Tesla Coil Hazards and Safety

WARNING - I disclaim all warranty for the information below, including but not limited to any and all warranties of completeness of this information! I will *NOT* be responsible if you or anyone dies, gets hurt, or has any damage to real or personal property if anything goes wrong. This is regardless of whether or not I warned of whatever goes/went wrong accurately, inaccurately, or not at all! Build, test, and/or operate Tesla coils only at your own risk!

In addition to any precuations mentioned or implied below, use common sense.

General hazards of the very high voltage

UPDATE 12/2/2024: I have noticed a Tesla coil by the Eastern Voltage company as appearing to me as being risky for electrocution, even though Tesla coils small enough to have oscillation frequency over 200 KHz and/or small enough to carry in a laundry basket have a lot of history of their high voltage output being typically unable to electrocute humans.

Although the current is usually low and the high frequency involved usually does not shock, things sometimes go a bit wrong. You may draw a spark beginning near time of a secondary voltage peak and ending at a time that the secondary instantaneous voltage is near zero, which gives some net DC charge transfer to you which will shock. This may be worse if you are grounded. The shock from this is usually minimal, but there is no guarantee of safety, especially with larger coils. Maybe nobody has ever died from this yet, but there is the chance the shock may jolt you into bumping something and breaking it, or contacting more hazardous points such as the primary circuit of a spark gap Tesla coil.

Hazards of you being in the electric field

If you are near the secondary, you become a small part of a resonant circuit. The secondary has capacitance to you, and you have capacitance to ground. If you touch something grounded, you can conduct more current through that point of contact than is normally available from the top end of the secondary. I have burned myself with nasty arcs of a kilovolt or two and currents in the tens of milliamps by touching ground while being close to the secondary of a solid state Tesla coil.

Ozone and nitrogen oxides

The sparks, brush discharges and corona produced by Tesla coils can produce ozone. Ozone is bad to breathe since it can corrode lung tissue. If you are going to breathe it a few hours a day, it can be unhealthful to breathe even if the concentration is too weak to smell. Ozone can also oxidize some rubber objects. If you are having brush discharges or corona, you should operate the Tesla coil only in a very well ventillated area, or expose youself to the ozone for no more than a few minutes a day.

Sparks and arcs also produce nitrogen oxides which can also be hard on lung tissue. Nitrogen oxides can be changed to other nitrogen oxides when mixed with ozone and enough heat or ultraviolet. The worst one is nitrogen pentoxide (N2O5) which combines with any moisture to form nitric acid. Some other nitrogen oxides form acid solutions in contact with moisture to a lesser extent. The most obvious nitrogen oxide is nitrogen dioxide, which is to some extent corrosive to lung tissue and maybe rubber objects.
Nitrogen oxides may be formed in very significant quantities by the high current sparks in the spark gap of spark gap type Tesla coils. You can largely confine the production of noxious gases by putting the spark gap in a closed container. This container should be either not perfectly airtight or be capable of withstanding the pressure when the heat of the spark makes the air expand. If you put the primary spark in a glass jar, you may see the air turn orangish brown from the nitrogen dioxide. Production of nitrogen dioxide will stop when nearly all the oxygen in the jar has combined with nitrogen.

Oddball hazards

One hazard is the possibility of deep burns. Burns penetrating through the dermis or affecting internal tissue can take a really long time to heal. Such burns can easily occur from radio frequency currents since they usually produce little or no electric shock effect, and people exposed to these can underestimate the risks of these currents. Furthermore, you may be able to heat up tissue where there aren't many heat-sensitive nerve endings and can cook part of yourself before you know it!

One really oddball hazard is any effects of putting your head in a strong enough radio frequency magnetic field. This is more a hazard of solid state coils which oscillate continuously, and probably requires being at a close range (maybe almost having your head inside the secondary). The problem is effects of a sudden temperature change in your brain. This may cause a seizure; in milder cases suddenly reduced mental function. In severe cases related to point-blank exposure to powerful microwave beams from radar transmitters, people have been known to pass out and stop breathing - obviously dangerous! Severe effects can occur from a sudden temperature change of only a degree or two of brain temperature.

Hazards specific to solid state Tesla coils

Solid state Tesla coils often oscillate continuously, or even when intermittently usually do so at a higher duty cycle than spark gap Tesla coils, which means much higher average currents and somewhat higher RMS currents everywhere. The same is true of vacuum tube Tesla coils. This makes corona and brush discharges more intense, meaning more ozone. This also makes sparks, arcs, brush discharges and corona more capable of causing burns and starting fires.

Hazards specific to spark gap Tesla coils

The main hazard specific to spark gap Tesla coils is the primary circuit. This can electrocute you. Neon sign transformers and sometimes oil burner transformers are often used for these. There is some popular misinformation around that these won't kill you, but they can. The current is below the most-deadly range of 100 to 1000 mA, but people sometimes die from getting shocked by these things. And with capacitors added to this which spark gap Tesla coils normally have, the effects of getting shocked are worse.

One more thing - although having someone nearby who knows CPR or who has a defibrillator handy is good, the success rate is less than 100 percent.

Hazards to property - including oddball

You should be well aware that Tesla coils make sparks and arcs that can burn things and start fires. Sometimes, intense corona can set things on fire, and this is especially true with solid state Tesla coils. Solid state ones usually oscillate continuously or nearly continuously, and the capacitance of a puff of corona or brush discharge can draw enough current for it to be quite hot!

Conductive objects in the coil's electric field can make sparks if they touch or get very close to other conductive objects that are grounded or unequally exposed to the coil's electric field. These sparks could start fires. One fear I have is that a stray wire, pipe, pipe support strap, or a long nail behind a wall could touch some damp wood behind the wall and make a spark and then a carbonized spot and then start a fire.

Closed loops of fine wire may burn up if they are close enough to the coil to get enough magnetic field to have a lot of current flowing through.

The strong AC electric field is probably not too good for electronic equipment, especially if the equipment is running. This is particularly true with computers! They may "latch up" and have even been reported to spontaneously reboot when exposed to the electric field of a Tesla coil.

NEWS 7/6/2005 - I have caused an answering machine to falsely detect an actually nonexistent incoming call when 5 feet (1.6 meters) from a slightly large size Tesla coil that developed peak voltage of merely 40, maybe 50 kilovolts!

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Written by Don Klipstein.

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