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.


(arranged alphabetically)

Chip Atkinson

Ed Phillips

Mark S. Rzeszotarski, Ph.D.

R.W. Stephens

>>>>>(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


1.0) Electrical Hazards, Fuses and Safety Switches

2.0) Burns

3.0) Induction Field Effects

4.0) Ozone, NOx, 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:

  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 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.


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.


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.


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 involuntary contractions.


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.

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.....

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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.

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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.

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4.0) Ozone, NOx, and Vapors

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.

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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 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!


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, nonetheless.


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.

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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.

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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.

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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 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!

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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?

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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!

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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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