New to this blog?
Sunday, May 23, 2010
Once upon a time, during a leisurely trip back to the home planet, the battery of my aging car died. I'd stopped for a burger in a little town on the Central Coast, and when I got back in the bug-splattered old Camaro and turned the key... nothing. It still had enough juice to run the radio, but that’s all.
No lights, no camera, no action.
Being a small town, there was a full-service gas station right around the corner.* The attendant on duty was a small man in his early forties, friendly, but with an oddly distant gaze, as if he’d witnessed something far bigger than most people could possibly imagine. Both his hands and arms were covered with terrible scars, leaving the skin withered and hard, like thick ropes of beef jerky. He moved with the cautious, deliberate manner of a man who had learned the hard way that moving too fast can be a terrible mistake.
I was curious about those scars, of course, but there are things you just don’t ask. We traded small talk as he went about pulling the dead battery, filling a new one with acid, then installing it. Eventually the small talk faded, and after a brief silence, he began to tell his story. I think he needed to. By then, everyone else in that little town had doubtless heard it ten times over, but I was a captive audience with a fresh pair of ears. It didn't take long to tell, and if some of the details have evaporated over the years, the central image remains branded in my memory. He’d been standing atop a gasoline tanker doing some kind of maintenance task with a long aluminum pole, unaware that the truck was parked directly beneath high tension power lines. For one reason or another, the pole came dangerously close to those lines -- and at that point, 17,000 volts of extremely big power screamed through the metal on its way to the ground.
For a brief, life-altering instant, this little man held the fire of the Gods in both hands.
That he wasn’t instantly fried like a mosquito in a bug-zapper by such a massive jolt is something of a miracle. In all likelihood, he survived only because he was not the primary path traveled by the high voltage surge as it headed for the ground. Had his body (rather than the metal pole) been the main transit route for all those electrons, this man would be nothing but a smoldering memory in that little town. As it was, he suffered horrible burns and god knows how much internal and neural damage. I have no idea what he’d been like before this incident, but there’s little doubt the course of his life had been radically changed in ways few of us can comprehend. We humans are bio-chemical machines that operate on minute micro-currents, and under the proper circumstances, a very small amount of electricity can kill. If the juice powering a household light-bulb can be lethal, imagine the damage 17,000 volts carrying a huge amp load could do. A shock of that magnitude would be something like dipping your head to sip from a drinking straw only to get blasted in the face by full steaming fury of Old Faithful.
After that – in the unlikely event you survived - you’d never look at a drinking straw quite the same way again.
But he here he was, still alive and working in the same gas station several years later. In a way, he was like some modern day Prometheus, punished by the Gods as a warning to respect the immense power of electricity. As one who handles the invisible juice on a daily basis in my working life (albeit much lower voltages), I took this warning to heart. Whenever I'm under pressure and trying to work fast with hot lines, I remember the little man with those big scars. That slows me down every time.
Most movie lamps run on 120 to 240 volts AC, either of which can be lethal when things go wrong, but in general, higher voltages deliver a meaner punch. I related some of my experiences “getting bit” in a previous post, and have heard many more from other juicers over the years, but the scariest stories always seem to involve 480 volt AC power. Until recently, set lighting techs didn’t have to deal with 480** – we left that to licensed industrial electricians -- but even they sometimes have problems with this unruly beast, and given the unforgiving nature of 480, any trouble is a Big Trouble. I heard one grim story of a guy who walked into a dark electrical closet unaware that a co-worker had left some loose cables dangling near the live 480 bus bars. One or two cables hit the bars, and the result was an explosive fireball that inflicted fatal burns.
Here's the technical description of "arc flash," and why it's so lethal:
“An electrical explosion, or "arc flash", occurs when one or more high current arcs are created between energized electrical conductors or between an energized conductor and neutral ground. Once initiated, the resulting arc(s) can bridge significant distances even though the voltage is relatively low.
The energy dissipated within a power arc is limited only by the fault current capability of the upstream power source and the duration before protective hardware "clears" (interrupt) the short circuit. In many low voltage (480 - 600 volt) electrical power distribution systems, fault currents can exceed 70,000 amps. The thermal energy liberated within a high current arc can be many tens of megawatts - equivalent to several sticks of dynamite. The arc core may reach 35,000 degrees F (four times that of the surface of the sun!). As the arc "roots" vaporize portions of the copper bus bars, the copper vapor explosively expands to over 60,000 times its solid volume. The incandescent copper vapor then combines with oxygen in the atmosphere, forming dense clouds of cupric oxide, blackening the air and covering nearby objects with black "soot". Globules of molten copper are also violently ejected, showering the immediate vicinity with 2,000 degree droplets that can approach speeds of 700 miles per hour.
Magnetic forces also propel the arc along the bus, extending it in the process, and the huge magnetic forces may be sufficient to actually bend bus bars or rip them from their mountings, possibly creating additional shrapnel. An unprotected individual unlucky enough to be anywhere near this event would be seriously injured or killed. Because of the extreme danger, most countries now require electrical workers to wear protective clothing and headgear whenever working near energized high energy equipment.”
The text above is used with permission from Bert Hickman, of Stoneridge Engineering, at Capturedlightning.com. A retired electrical engineer who worked for Bell Labs and AT&T, Mr. Hickman knows what he’s talking about – and given that he refers to anything below 600 volts as “low voltage” power, it’s safe to assume he’s forgotten more about electricity than I will ever know. For some truly eye-opening examples of what big power can do, check out the Arcs and Sparks section of his website. One of his more astonishing video clips shows an entire substation shorting out and melting down. It’s scarily impressive stuff, and a reminder that big power is extremely serious business.
Me, I'll stick with 240 and under, thankyouverymuch...
* This was in the mid-80’s, when many gas stations still had a garage, hydraulic lifts, and a mechanic on duty to fix blowouts, mount and balance new tires, replace a broken fan belt, install a new water pump, or put in a fresh battery. Nowadays, most gas stations are nothing more than eight Serv-Ur-Self pumps and a kiosk selling newspapers, cigarettes, breath mints, and condoms.
** A new lamp called a SoftSun came into use during the past few years, a 320 pound monster that turns 480 volts of electricity into 100,000 watts of daylight. Properly hooked up and fed by a generator, it's perfectly safe to use, but personally, I'd be leery of energizing 480 for one of these lamps via step-down transformers connected to city power lines. Given the immense electrical capacity of high tension lines, the potential for trouble should something go wrong is scary.