Tesla Coil Safety

Safety in use of our Tesla coils is a subject that hasn't been talked about much. 
With the large number of newcomers to the hobby I thought this a good time to
explore this topic.

The dangers involved with operating a Tesla fall into several categories.
Lets explore each of these categories in more detail.

1. Electrocution

This is the number one danger. The voltages we are dealing with are
deadly. The neon sign transformers have voltages from 6000 - 15000 volts
and this voltage alone can kill you. Its made even more dangerous in the
primary circuit of a Tesla coil due to the storage of energy in the main caps.

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It is the current which actually kills you. But as you know
the human body has some resistance, and using ohms law you can
see that these voltages will generate a lethal current through 
the body. Here's a simple chart showing the various effects on
the body if a current of this value was placed through it.

 >= 1 milliamp    - Tingling sensation felt.
 >= 10 milliamps  - Muscle contraction, twitching, shaking, etc.
                    You will probably not be able to let go of
                    anything you are holding.
                   
 >= 100 milliamps - If this current passes through the heart for
                    one or more seconds, you will DIE. The heart
                    will go into ventricular fibrilation and will
                    be unable to pump any blood. 

If this does not seem significant to you, then realise that under
certain conditions the resistance of the human body may be as low
as 300 ohms. In this case, 30 volts can produce a LETHAL current. 

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The secondary coil output voltage is much less dangerous although it
can be deadly under the right circumstances. Its is primarily RF energy
and tends to flow along the outside of conductors like the human body.
But the output often contains a considerable 60 hz component and its this
60hz energy that is deadly.

Plus in our effort to increase the stength and length of the output streamers
from our coils we are pumping more current into those streamers. The use of large
toroids and high power make the streamers just that much more dangerous.

Often some coils have a tendency to arc down into their primary coils.
This arc can connect the secondary directly to the deadly 60hz energy in
the primary coil via the ionized air path. If you are attached to another
streamer at the same time you are effectively connected to the primary.
This could be a deadly situation.

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I have seen many Tesla coil schematics showing the base of the 
secondary coil attached to the primary coil. This is WRONG! In this
configuration, LETHAL 60 hz line current can potentially make it to
the output of your secondary. Under no circumstances should the
secondary coil come into contact with any primary component. The
secondary should be connected to a suitable RF ground instead.

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Here are a few dos and donts:

Do build a control system for your coil. House your variac into this controller
and provide proper fusing and power switches. Provide a power on indicator so
you KNOW when power is applied. A keyswitch on/off switch will also keep someone
from accidently turning on the power while your are making adjustments.
Make it a habit of keeping the key WITH YOU.

Do not make any kind of adjustments with the power on. It's easy to forget sometimes.
Make it a habit of turning the power OFF. Just turning down the variac to zero is
not good enough. Some variacs will put out 1-2 volts at their zero setting.
2 volts into a 15kv neon transformer could result in 250 volts. Enough for
a nasty shock. Never assume anything is OFF. Check for yourself.

Do use a little care in your construction. Jerry rigged setups are OK for a short
test but something a little more permanent should be a priority. Having high voltage
leads coming loose under power can be quite dangerous.

Do discharge any high voltage capacitors before working on them. In most cases
the main cap in a Tesla coil is discharged as soon as the power is removed the
path being through the  transformer winding. If your neon secondary should fail
and open up during a run a lethal charge could remain in your caps. If you 
are going to do high voltage work utilizing capacitors make and USE a shorting
wand with a 100K or more power resistor to safely discharge your caps.

Capacitors can still retain a charge after being discharged. This
effect is known as dielectric memory. And  sometimes capacitors sitting idle
can build a charge. The safest thing is to keep all HV caps shorted when
not in use.

2. Ozone

Ozone is classified as a health hazard at quite low concentrations.
If you can smell it the concentration is already well above safe limits.
The smell is a slightly sweet bleach type odor. You may also notice some stronger
biting odors. These are caused by various oxides of nitrogen. These are even more
noxious than ozone. These gases are produced in great quantity by our spark gap
and secondary output. They are a fact of life with high voltage and should be
dealt with.

Provide some form of airflow to the outside if you coil indoors. Open a window
for fresh air. If all else fails limit run times and remove yourself to fresher air.

3. Fire or explosion

The high power streamers from our coils can set objects on fire.
Be aware of what objects are getting struck by the output streamers.
Remove any flammables from the vicinity of the coil.

Make sure your AC wiring is adequate for the expected load and properly fused.
Overloaded circuits can overheat.

Keep a fire extinguisher in the coiling room at all times.
Make sure you get the type that is safe for use on electrical fires.

The main tank circuit capacitors are storing terrific amounts of energy.
If the cap fails internally that energy could cause the capacitor to explode.
I have seen SW bottle caps blow up and heard of several commercial pulse caps
exploding in Tesla use.

One of my poly and oil rolled caps developed an internal problem and began to overheat.
I had drilled a small vent hole in the top cover. After one rather long run enough
pressure had built up that oil was squirting out the vent hole. Without the vent
it would have probably exploded. That cap went into the junk pile.

Do not use metal cased caps like typically found in microwave ovens. These are meant
as power supply filter caps and are not suitable for Tesla use. They use mylar
dielectric and will overheat and likely explode.

Vent your homemade caps and place others in a strong container to contain the
blast if a cap should explode.

4. Noise

Tesla coils are noisy devices. Small coils are just loud. Larger coils are absolutely
deafening. It's a good idea to wear some sort of hearing protection while running
your coil indoors. I wear a pair of ear muffs meant for use on a pistol range.

I'm sure there are other hazards to avoid when building and using Tesla coils.
Use your head and if you dont have any idea on how to do things safely then
get assistance.

The bulk of this text was originally posted by me on the Tesla-2 coil
builders mailing list with additional comments provided by list member
Dave Bronakowski.