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Safety sheet, revised
Greetings,
Mark Rzeszotarski was kind enough to put in a bunch of work to enhance the
safety sheet.
Here is the latest version. Note that I haven't yet incorporated the
suggestions that some of you sent in.
Chip
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
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:
A. Never adjust tesla coils when the power is turned on.
B. High voltage capacitors may hold a charge long after power
is turned off. Always discharge capacitors before
adjusting a primary circuit.
C. Make sure the metal cases of transformers, motors, control
panels and other items associated with tesla coils are
properly grounded.
D. 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.
E. 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.
F. A safety key should be used in the low voltage circuit to
prevent unauthorized use.
G. 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!
H. Never operate a tesla coil in an area where there is
standing water, or where a significant shock hazard
exists.
I. Do not operate a tesla coil when pets or small children
are present.
J. 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 usually 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 (I^2 x R), 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 I^2 x
R 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 know 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, and 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.....
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.
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.
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.
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.
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.
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.
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!
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?
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!
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.
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