Re: RF safety

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

Hi Jim,

	MANY thanks for this wonderful insight!!!  It will take me a day or so to
absorb all of it's implications, but the info is priceless!!

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
>
>