High voltage safety was: RE: [sdiy] New Vco

[KHeck73 at aol.com]

In a message dated 9/28/2001 3:19:15 PM Eastern Daylight Time, 
uzs159 at uni-bonn.de writes:



> Right, its knowing which places you can safely touch and which you better 
> not touch. THere are books on electrical safety, mainly with respect to 
> mains wireing, but these are good guidelines. Keep as much insulated, so 
> that there is little chance that you accidentaly touch anything.
> 


Hi. I learned to fix tube-type TV's in high school. The teacher stressed the 
safety aspects. (He also sponsored our high school ham station which was 
dedicated to a student who was electrocuted while adjusting his homebrew 
linear amp). 
It's been a long time but a few safety details I remember are:
1) If at all possible, don't work on a powered circuit. Don't rely on the 
power switch, always unplug from the mains.
2) Work on an insulated (wood) workbench and wear insulating shoes. Stand, or 
if you sit, use a wooden stool.
3) Always discharge the filter capacitors in the HV supply with an insulated 
screwdriver before doing anything else, even if the power has been off for 
days or weeks. Use one hand only (see #5).
4) If you must apply power to make an measurement, clip the probes in place 
before applying power, then keep your hands away when energizing. Turn power 
off and discharge capacitors again to remove the clips. This may seem a 
little silly, but if you don't need your hands in there, don't put them in.
5) If you must make an adjustment while power is applied, here's a good 
technique to remember. Use only one hand. Take your other hand and put it 
inside your back pants pocket (thumb included), and make a fist. You will 
feel a tug at your pocket if you unconsciously start to reach that hand into 
the circuit or to accidentally touch a chassis ground. You want to prevent 
any chance of current passing through your chest.
6) Remember that high voltage can arc through air, so you don't have to 
actually touch something to get a shock.
7) If possible, let someone else (wife, roommate, etc.) know what you're 
doing so they'll check on you from time to time.

It isn't the voltage that gets you, it's the current. Of course, higher 
voltage moves more current through the resistance. I think it takes only 10 
or 15 milliamperes through the heart to stop it.

There are probably other important issues to learn about. I'd follow the 
recommendation to find a good book, or a good teacher like I had.
-Karl.
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