other hand or being barefoot on a tile floor), it can be anything from a nasty shock to a life-ending event.


Pick your poison, fire or electrocution.


How come electric train derailments almost never start fires, and people are almost never electrocuted by touching live electric train tracks?  Of course I’m speaking of toy or model trains here.  The third rail of the New York City Subway system is at 550 volts DC and while, if you’re lucky, a nasty shock may be the only consequence of inadvertent contact with 120 volt house wiring, 550 volts DC will almost certainly kill you.


Well, to understand electrical fires and electrocutions, we have to understand two things – the tremendous amount of energy deliverable by electricity and how easily neurological “wiring” becomes short circuited on contact with low-levels of electric current introduced into the human body from exterior sources.


First, electric energy and fires.


The flip side of the lack of damage caused by low-voltage electric train derailments is the 120 volt extension cord explosion and fire in the Mythbusters episode which was the starting point for this issue.


To get back to our water analogy, the water pressure (pounds per square inch) which forces a bunch of flow (gallons per minute) through a fire hose is analogous to the volts of electricity which force the amperes of current flow through a wire.  Since a fire hose is a lot bigger than your kitchen faucet, the, oh, say 50 pounds per square inch of street pressure forces a lot more water through the former than it does through the latter.


It’s also why a knocking over a fire hydrant results in a geyser spewing hundreds or thousands of gallons per minute into the air.


Just as your house piping is so skinny as to not pass much more than a few gallons per minute from your kitchen tap when pushed along by this street (or well booster) pressure, the filament of a lamp or windings of an electric motor will only pass a few amperes of current when pushed along by 120 volts.  Bypass the downstream filament(s) or windings, however, and there will pass directly through the circuit’s extension cord (which may be hundreds or thousands of times the cross-sectional area of the bypassed elements),  enough hundreds or thousands of amperes to melt metal, set insulation afire, and generate the thunder which accompanies the miniature lightning bolts of a line-voltage short circuit. 


Hence the explosion.


If you want to put this in terms which may be somewhat more familiar, electric power, measured in watts, is the product of volts times amps, thus, a 150 watt light bulb (much too hot to touch) is the result of one and a quarter amperes being pushed through the filament by 120 volts.  That is, 120 (volts) x 1.25 (amps) = 150 watts.  Now imagine a short circuit of, oh, say, 3000 amperes (not actually a terribly big short circuit) on a 120 volt circuit.  Well 120 x 3000 = 360,000 watts.  We engineers and scientists (actually, I think it was a guy named Joule, some time back) have determined that 746 watts is equivalent to 1 horsepower, thus, 360,000 ÷ 746 ≈ 483 horsepower.


Chrysler Hemi, anyone?


In the low voltage conditions of an electric train set, 12 or 24 volts is not enough to force a whole lot of current through the frame of a set of wheels shorting out tracks, especially when also limited by the very long and very skinny wiring of the upstream transformer’s coils which are knocking the outlet’s 120 volts down to the 12 or 24 volts used by the train set.


With regard to electrocution, you and I are (regardless of the fact that we’re mostly salt water), because of the insulating qualities of our skins, conductors equivalent to the skinniest of low-wattage lamp filaments, so what we feel as nasty shocks are actually only a few thousandths of a single ampere passing through us.


The problem is that it can be  enough to screw up your nervous system signals to the point of not allowing you to let go, and to cause your heart to flutter instead of pump.        Bummer.

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