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Re: [TCML] Re: Sculpture with Tesla coils
Jim Mora wrote:
Hi Dex, Jim,
I assume the magnetic field would decrease by the inverse square, so what is
a safe distance?
Actually not inverse square (that would be radiation from a point source
in the far field).. more like 1/r^3
What is a "safe" distance depends on what you want to define "safe" as.
From a human safety standpoint, it's pretty easy to stay below the
IEEE/ANSI C95.1 limits, mostly because they use averaging and RMS
measurements, and though a TC has high peak power, it has a low duty
cycle, so the average energy in the magnetic field is fairly low.
The regulatory limit for the H field at 150kHz is 108 A/m, and at the
center of a pancake primary (the worst case) for a 4 turn coil with
radius 20cm carrying an amp RMS is about 10 A/m (using Biot-Savart).
Double the turns, you double the field, etc.: H = N*Irms/(2*r), and it
only goes down as you move away.
However, if you are concerned about peak fields (e.g. EMI/EMC and/or
equipment damage), then you need to look at the peak primary current.
Taking some more numerical examples.. say your primary is about 12 uH
and your cap is 0.1 uF (fres around 140kHz).. If you charge this with a
14.4kV rms transformer to around 20kV, the peak primary current is
around 1.8kA. That makes the peak field on the order of 18,000 A/m at
the center of the coil. A pretty big difference..
I am wondering how the coil at Fry's doesn't kill all the
electronics inherent in their stores.
Distance? I doubt Frys would know or care if electronics suffered
non-fatal degradation (sort of like ESD causing latent damage). Bill
Wysock might be able to answer if they did any testing.
I still think this is doable following
proven safety methods. That is why I suggested a field trip to the Griffith
Park Observatory and Fry's in Fremont for the artist.
yes.. but let's not confuse "people safety" for which the suggestion of
a grounded screen is a good one with "electrical equipment safety" for
which the screen doesn't do a heck of a lot of good.
A significant other wrench in the works is what the risk and liability
policy of the venue is. If they want a "guarantee" of safety, as in an
analysis by someone qualified to do so, that's a different matter from a
"we did what worked before, and we have no assets so nobody is going to
sue us anyway."
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