On 01/27/05 12:59 ET, Tom Ciaramitaro said...
Not always!
In a television servicing class, a newbie hooked
his oscilloscope up to a hot chassis TV and the
ground wire exploded, vaporized, you classify
it. There were several inches of the ground wire
missing and several more where there was insulation
and no copper inside. Not a trace of melting
there!
A music professor I once knew could not tell this story without a look of glee:
He was on a ship during the Korean War, and there were repeated problems with the radar system. At one point they brought it down to do some work on the high voltage system.
A "technician," obviously not (first-class), was in a bit of a hurry to get things to a safe condition, so he entered the room that was basically occupied by twenty or so HUGE capacitors, the oil-filled kind with two-inch glass insulators on the terminals.
Let's give a little perspective to the young guys here. Back then, if you thought of a capacitor along the spec of the one farad cap sometimes used in car systems, you would be envisioning an entire room-sized capacitor. Then, as now, higher voltage meant larger size.
Bakc to the story -- so this idiot looks at this row of capacitors, something out of Frankenstein's lab, each about eight inches wide, two inches thick, and ten inches high. They are all bussed together. He decides the thing to do is to simply short out the capacitors. THAT'LL DO IT, just rapidly bring the voltage to zero.
(Keep in mind here that steel has higher resistance than copper.) So he whipped out a ten-inch screwdriver and gently laid it across the terminals of a capacitor. Since they were strung together, he shorted out the entire mess.
My professor friend did not see what happened, because at that moment he was some thirty feet away, around a corner, walking toward the equipment room. What he DID see was an intense green flash (yes, green) and an explosion.
The tech was thrown against the wall and ended up in sick bay. About six inches of the screwdriver was vaporized. At this point I can't remember whether the story says that the tech's silhouette remained on the wall where he fell after he was thrown into it, or if that is part of the story of the explosion in my high school chemistry lab's stockroom. I digress, but I am good at that.
Where was I going? Oh, yes: the screwdriver did not get hot. Its handle was not even warm. It simply disappeared.
Haven't you ever been tempted to clear a short circuit in speaker wiring by supplying, say, 120 volts at up to 20 amps to just blast the short circuit away?
