Re: Van de Graaff generator safety question (fwd)

[Steven Roys <sroys@xxxxxxxx>]

---------- Forwarded message ----------
Date: Fri, 26 Dec 2003 09:40:32 -0700
From: Gomez <gomez@xxxxxxxxxxxx>
To: High Voltage list <hvlist@xxxxxxxxxx>
Subject: Re: Van de Graaff generator safety question (fwd)

On Thursday, December 25, 2003, at 08:13  PM, High Voltage list wrote:

> Original poster: Steven Roys <sroys@xxxxxxxx>
>
>
>
> ---------- Forwarded message ----------
> Date: Thu, 25 Dec 2003 10:56:09 -0700
> From: Terrell Fritz <terrellfone@xxxxxxxxxxxxxxxxxxxxxxx>
> To: High Voltage list <hvlist@xxxxxxxxxx>
> Subject: Van de Graaff generator safety question
>
> Hi All,
>
> I was thinking of making a Van de Graaff generator out of a 50cm
> diameter
> (20 inch) stainless steel sphere I have laying around.  I got it from
> Waterscapes:
>
> http://hot-streamer.com/temp/Old2001Files/2001-04/Orb.jpg
>
> http://www.waterscapesweb.com/orbs.html
>
> If I calculate the top voltage assuming 3MV/m radius:
>
> 3MV / m x 25cm = 750VDC
>
> If I calculate the top capacitance I get:
>
> C = 4 x pi x Eo x R  =  4 x 3.1415... x 8.85E-12 x 0.25  ==  27.8031pF
>
> Where I got worried was the stored energy:
>
> E = 1/2 x C x V^2  =  1/2 x 27.8pF x 750000^2  ==  7.81 Joules!!
>
> Assuming one is grounded and the thing arcs to one's forehead,
> assuming the
> human is the biggest resistance in the path, then all the 7.8 joules is
> dissipated in the human....
>
> My question is how dangerous is it??  Making this thing would not be
> fun if
> it was capable of killing anybody (well...  "easily").  If it would be
> really dangerous, I would not want to build it since it would not be
> fun to
> demonstrate if I had to really worry about it.
>
> It is pretty much like a 30pF 750kV cap I would think.  But I am not a
> great expert on this stuff so I thought I would ask.

  I have been wondering this too, since I have (on the back burner)
plans to build
a fairly ridiculous Van de Graaff myself.  (24" reentrant sphere, in my
case)

  I have tried to do a bit of research on the effects of short pulse
discharges
on people, but I haven't been able to find any conclusive guidelines,
other than
safety pages which simply say don't come in contact with any high
voltage, period.

  Yes, 7 - 8 Joules is a fair amount of energy to be dumping into the
human body,
and at the 500 - 750 kV you can expect on your terminal, the momentary
current
pulse ought to be ridiculously high.

  I think that, just as with large Tesla coils, the best policy is
simply to
not come in contact with it, and to always provide a grounded discharge
electrode
which is closer to the HV terminal than anything (or anyone) else.

  Obviously, very large VDG's _can_ be demonstrated safely with the
right precautions,
since the Boston Museum of Science has been demonstrating the Round
Hill machine for
years. (without, so far as I have been able to find, any incidents -
does anyone
know of any?  I'd like to hear about them if so!)
http://www.mos.org/sln/toe/toe.html

  - Gomez

..............................................................
Once when told that Apple Computer bought a Cray supercomputer
to simulate their next Apple computer design, Seymour Cray
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