[Elecraft] Dangerous Low Voltage (WAS: Power pole for K3?)

[Ron D'Eau Claire]

I never use a wrench  which has exposed metal surfaces long enough to  short

a battery, and I always disconnect the negative terminal (- ground assumed  
here) and cap it before working around them.
 
Al WA6VNN

--------------------------------

Disconnecting power before working on the gear to remove/replace parts is
basic to both low and high voltage supplies.

My one up-close and frightening experience with low-voltage high-current
supply shorts occurred on an F-86 (Korean war era jet fighter) flight line
at Lockheed. It was the middle of the night and bitterly cold as I and
another tech worked on two fighters sitting side-by-side. They were powered
by ground  carts. We had lowered the canopies almost all the way down to
conserve a little heat in the frigid cockpits as we ran through the avionics
ground checks. 

Suddenly I heard one of the power carts groan under heavy load. It caused me
to look up and out of the cockpit to see what was going on. The plane next
to me had smoke billowing from the cockpit, coming out around the
almost-closed canopy. I could see the other tech's head lying against the
canopy. He was apparently unconscious. 

I hit the canopy button and clambered out as it slowly wound open, yelling
for help, very conscious that these planes were fully-fueled bombs sitting
wingtip to wingtip waiting for a fire to set them off. We got the guy out.
He was splattered with burns but the plane wasn't on fire.

Later we learned that he had discovered a defective circuit breaker. He had
a replacement with him, but decided he didn't want to open the canopy and
climb down to disconnect the power cart. The circuit breaker connected to
power by a wire with a lug on the end that attached with a screw into a
threaded hole in a large copper bus bar. As he was turning the screw, he
dropped the screwdriver and it fell touching the power bus bar and the
aluminum side of the console. He got a face full molten metal blobs as the
shaft of the tool literally exploded in his face. He instinctively stood
bolt upright in the cockpit, smashing his head against the closed canopy and
knocking himself unconscious. 

Lockheed management and the US Air Force were not impressed. They had a
badly damaged jet fighter with molten metal drooled down the side of the
console and cockpit and it could have been much, much worse. 

I think he found a new line of work.

Even "minor" short circuits can cause lots of grief. It's very easy to
instantly fry a PC board trace with a temporary short circuit. Those can
usually be fixed (once you find the break), but it's still a very
unnecessary and easily avoided damage by following simple safety protocols,
including removing power before touching the circuit to remove parts or
taking any other short-cuts.  

And for those of us who regularly work around equipment with high voltages -
a few hundred volts and up - allowing ourselves to be lulled into complacent
behavior by the low voltages in most gear today can be dangerous. One time I
was troubleshooting a receiver that had something that smelled hot so I was
gingerly touching resistors to see if one was overheating when I suddenly
remembered I was working on a tube-type receiver with a 250V supply!

Ron AC7AC