TESLA COILS SAFETY INFORMATION
This document is provided to assist the amateur in understanding
the
significant dangers associated with tesla coils.
Disclaimer: The authors of this document are amateurs, not
professionals. The safety information provided in this document
should
be interpreted with this distinction clearly in mind. The authors
hereby
disclaim any liability for injury to persons or property that
may result
due to the construction and use of tesla coils and other high
voltage
apparatus. This document is for informational purposes only,
and makes
no claims to its completeness or accuracy. While many of the
dangers
associated with the construction and the use of tesla coils have
been
pointed out in this document, other potential hazards may exist.
Tesla
coils are inherently very dangerous devices and should only be
constructed and operated by individuals familiar enough with
these
dangers.
CONTRIBUTORS
(arranged alphabetically)
Chip Atkinson
Ed Phillips
Mark S. Rzeszotarski, Ph.D.
R.W. Stephens
>>>>>(Many others have contributed . . . Who
are you?)
REVISION HISTORY
Version 1.0 - original posting 27-July-1996
Version 1.1 - 4-August-1996 rearranged text, added disclaimer,
general
cleanups and a few new sections
CONTENTS
1.0) Electrical Hazards, Fuses and Safety Switches
2.0) Burns
3.0) Induction Field Effects
4.0) Ozone, Nitrites, and Vapors
5.0) Ultraviolet Light and X-ray Production
6.0) Radio Frequency Interference
7.0) Fire Hazards
8.0) Chemical Hazards
9.0) Explosion Hazards
10.0) Noise Hazards
11.0) Neighbors, The Spouse, and Children
12.0) Other
------------------------------------------------------------------------
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1.0) Electrical Hazards, Fuses and Safety Switches
Tesla coils use high voltages, and the risk of death or injury
is
significant. The following general guidelines are suggested:
1.Never adjust tesla coils when the power is turned on.
2.High voltage capacitors may hold a charge long after power
is turned
off. Always discharge capacitors before adjusting a primary circuit.
3.Make sure the metal cases of transformers, motors, control
panels and
other items associated with tesla coils are properly grounded.
4.Make sure that you are far enough away from the corona discharge
so
that it cannot strike you. Do not come in contact with metal
objects
which might be subject to a strike from the secondary.
5.The low voltage primary circuit is extremely dangerous! These
voltages
are especially lethal to humans. Make sure these circuits are
well
insulated so users cannot come in contact with the A.C. line
voltage.
6.A safety key should be used in the low voltage circuit to prevent
unauthorized use.
7.Use adequate fusing of the primary power and/or circuit breakers
to
limit the maximum current to your control panel. Do NOT count
on your
home circuit panel to provide adequate protection!
8.Never operate a tesla coil in an area where there is standing
water,
or where a significant shock hazard exists.
9.Do not operate a tesla coil when pets or small children are
present.
10.Spend some time laying out your circuits. Hot glue, electrical
tape
and exposed wiring are quick and easy, but could be lethal.
Information about electricity and humans
Lightning kills about 300 people each year in the United States,
and
injures an additional three to four times this number. (Sorry,
I have no
data for the rest of the planet.) More than one thousand people
are
killed each year in the U.S. due to generated electric current,
and
several thousand more are injured. (This would include potential
tesla
coilers.) In the case of lightning, the voltage and current are
extremely high, but the duration is short. The current tends
to flow on
the outside of the body and may cause burns, respiratory arrest
and/or
cardiac arrest. Many die from lightning due to respiratory arrest
rather
than cardiac arrest. (The portion of the brain controlling breathing
is
often severely affected in a lightning strike.) Power line deaths
usua
lly involve lower voltages and currents, but the duration may
be
significant. Often the current flows inside the body, causing
deep burns
and cardiac arrest. Frequently, the individual cannot let go
of the
power source due to involuntary muscle contraction. The brain
and heart
are the most sensitive organs. The dose response for animal and
human
data suggest the following: for less than 10 mA hand to foot
of 50-60
cycle line current, the person merely feels a "funny"
sensation; for
currents above 10 mA, the person freezes to the circuit and is
unable to
let go; For currents of 100 mA to one ampere, the likelihood
of sudden
death is greatest. Above one ampere, the heart is thrown into
a single
contraction, and internal heating becomes significant. The individual
may be thrown free of the power source, but may go into respiratory
and/or cardiac arrest.
Six factors determine the outcome of human contact with electrical
current: voltage, amperage, resistance, frequency, duration and
pathway.
I will discuss each individually.
Voltage
Low voltages generally do not cause sudden death unless the
external
resistance is low (so don't fire up your coil in wet areas).
As the
voltage is increased, more and more current passes through the
body,
possibly causing damage to the brain, heart, or causing involuntary
muscle contractions. Perhaps 100-250 volts A. C. is the most
lethal
voltage, because it is high enough to cause significant current
flow
through the body, and may cause muscles to contract tightly,
rendering
the victim incapable of letting go. Lower voltages often are
insufficient to cause enough current flow, and higher voltages
may cause
the victim to be thrown clear of the hazard due to the particularly
fierce involuntary muscle contractions. Arcing may occur with
high
voltages, however. Naturally, burns become more severe as the
voltage is
increased.
Amperage
Greater amperage means greater damage, especially due to heating
within
tissues. As little as 10 microamps of current passing directly
through
the heart can cause ventricular fibrillation (heart muscle fibers
beat
out of sync, so no blood is pumped) and cardiac arrest. Because
of the
air filled lungs, much of the current passing through the chest
may
potentially pass through the heart. The spinal cord may also
be
affected, altering respiration control. 100-1000 milliamperes
is
sufficient to induce respiratory arrest and/or cardiac arrest.
Thermal
heating of tissues increases with the square of the current (I2R),
so
high current levels can cause severe burns, which may be internal.
Resistance
A heavily callused dry palm may have a resistance of 1 megohm.
A thin,
wet palm may register 100 ohms of resistance. Resistance is lower
in
children. Different body tissues exhibit a range of resistances.
Nerves,
arteries and muscle are low in resistance. Bone, fat and tendon
are
relatively high in resistance. Across the chest of an average
adult, the
resistance is about 70-100 ohms. Thermal burns due to I2R losses
in the
body can be significant, resulting in the loss of life or limb
long
after the initial incident. A limb diameter determines the approximate
"cross section" which the current will flow through,
(for moderate
voltages and low frequencies). As a result, a current passing
through
the arm generates more temperature rise and causes more thermal
damage
than when passing through the abdomen.
Frequency
The "skin effect" also applies to a human conductor,
and as the
frequency gets above about 500 kHz or so, little energy passes
through
the internal organs. (I unfortunately have little data in the
50-250 kHz
range, where we operate most tesla coils. I'll check another
reference I
have at home.) At a given voltage, 50-60 A.C. current has a much
greater
ability to cause ventricular fibrillation than D.C. current.
In
addition, at 50-60 Hz, involuntary muscle contractions may be
so severe
that the individual cannot let go of the power source. Higher
frequencies are less able to cause these involuntary contractions.
Duration
Obviously, the longer the duration, the more severe the internal
heating
of tissues. Duration is particularly a problem when working with
110-240
volts A.C., which can render the individual incapable of letting
go.
Pathway
If the current passes through the brain or heart, the likelihood
of a
lethal dose increases significantly. For example, hand to hand
current
flow carries a 60% mortality, whereas hand to foot current flow
results
in 20% overall mortality. Be aware that foot to foot conduction
can also
occur, if a high voltage lead is inadvertently stepped on or
if
grounding is inadequate.
Electrical Precautions
Obviously, the A.C. line voltage, the high voltage transformer
and the
high voltage R.F. generated by a tesla coil are each potentially
lethal
in their own unique ways. One must always respect this extreme
danger
and use high voltage shielding, contactors, safety interlocks,
careful
R.F. and A.C. grounding, and safe operating procedures when working
with
coils. A safety key to prevent inexperienced operators from energizing
a
coil is essential. High voltage capacitors can also retain lethal
energies (especially in the "equidrive" configuration)
and should always
be grounded before adjusting a primary. Whenever possible, have
a buddy
around to assist you. Place one hand in your pocket when near
electrical
components so the current won't pass through your chest, and
use the
back of your hand to touch any electrical components so you can
let go
if it happens to bite you. Remember that most deaths are caused
by
regular 110 A.C. power! Never perform coiling when overtired
or under
the influence of mind altering drugs. Watch a tesla video instead!
More Tesla coils electrical danger information
The previous article mentioned some of them in a general electrical
hazard context, while this article will attempt to discuss the
dangers
from a tesla coil point of view.
Electrical Dangers
Exposed wiring on transformers. Most transformers have exposed
high
voltage lugs.
Most neon sign transformers that I have seen used for tesla
coil usage
have exposed lugs. A 15000 volt transformer has a turn ratio
of 125:1
(assuming 120 volts in). If you haven't disconnected your input
power
from the source (unplugged your variac), you may be in for a
surprise. A
variac that is putting out two volts will give you a 250 volt
shock if
you touch the high voltage outputs of the neon sign transformer!
Pole pigs (also known as distribution transformers, such as
the one that
is probably hanging on a utility pole near your home) have the
same
dangers as mentioned above, as well as having much more current
available. At the output voltage of a pole pig, the current that
can go
through you is not really limited by anything other than the
current
regulation that you attached to the pig.
Once I shocked myself with one end (7500 volts) of a 60 mA.
neon sign
transformer. I just brushed against an exposed end, so I wasn't
gripping
anything. It was quite painful, much more so than touching a
sparkplug
wire. I felt the path of the current follow my arm, and go down
my leg.
Keep one hand in your pocket when working near or with charged
items.
(Capacitors, secondary coils, etc.)
Richard Hull's "Tesla Coil Primer" tape has some
excellent safety
suggestions in it, is entertaining, informative, and well worth
the
money. One of his best suggestions is the one of holding the
power plug
to the power transformer in your hand whenever you are putting
your
hands around the circuit.
The transmission line between your high voltage transformer
and your
tesla coil is another potential source of electrocution. This
should be
constructed using neon sign wiring (rated to 40 kV) or thick
coaxial
cable like RG-8A/U or RG-11A/U. If using coaxial cable, use the
inner
conductor for the high voltage, and strip back the outer braid
about
6-12 inches from each end. Connect one end of the braid to your
RF
ground. Leave the other end unconnected so it does not form a
current
loop. Some coilers also place their high voltage cables inside
a plastic
conduit, which is laid on the floor. This also protects the cable
somewhat from strikes.
Charged capacitors
"Equidrive" systems will almost always have a residual
charge remaining
on the capacitor when the system is turned off. The "equidrive"
system
uses two capacitors in the primary coil circuit. The gap is across
the
transformer, and the capacitors extend from the gap to each side
of the
primary coil. Even with the gap shorted, the capacitors can hold
a
lethal voltage. If you use this configuration, make yourself
a shorting
rod using a piece of copper tubing or wire with an insulating
handle
attached, and always short out each capacitor at the end of each
run,
and again each time you plan to touch the primary system.
Capacitors can also build up a residual charge from electrostatic
sources.
Capacitors have been known to accumulate a charge from various
sources
such as static electricity and electric fields. IF YOU STORE
A
CAPACITOR, STORE IT WITH A WIRE ACROSS THE TERMINALS. (MAKE SURE
YOU
DISCHARGE THE CAPACITOR BEFORE PUTTING THE WIRE ON!!!)
Capacitors can "regain" charge from dielectric "memory".
The dielectric
in a capacitor is put under electrical stress during use. During
operation, this stress may cause the molecules in the dielectric
to
orient themselves in such a manner that they store this charge
in their
structure. The charge remains after the capacitor has been discharged.
Later the molecules return to their original states and the charge
that
they "captured" ends up on the plates of the capacitor.
This charge is
then available to shock you.
Other sources of danger
You are literally playing Russian Roulette when you stick
a hand held
metal rod into the output streamer of your coil running at 3kvA,
while
standing on a concrete floor!!! When you start running these
kind of
power levels (or even less) some coils have a tendency to form
a corona
or even send a streamer down to their own primaries every once
in a
while. A grounded strike ring is often added around the primary
to try
to prevent this self striking streamer from hitting the primary
coil and
thus introducing a high voltage pulse into the 'bottom end electronics'
where it could do damage to components. These strike rails are
not 100%
effective. The streamer can still, and sometimes does strike
a point
downstairs that is part of the LETHAL high voltage 60 Hz circuitry.
When
such a contact is made, any person also connected to a corona/streamer
link to the secondary at the same time will, via the ionized
air path,
become connected to lethal 60 Hz mains current. You could try
the trick
you described standing on the cement floor in your tennis shoes
half a
dozen times and live, or be killed the very next time you try
it. The
fact that the bottom of your secondary is tied to ground will
not save
you!
If you isolate your own body well away from the floor and
any other
potentially conductive objects in the vicinity, such as sitting
or
standing on an elevated insulated platform (I would NOT consider
a
plastic milk crate adequate!), then you will probably survive
if 60 Hz
is introduced into the streamer you are in contact with by the
mechanism
described above. However, in setting up this insulated platform
you must
consider the path that may be taken from streamers that will
re-emerge
from your body and head off looking for other targets, which
could
result in direct contact with earth ground again.
In a safety warning I have about the potential hazards of
Tesla coils
mention is made of a stage lecturer while demonstrating how he
could
cause long sparks to come out of his fingers (by standing on
a specially
constructed coil), was electrocuted when the discharge created
an
ionized path to grounded overhead pipes supporting stage back
drops, and
the lower voltage but far more deadly 60 cycle current passed
through
his body along that path. The name of this lecturer is believed
to be
Transtrom.
I was dinking around once with a vacuum tube coil drawing
15 inch
streamers to a hand-held, 10 megohm metal film porcelain resistor
about
a foot long while standing on a carpeted, elevated wooden floor
in
composition rubber soled dry shoes. I inadvertently got the resistor
too
close to the primary tank coil (the top end directly connected
to the 3
kilovolt output of the plate supply transformer) and the high
voltage RF
closed a path to the primary. I felt an uncomfortable 60 Hz shock
through my entire body. Had that resistor been a solid metal
rod I would
have experienced a very painful jolt or worse, and had I been
standing
on a cement floor, I'd probably be 'worm food'.
I think the danger of electrocution is just as real by making
contact
with a hand held florescent lamp tube, as any solid conducting
metal
object.
I cringe when I hear of some body contact stunts proposed
by people on
this list! The potential (no pun intended) for death is very
real. Be
EXTREMELY careful!
Another viewpoint
The 60 cycle side of things is where electrocution can happen.
Keep well
away from any 60 cycle leads, use grounds and cages as appropriate.
Bear
in mind that if a radio frequency arc starts from a place which
also has
60 cycles on it (one side of a primary circuit, for example)
there is
the possibility of high-current 60 cycle conduction along the
ionized
path. That could be deadly.....
Back to contents
2.0) Burns
Tesla coils can cause burns, especially due to RF discharges
from the
secondary. Stay out of the immediate vicinity of a tesla coil.
Remember,
if you do get zapped by a large coil system, the heating effects
may be
mostly internal, causing lasting damage! Also remember that spark
gaps
and rotaries get hot and are a potential source of burns.
Back to contents
3.0) Induction Field Effects
Tesla coils operate in a pulsed mode, and strong electric
and magnetic
fields are locally produced. In addition, significant amounts
of RF may
be produced if the grounding is poor, or before spark breakout.
This can
result in induced currents in other conductors, like test equipment,
nearby computers and electronics, and metal structures in the
facility.
The end result is generally bad. Turn off computers and sensitive
test
equipment, and move it away from the vicinity of your coils.
If you
foolishly choose to use your house electrical ground as your
RF ground,
you are asking for trouble. Currents may be induced anywhere
in the
building, and voltage standing waves along the wiring may destroy
electronics far from the coil location. Construct a dedicated
RF ground,
and make sure it is properly connected before firing any coil
of
substantial size.
Fire from other induced currents.
Tesla coils are good at inducing currents. Beware of metal
things that
are connected to the same ground as a tesla coil. For example,
I run my
coil in my garage, which has a wooden door on metal tracks. The
tracks
are against the concrete floor, and near the strap that serves
as a
ground for my coil. When the coil operates, it causes sparks
to jump
between the running hardware of the door and the tracks.
Static charges
During the operation of the tesla coil, significant static
charges can
build up on the secondary. If you need to move the secondary
(say you
are adjusting the coupling), you may get a nasty zap right across
your
chest when you pick it up with both hands. Before you touch the
secondary, wipe it lightly with a grounded wire. You can sometimes
hear
the crackling as you do so. Besides the shock hazard, there is
the
physical hazard caused by the shock. You will likely drop the
secondary
or jump onto something that isn't soft.
Hazards to electronics
Strikes to house electrical ground -- also goes to power(?)
A tesla coil
must be connected to a ground that is separate from the house
ground or
water pipes. Connecting your coil to either of these grounds
is a recipe
for disaster. Notice that your stereo, computer, VCR, etc., have
three
prong plugs. Also, note where your telephone box is grounded.
It is
likely grounded to the water pipes.
Consider what happens when your coil strikes the grounded
strike rail,
or an unexpectedly long spark that hits an electrical receptacle.
That
enormous voltage at high frequency will now be connected to the
grounds
of all your electronic goodies or your telephone. Furthermore,
a spark
is a conducting path in the atmosphere. By creating this path,
you open
your electrical system up to connections among the 120/220v house
system
and ground.
Strikes to garage door opener rails. Since many people do
their coiling
in the garage, this topic deserves individual consideration.
If you have
a garage door opener, or are installing one, you should put in
a
mechanism, such as a switch or plug and socket, that allows you
to
disconnect the opener from the house power.
My garage door got zapped by my coil. The door is connected
to the
opener track so the opener got zapped too. The strike caused
the opener
to attempt to open the already open door. Since the door couldn't
go any
further, the opener started binding. I was able to unplug the
opener and
keep the thing from smoking.
More than one person on the list has replaced their opener
as a result
of their coiling activity. Be warned of the dangers to the equipment.
An
untested suggestion is to put a grounded wire underneath the
rail and
opener to draw the sparks to the wire.
Electric fields inducing currents and killing sensitive meters.
Oddly
enough sensitive meters and measuring equipment are just that
--
sensitive. Solid state instruments are much more susceptible
to damage
from being near tesla coils than are vacuum tube items. Consider
purchasing a cheap volt-ohmeter (VOM) with an analog meter movement.
If
will survive in places many digital units will not. A used vacuum
tube
oscilloscope is also more likely to survive the tesla coil environment
and can be obtained cheaply at hamfests.
Good electrical practice
Place your coil in a location that will prevent the strikes
from hitting
electrical outlets, people, animals, and sensitive electrical
equipment.
Turn off and unplug computers in your house.
Back to contents
4.0) Ozone, Nitrites, and Vapors
A sparking tesla coil produces ozone, nitrites, and probably
a host of
other potentially toxic substances. Do not operate a large coil
in an
enclosed area for long periods of time. Make sure ventilation
is
adequate at all times. There have been anecdotal references to
people
becoming ill due to ozone toxicity. The long term bioeffects
are
unknown. (On the other hand, it does help out the ozone layer!)
When
constructing secondaries, use adequate ventilation when coating
coils
with varnish, etc. Some of these materials are also quite toxic.
The
flux from solder is also potentially hazardous.
Back to contents
5.0) Ultraviolet Light and X-ray Production
Ultraviolet light may be produced by the spark gap during
operation of a
tesla coil. The human eye has no pain sensors within it, so the
bioeffects are felt later, when it is too late. (Ever look at
the sun
for a while, or watch a welder at work?) The light produced in
a spark
gap is essentially identical to that produced by an arc welder,
containing substantial amounts of hard ultraviolet light. As
any
professional arc welder will tell you "Don't Look At The
Arc!" Spark
gaps produce a large amount of UV and visible light. The visible
light
is extremely bright, and the ultraviolet light will damage your
eyes,
and can cause skin cancer. The arc is so bright that you couldn't
make
out any detail anyway, so why bother? If you must study your
spark gap,
use welder's glasses. Generally, it is not too difficult to rig
up a
piece of plastic, cardboard, etc. that will shield yourself and
others.
X-rays
X-rays can be produced whenever there is a high voltage present.
Although a number of coilers have tested their coils for x-ray
radiation
and found none present that is not to say that x-rays cannot
be
produced, especially if vacuum tubes, light bulbs, and other
evacuated
vessels are placed near a coil. Here is a little information
about
X-rays.
X-ray Production
A number of vacuum tubes work pretty well as X-ray tubes,
and several
articles have appeared in Scientific American magazine in the
distant
past. X-rays are typically produced by slamming electrons into
either
the nuclei or inner shell electrons of atoms. The source electrons
are
usually boiled off a heated filament (cathode), and accelerated
toward
an anode via some large potential difference, typically 25-150
kV in the
medical world. Basically, any time the voltage gets above 10
kV, there
is a significant risk of X-ray production, and the risk increases
with
increasing voltages. You can also get some X-ray production via
field
emission, whereby electrons escape a cold metal due to very high
local
electric fields (the Schottky effect). This was probably the
type of
emission obtained by an amateur described recently on the list.
For the
remainder of this discussion I will limit my comments to conventional
X-ray tubes, using a filament and anode, although most of it
applies to
both forms. The target or anode is normally a high atomic number
material like tungsten. X-ray production is relatively inefficient,
so
most of the energy is wasted as heat (typically about 99% with
good
X-ray tubes). Tungsten works well because of its high melting
point (to
absorb all that wasted heat energy). If the potential difference
between
the anode and cathode is +100 kV D.C., a spectrum of X-rays will
be
produced with energies from zero to 100 keV. The graph of the
number of
X-rays produced (y-axis) versus X-ray energy (x-axis) has a negative
slope with a Y=0 point at x = 100 keV. Hence, many more low energy
X-rays are produced than high energy X-rays. Some of these low
energy
photons are absorbed by the tube housing. In a clinical X-ray
machine,
the tube is placed in a leaded shield with a window (hole) in
it for the
X-rays to escape through. This window has a piece of aluminum
over it to
further attenuate the low energy X-rays. In conventional equipment,
the
tube, housing and aluminum filter accounts for about 2.5 - 3.5
mm of
aluminum equivalent material in the exit port. This effectively
knocks
out most of the low energy (<10 keV) radiation, which would
be absorbed
in the patient and could not contribute to producing an image
anyway.
X-ray Absorption
High atomic number materials readily absorb x-ray radiation.
There is an
energy dependence here, as high energy X-rays are more penetrating
than
low energy x-rays. For example, the percentage of radiation which
will
pass through 10 cm (about 4 inches) of water is 0.04% at 20 keV,
10% at
50 keV and 18% at 100 keV. Compare this with 1 mm of lead (about
0.04
inches), which transmits 0.02% at 50 keV and 0.14% at 100 keV.
The human
body absorbs X-rays pretty readily (similar to water), but becomes
more
transparent as the energy of the X-ray increases. That is why
we use
50-150 keV for many clinical procedures. The low energy X-rays
are
filtered out of the spectrum before they enter the patient, usually
through the use of an aluminum filter, which lets the high energy
X-rays
pass through with little attenuation (except possibly to give
you enough
contrast to see what you want). Most of the x-rays are absorbed
in the
patient, with 1-5% exiting the patient typically. Low energy
X-rays
(0-15 keV) are totally absorbed in human skin near the skin surface,
and
would contribute substantially to patient dose if allowed to
reach the
patient. This is to be avoided in general!
Shielding
The best materials are lead or depleted (nonradioactive) uranium.
Concrete and steel also work pretty well. Aluminum is a poor
absorber of
radiation, unless the radiation is very low in energy. Most plastics
are
similar to water in attenuating properties (quite poor).
Hazards
X-rays are capable of producing ionizations, which means that
the
electrons can be stripped off of atoms when an x-ray is absorbed
in a
material. This results in the production of chemically reactive
free
radicals, and the direct disruption of chemical bonds. This is
generally
bad in humans, causing cancer, leukemia cataracts, etc. However,
due to
natural background radiation levels, humans have built in radiation
repair mechanisms and can handle low doses of radiation quite
well.
Bio-effects are not generally observed for doses of less than
25 rem.
Skin reddening occurs with doses of around 300 rem or so. Natural
background radiation levels typically contribute 0.2 - 0.5 rem
per year.
Most regulatory agencies recommend no more than 0.5 rem per year
above
background radiation levels for the general public. Occupational
radiation workers can get 5 rem per year above background. The
radiation
from a well designed X-ray tube can be as high as 10-50 rem per
minute
of exposure, at a distance of 1/2 meter. The radiation source
acts like
a light bulb, decreasing in intensity via the square law with
distance.
Hence, don't stand close to a possible radiation source, use
adequate
shielding and minimize the exposure time. Incidentally produced
radiation from metal objects other than X-ray tubes will generally
be at
much lower production levels, but should be avoided, nonetheless.
Regulations
In the U.S. the individual states regulate X-ray machines.
They
generally keep close tabs on clinical and industrial X-ray machines
and
aren't too impressed to see them in the hands of people without
the
appropriate licenses. If you happen across an old X-ray tube,
you might
consider releasing the high vacuum inside (very carefully, please)
so
that it is inoperable, and a little safer to handle for show
and tell
(and much more acceptable to the regulators). This can be done
by making
a small hole in the glass envelope with a file, keeping the tube
wrapped
in a large quantity of towels for implosion protection during
the
process. (It goes without saying that you should always have
your
favorite towel handy anyway [for you Doug Adams fans]).
Monitoring
At this point I presume you are wondering how to tell if that
great
apparatus in your basement or garage is producing X-rays. There
are
several ways to tell. First, go look for a surplus Geiger-Mueller
counter at your local hamfest or make friends with someone in
your local
fire department, since many fire departments have radiation survey
meters at their stations (in case we have a nearby nuclear explosion,
etc.). (Don't bother with the fire department if your apparatus
is
likely to upset them!) In addition, nearly every hospital has
a
radiation safety officer who is likely to be more than willing
to take a
look at your toys, and will bring a radiation survey meter along.
The
standard method for monitoring radiation dose is via film badge
and/or
thermoluminescent dosimetry monitors, but these are not all that
useful
to the experimenter since they must be mailed back to the dosimetry
lab
for reading. In general, film is quite insensitive to radiation,
and is
of limited value in the experimenters setting unless you can
leave the
equipment on for a long time to get adequate exposure. Cloud
chambers
are great fun and can detect a variety of radiation particles,
but get
easily overwhelmed by devices that put out even low radiation
levels. If
you don't expect any radiation but still want to check, a cloud
chamber
can be used. Buy a thorium doped lantern mantle at your local
camping
store to use as a radiation check source to make sure your chamber
is
working okay before you power up your equipment. Another possibility
is
to construct an electroscope and place it near your apparatus.
An
electroscope measures the amount of charge using two thin metal
foils
which are charged up to a high potential, causing them to swing
apart
due to repulsion of like charges. Radiation ionizes the air in
the
electroscope chamber, causing a loss of charge on the foils.
Naturally,
this type of equipment has limited utility in the direct vicinity
of
high voltage equipment if electric fields are significant.
X-rays and Tesla Coils
I have monitored my various tesla coils using a number of
different
radiation instruments and have not seen measurable radiation
levels. My
coils produce 3 to 5 foot sparks in magnifier and conventional
forms
using up to 15 kV input, with power levels of no more than 1.5
kVA.
Obviously, you don't want to get a survey meter too close to
an
operating tesla coil.
Finally, always keep safety in mind with all of this equipment.
Humans
are not able to sense X-ray and ultraviolet radiation. If you
think you
are producing some, use an appropriate instrument to find out
for sure.
Back to contents
6.0) Radio Frequency Interference
Tesla coils are generally inefficient as antennas go, but
can still
produce a fair amount of RF, especially if operated with a large
top
capacitance, before spark breakout. Significant quantities of
RF can
also be produced if the RF grounding is inadequate. This can
cause
interference with TV's, radios, and other electronics. If you
note
interference, try to improve your ground first, since that is
likely
where your problem is. In addition, every tesla coil should be
wired
with a power line conditioner in series with the primary circuit.
These
are relatively inexpensive and are very effective in keeping
RF out of
the house wiring.
Legal dangers
In the United States, RF transmitters are regulated by the
Federal
Communications Commission ( FCC), and they generally aren't keen
on any
type of RF interference. They have specific rules which prohibit
the
operation of spark gap type damped oscillators, dating back to
the early
days of radio. Make sure you operate your coil with a good RF
ground. If
interference still exists, construct a Faraday cage from chicken
wire or
similar material, which should eliminate the interference.
Other Comments
When I first got interested in tesla coils, I called the FCC
to ask
about the legal aspects of coiling. While the man that I talked
to
wasn't too sure about the potential interference, he did say
that
modulation of the output is definitely illegal. Of course, if
you shield
your coil from emitting RF to the outside world, you can do anything
you
like.
Try to be aware that your coil may cause various interference
problems.
If you know about any, take care to eliminate them if possible
before
they figure out who caused it.
7.0) Fire Hazards
The danger of fires is substantial with tesla coils! Make
sure you have
a functional fire extinguisher designed for fighting electrical
fires
handy. Fires can be caused by an overheated spark gap, equipment
failure
(e.g., shorted transformer), corona discharge, induced currents,
to name
a few causes.
Fire starting from sparks to flammable points. The sparks
from a tesla
coil are hot. Depending on where they strike, these sparks can
cause a
fire. Richard Hull has captured fires caused by sparks from his
coils on
video tape. (This was due to a failed power line conditioner.)
Be sure that when you run your coil, that there are no flammable
substances around. For example, gas cans (e.g., for a lawnmower),
ammunition, sawdust, fireworks, etc. Walls and ceilings can also
be
ignited, so keep the fire extinguisher handy.
Gasoline on premises (mowers, etc.) Without a spark, what's
a tesla
coil? What's it take to ignite gasoline? Consider the location
of gas
cans, lawnmowers, etc. when operating your coil. Remember that
when you
operate your coil, it's usually in the dark with plenty of exposed
high
voltage wires. Not a good combination for fighting a fire in
your
garage.
In addition, most coilers use polyethylene and other plastics
in
constructing their coils, capacitors, and other equipment. These
plastics ignite at relatively low temperatures, and produce large
quantities of toxic smoke.
Back to contents
8.0) Chemical Hazards
Old capacitors and transformers often used PCB oils for insulation.
This
oil is a known carcinogen. Similarly, the materials used to coat
coils
(e.g., varnish) may contain hazardous chemicals. Consult a Material
Safety Data Sheet (MSDS) for any materials you have questions
about.
(Many of these are available via Internet. Use your favorite
Web search
engine with the key word MSDS'.) Some forms of solder contain
lead,
which is also generally bad for humans.
Back to contents
9.0) Explosion Hazards
Explosions can and do occur with tesla coils! The rotary gap
and
capacitors are the most frequent culprits, but nearby flammables
are
also at risk.
Rotary gaps
During operation, rotary gaps spin at high speeds. The spinning
rotor or
disk is subjected to tremendous force. At a modest 3600 RPM,
the
periphery of a 10" disk is subjected to a force of 1835
G's. A 5 gram
(0.011 lb) 1/4-20 brass acorn nut used as an electrode will exert
a
force of over 20 pounds. The peripheral speed of the 10"
disk is 107
MPH. At 10000 RPM, the edge of the disk is running at about 300
MPH!
All these numbers translate into one thing: Danger.
The best way to guard against this danger is to shield the
rotor and
build the entire system carefully and take pains to balance it.
The
shielding must be nearly bullet proof (literally). Lexan (polycarbonate)
is an excellent plastic for shielding. It is non-conductive,
strong, and
tough. Consult with your plastics dealer to determine what thickness
you
need.
Capacitors
Capacitors are great at releasing energy very quickly. The
explosion
danger in a capacitor occurs when it shorts out and suddenly
produces a
large volume of hot vaporized gas. Since capacitors are usually
in an
airtight container, the volume of gas will cause the container
to
explode, sending pieces of solid cap guts and oil all over.
One recommended method of shielding capacitors is in an HDPE
(High
Density PolyEthylene) pipe. These pipes are used in the pyrotechnics
industry as mortars because of their strength and the fact that
they
don't create shrapnel as steel or PVC pipes do.
Also, avoid storing gasoline or other flammables near a tesla
coil!
Back to contents
10.0) Noise Hazards
Tesla coils produce a lot of noise, and large coils can damage
one's
hearing. Go to your local gun shop and buy ear protection if
you operate
large coils.
One type of spark gap, the air blast gap, produces a loud
noise. Buy and
use a set of ear muffs or ear plugs. There are a wide variety
of types
of ear plugs and muffs, so you will likely find one that works
well and
is comfortable. I prefer the roll up foam type myself. If you
are on a
tight budget (blew all the $$$'s on the pig), you can wash the
foam ear
plugs. Just put them in a pants pocket (one that closes is best)
and run
the pants through the wash. Works great.
When a coil is in tune, you will notice a dramatic increase
in the noise
level as it sparks. This noise is loud enough that it can damage
hearing. See the warnings in the previous paragraph.
Hearing is important -- how will you tell if your teenager
is mocking
you behind your back without it?
Back to contents
11.0) Neighbors, The Spouse, and Children
While the beauty of a tesla coil firing outside is something
to behold,
often your neighbors will not see it that way, and your local
police
will make a personal house call. Be cognizant of your possibly
unreasonable neighbors, and do your work inside if possible,
or invite
them over and explain things before you start. Attitudes are
a lot
different if a little common sense is used first.
Coils are noisy
Please consider your neighbor's sleep habits.
Remember the following:
¨ For new parents, sleep is the most precious commodity
that they have.
¨ Not everyone works 8am to 5pm.
¨ Not everyone is tolerant or nice.
A potential secondary hazard would be from enraged neighbors
if radio or
TV interference was generated often enough to be a nuisance,
and said
neighbors could trace it to its source. Good citizenship will
solve this
problem (or a large building with a good RF ground and a batch
of power
line filters).
Kids, small pets
Kids and small pets are quite curious, innocent, and ignorant.
(Note the
similarity!) Their judgment isn't the greatest either. If you
have
children and they have access to your coil, install some sort
of key
lock on your power cabinet, variac, or whatever. Killing or injuring
a
child or pet, be it yours or neighbors, will most likely be the
worst
thing that will happen to you in your life.
The Spouse
Another potential hazard is if the spouse thinks one is spending
too
much time on his or her hobby. ANY HOBBY!!!! Expect the wife
to not
understand!
Back to contents
11) Other
Whenever possible, have a buddy assist you. Most coilers prefer
to
operate their coils with the lights off, which is inherently
dangerous.
This situation can be improved by having an assistant around
to operate
the lights and/or power switch. Also, have your buddy learn CPR,
and
post your local emergency telephone numbers, just to be safe.
The layout of your apparatus is also a safety consideration.
Many
coilers throw their systems together using electrical tape, hot
glue,
and assorted bits of plastic. If things move around a bit during
firing,
the risk of something bad occurring is increased significantly.
Spend a
little time to construct yourself a nice power cabinet with a
safety
switch, and construct a safe high voltage transmission line to
your
coil.
Drinking and coiling can be lethal! If you feel the need to
consume some
mind altering drugs, watch a tesla video instead! Never operate
a tesla
coil while under the influence! Quaff the ales later during bragging
hour, not when you are actually working.
Back to contents
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Nikola Tesla
Science
& Mathematics
The
Uncle Taz Library