Re: RF safety

[Terry Fritz <twftesla-at-uswest-dot-net>]

Hi Jim,

	It appears that normal TCs at reasonable distances have a very minimal if
any risk to the operator from this great data.

	I would also like to mention that the individuals that charge them selves
up directly to the output potential (obviously, not recommended for the
average coiler!), like Dean Ortner and Robert Krampf, have not had any
problems from perhaps 20 years of doing shows.  Their exposure is actually
"being the source"!  Robert has mentioned headaches but it is hard to say
if that is from the noise, O3 and Nx gasses, or the current going through
his head.  The fact that their exposure is far far greater than any casual
observer and they don't have problems, pretty much relives any concerns "I"
have.  They have sort of done the high-exposure "white mice" test for us...

	Of course, there are simple elctrical and other dangers that are very real
with Tesla coils, and the pacemaker type concerns need to be considered.
However, risks of a Tesla coil cooking a person like a microwave oven, or
doing damage as X-rays would, seems tiny to non-existent...

Cheers,

	Terry



At 01:46 PM 7/21/00 -0700, you wrote:
>A couple of days ago, I attended an RF exposure safety class at work. A lot
>of interesting information was presented, particularly with respect to tesla
>coil operations.  I had an opportuntity to talk to the instructor (who is
>quite knowledgeable about RF effects on humans, etc.) about this in some
>detail.
>
>The significant figure to remember here is that NO repeatable effects have
>been noticed at less than 3-4 Watts/kg specific absorption rate.  The ANSI /
>FCC / OSHA standards are based on limiting occupational exposure to 1/10 of
>that, i.e. 0.4 W/kg. For the uncontrolled general public, the limit is 1/5
>of that.  Coilers would fit in the occupational category, I should think.
>
>At low frequencies (i.e. <3 MHz), the limits are formulated more to limit
>the conducted or induced currents
>
>Some general comments:
>
>1) At TC frequencies (hundred kHz area) ohmic heating will be the primary
>concern.  The varying field will induce currents in you  (just as it does in
>any conductor), those currents will result in I^R heating. In other cases
>where people have had significant induced current dosages (typically such as
>working around aircraft on the surface of a carrier near transmitting
>antennas), the primary symptom is heating, discomfort, or sensation in the
>wrists and ankles.  The current path (capacitively coupled) is through the
>hand, then the body, then through the legs to the ground.  The wrists and
>ankles have the highest resistance (lots of bone), so the most power is
>dissipated there.
>
>2) The ANSI specs (IEEE C95.1-1991) provide a way to relate E or H field
>strength to power density.  For TC frequencies, the maximum field is 614
>V/meter (or 163 A/m for H field).  A program like Terry's Efield will allow
>you to calculate the efield around a TC.  You can also do a quick
>approximation: a 1.5 meter coil with 1 Megavolt on it (typical numbers)
>would have a field strength of 660 kV/meter (roughly 1000 times the ANSI
>limit).  However, the field drops off fast as you move away.
>
>We'll make an assumption that the field is due to the topload considered as
>a point source against an infinite ground plane.  We'll also make the
>assumption that we're interested in the E field on the ground (actually, on
>a line halfway between the point source and it's image)
>
>Call the height of the topload above the ground "H"  (That is, the distance
>between the point and its image is 2H.) Call the distance from the line
>between the topload and ground (how far you are from the coil), "R".  The
>voltage on the top load is V
>
>The E field will vary as Emax/sqrt(H^2+R^2)  (note that at R=0, the field is
>Emax=V/H)... So, using our example where the 1.5 meter high coil has a field
>of 660 kV/meter.  To get the field down to to 600 V/m, then 1/(H^2+r^2)
><.001, or roughly, R has to be 30 H... 45 m away!!
>
>But wait, there's more.. you can average over 6 minutes, and you can take
>the duty cycle into account.  A typical disruptive TC actually only has the
>peak E field for a very short time, and then has several milliseconds of
>dead time.  A duty cycle of 10% or even 1% might not be a bad estimate for a
>typical TC, when actually making sparks ( for a loaded Q of 10, at 100 kHz,
>the RMS value of the first 100 cycles (1 mSec) is 0.22 times the peak value
>of the first cycle, and an additional cycle only raises that about .001)..
>So a reasonable RMS power estimate for a synchronously fired tc at 120 Hz,
>with a 100 kHz fRes is about .026.  This means that your integrated field
>limit will be reached at a much closer distance.  1/(H^2+R^2)<.05... or R
>about 4H or 5H, that is.. 6-7 meters... a much more reasonable number.
>
>If you are running a DC coil or running a high break rate, you'll have to
>take this into account... Increasing the break rate to 1000 pps (from the
>120 pps, above), would significantly increase the RMS field.
>
>The other issue to consider (if you want to flog the deceased equine) is the
>field changes due to the sparks.  And, a coil that isn't breaking out could
>have significantly higher RMS fields (because the Q is much higher without
>the load of a spark, so the duty cycle is higher...) This might account for
>the anecdotal observation of more RFI when the coil is out of tune......
>more power is actually radiated, instead of being dissipated in the spark.
>
>Note also that if there is a conductive membrane between the observer and
>the coil (i.e. a chicken wire mesh) then all these field strength
>calculations go out the window....
>
>
>3) The other part of the ANSI  MPE (maximum permissible exposure)  limits
>induced and contact RF currents to 450f mA (*2 for both feet) (f is
>frequency in MHz), for f<100 kHz, and a flat limit of 45 mA for frequencies
>over 100 KHz.  I haven't done the calculations yet on induced currents, so I
>don't have any feel for whether this is a significant limit.
>
>
>
>
>
>
>
>
>Jim Lux
>phone:818/354-2075  fax:818/393-6875
>Spacecraft Telecommunications Equipment Section
>Jet Propulsion Laboratory   M/S 161-213
>4800 Oak Grove
>Pasadena CA 91109
>
>