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Re: Grim Safety Reminder
Original poster: "Terry Fritz" <twftesla-at-qwest-dot-net>
At 10:34 AM 5/9/2002 -0400, you wrote:
>Thank you for this safety reminder. I, too, am a firefighter and have been
>for over 13 1/2 years, but I have been fortunate enough not to personally
>witness gross electrocution(s) YET. However, I have definitely witnessed
>the awesome power of shorted primary power lines in the course af my
>job. I've seen a downed, energized 13,280 volt primay line laying on a
>rain-soaked asphalt street and it resembled a giant flaming r/r flair or
>blowtorch melting the asphalt and blowing a falme about 2 ft. into the air.
>Another time a 13280 volt primary line fell off of the insulator and was lying
>on the rain-soaked wooden cross arm of a utility pole. The pole had a
>grounded bracket holding three pole pig transformers and the entrie pole
>between the point of wire contact on the cross arm and the grounded
>transformer bracket was lit up like a Christmas tree. The utility shut it
>off soon, but then when they turned it back on, it made a plasma blue
>fireball about 6 to 8 ft. in daimeter with a noise like a small atomic bomb!
>I suppose the short had already established a carbon path in the wood
>restoring the power basically dead shorted it. Of course, the substaion
>safety circuit breakers quickly shut the power back off, but this process
>continued about two or three times (cut off and atomic bomb!)
>I couldn't help but think if an unfortunate lineman had been even close to
>that power arc, just the blast in close proximity to that would have most
>likely been fatal, even without electrical contact! It definitely gave me a
>renewed respect for high voltage!
Of course, the local 14.4 kV lines are the "small" ones. The big high
tension lines I used to work around were up to 375kV and 1000amps three
phase. That's many hundreds of megawatts of power "continuously". A Tesla
coil may get 1000 amps in the primary and that kind voltage on a secondary
for a few hundred microseconds, but power lines are at those levels all the
time!! Even though wires look nice and tame, it is hard to comprehend that
there may be a quarter of a million horsepower going through them! If one
"shorts" one of those things, the short circuit current is "high"... A
cotton wood tree tried on of our lines once and it turned it into
toothpicks (they had trouble finding where it "was"). The line monitors
showed a tiny 1mS "tick" on one of the phases that you had to look hard to
see, that was the tree exploding... We often think of Tesla coils as super
high power high voltage machines but when you compare them to electrical
distribution systems they seem pretty pale.
In many ways the lower powered 14,400 volt lines are more dangerous in that
they don't have the hum and electrostatics effects of the big lines to warn
you. Local lines just sit there quietly but they can pump 20,000
horsepower in the blink of an eye! (about 10 diesel electric locomotives).
The self resetting circuit breakers are to protect the "electrical
equipment and system". It is assumed that what ever the short was, it is
"gone". In fact, the system is designed to "blow shorts clear" by limiting
the power enough to protect the lines but delivering enough power to clear
the common shorts.
At our level, our stuff is fairly current limited to maybe 10 horsepower
for a pig system. So we don't get blown to bits but rather just killed
instantly and/or very seriously burned. Of course, death is not on a scale
of 1 to 100. If you touch an NST the wrong way or parachute onto a high
tension line, your still dead. We seem to control Tesla coils and there
power so easily, but never forget they can kill you instantly. You can't
see the power and energy in a high tension wire like you can say in a mad
Grizzly bear, but your far better off being attacked by the bear than
touching that little wire...