K7DD, Mike Baker, has printed a very detailed and wise treastise on
the hazards of high voltage. Everything he said reminds me of various
accidents and scenarios for accidents which I can remember. I have
never felt the jab of derious DC HV supplies, but have seen others do
it. Plenty of AC mains shocks as a kid, and lots of HV from tesla
coiling and spark plugs, induction coils, etc. But everytime I go
into a many kilojoule bank of capacitors that had been charged up to
80 kV minutes before entry (and properly discharged), I consider it
like walking in a mine field. Our procedures at work are sometimes
dozens of pages long, with annual retraining required to be qualified
to work. Also, we have annual CPR retraining as a job requirement.
All of this is now necessary as the current crop of electronics
technicians come into this field with backgrounds working at less
than 20 volts.
At home, when I was rebuilding an SB220 ~7 years ago, I did find it
awfully easy to expose myself to hazardous voltage. Ham gear is so
easy to have interlocks cheated (what interlocks?). And it was late
at night - I was alone. When you work a full day and then do your
home projects into the wee hours, this is a possibility. I had to
remember the same 'fear of god' (sorry I am not extremely religious,
just an expression) feeling that I get at work when I walk into a
capacitor room the size of my own living room. But on the bench at
home, it is so easy to think "how can such a small chassis kill me?".
When you short out a charged HV capacitor with your stick it becomes
very obvious!
Broadcast gear (tube transmitters mainly) is similarly very
dangerous, esp the older Gates, Collins, etc, where the door
interlock button could be pulled and locked to operate with the rig
open. Add to the hazard the fact that most transmitters get repaired
after midnight when everyone is asleep (except the engineer on duty)
and that most broadcasters are so cheap that they cannot afford two
workers doing the task. I remember neutralizing the BC-1G alone, one
hand behind my back, the 833s glowing at eye level. HV was off, as
prescribed, but nothing would prevent a relay or short from
accidentally closing. Worse than this was the SCR controlled rigs,
like the Collins 2.5 kW FM transmitter, which would ramp up without
having the klunk of a mains contactor. Easy to accidentally power it
up from a simple 24 volt contact closure at the remote panel (did I
remember to switch to local before working on it? Oh its OK, there is
no one else in the building at this hour. RIGHT)
Once I stuck my grounding stick on the terminals of a 20 uF 5 kV oil
capacitor before reaching in to a rig, and it nearly blew the end
off, as the bleeder had failed open. This was at least 5 minutes
after power down. Yikes, these things do happen. That could have
easily been my finger/arm.
73 to all, and please be careful.
John
K5PRO
> 1. HIGH VOLTAGE is not just a sticker on the side of a cabinet. It
>is a seductive lover that will KILL you and bring you to room temperature if
>not respected and the rules of working around it obeyed to the letter. You
>seldom get a second chance let alone a third to violate the rules. I have
>been blessed with having survived lethal shocks twice and figure that a
>third is not likely so be aware.
.....
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