|[AMRadio] ground sticks again|
jcandela at prodigy.net
Mon Jun 5 14:22:24 EDT 2006
--- Donald Chester <k4kyv at hotmail.com> wrote:
> >From: "Jim candela" <jcandela at prodigy.net>
> >Humans are expendable, but don't destroy them hard
> to find capacitors! :-)
> It's no less safe to discharge them gently, first
> using a resistor then
> discharging directly, than discharging them with a
> Besides, if you discharge them with a "bang" a tiny
> piece of molten metal
> could hit you in the eye.
> I don't normally wear safety goggles while working
> inside a transmitter.
> That itself could be a hazardous distraction.
> Don k4kyv
All good points. I was trying to put this into
perspective where safety concerns must prevail over
equipment concerns every time. That said, we can
design gear and write safety policies to achieve both.
I like to put a large wire wound resistor in series
with a capacitor bank before the DC goes to a RF PA or
modulator. It could be a 50 watt 50 ohm resistor. This
will have little impact to normal operation, but in
case of a fault by either a chicken stick, or a
internal tube arc, the peak current is defined as I =
E/R instead of near infinity and beyond. This approach
will protect your precious capacitors too. A bleeder
should be used before the series resistor with
something like a 5 minute discharge time. So combine
this with a sign saying to wait 5 minutes or longer
after removing AC power, and to short out the B+ with
a chicken stick. When done, safety requirements are
met. Of course any Yahoo can still get around, and
either kill themselves, and or destroy the rig. It's
hard to make something idiot proof.
At the company I work for, some Yahoo used a
chicken stick to defeat a gravity assisted grounding
bar. Then a 90kv 100ma supply turned on, and arced to
his foot, and out his right butt cheek into the floor.
His life was saved when a metal door slowly closed,
and interrupted the arc, which allowed him to limp
away. The need to defeat interlocks did not exist, and
the whole issue was he was not aware of the danger,
and work arounds so that he didn't need to risk his
At an old job I once had, an engineer (a ham to
boot) would routinely isolate his chair, and
workstation, and go up to 40,000 volts to work on
something HOT. When he was done, he'd use a fiberglass
rod to hit the HV off button, and then take a 200
meg-ohm resistor in his hand and touch the far end to
earth ground. Then he'd step down, and go do something
else. He did that all the time. You didn't dare get
too close to him, or try to shake his hand!
I saw him get nailed one day where he was at 40kv,
and reaching up to something at 150kv, and he got too
close. I saw no less than 6 long snaps into his hand
as he waited for the Hippotronics 1D21 thyratron to
kick in, and disable the supply. He then stepped down
and walked away...white as a ghost.
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