From http://www.pupman.com/safety.htm – The origional is here
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
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:
- Never adjust tesla coils when the power is turned on.
- High voltage capacitors may hold a charge long after power is turned off.
Always discharge capacitors before adjusting a primary circuit.
- Make sure the metal cases of transformers, motors, control panels and
other items associated with tesla coils are properly grounded.
- 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.
- 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.
- A safety key should be used in the low voltage circuit to prevent
- 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!
- Never operate a tesla coil in an area where there is standing water, or
where a significant shock hazard exists.
- Do not operate a tesla coil when pets or small children are present.
- Spend some time laying out your circuits. Hot glue, electrical tape and
exposed wiring are quick and easy, but could be lethal.
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
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
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
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.
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
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.
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.
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
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.
Exposed wiring on transformers. Most transformers have exposed high voltage
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
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
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
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.
“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
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
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
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.
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.
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
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
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.
A sparking tesla coil produces ozone, nitrogen-oxygen compounds,
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.
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 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.
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.
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!
The best material is lead. 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).
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,
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]).
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.
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.
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.
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.
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
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
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.
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
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.
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 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!
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.
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?
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
Coils are noisy
Please consider your neighbor’s sleep habits.
Remember the following:
¨ For new parents, sleep is the most precious commodity that they
¨ 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
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!
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.
Mark S. Rzeszotarski, Ph.D.
>>>>>(Many others have contributed . . . Who are you?)
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