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Re: Safety FAQ (Electrical Hazards)
Hello Coilers,
Chip's query regarding the following stimulated this response:
>1) Is it true that it takes 10 Joules of electricity to kill someone?
> (If not, how much is it?)
The short answer is no. The long answer follows!
Electrical Hazards
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.) Over 1000 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 1 ampere, the
likelihood of sudden death is greatest. Above 1 ampere, the heart is thrown
into a single contraction, and internal heating becomes significant. The
individual may be thrown free of the power source, but may go into
respiratory and/or cardiac arrest.
Six factors determine the outcome of human contact with electrical
current: voltage, amperage, resistance, frequency, duration and pathway. I
will discuss each individually.
Voltage
Low voltages generally do not cause sudden death unless the external
resistance is low (so don't fire up your coil in wet areas). As the voltage
is increased, more and more current passes through the body, possibly
causing damage to the brain, heart, or causing involuntary muscle
contractions. Perhaps 100-250 volts A. C. is the most lethal voltage,
because it is high enough to cause significant current flow through the the
body, and may cause muscles to contract tightly, rendering the victim
incapable of letting go. Lower voltages often are insufficient to cause
enough current flow, and higher voltages may cause the victim to be thrown
clear of the hazard due to the particularly fierce involuntary muscle
contractions. Arcing may occur with high voltages, however. Naturally,
burns become more severe as the voltage is increased.
Amperage
Greater amperage means greater damage, especially due to heating
within tissues. As little as 10 microamps of current passing directly
through the heart can cause ventricular fibrillation (heart muscle fibers
beat out of sync, so no blood is pumped) and cardiac arrest. Because of the
air filled lungs, much of the current passing through the chest may
potentially pass through the heart (pun intended). The spinal cord may also
be affected, altering respiration control. 100-1000 millamperes is
sufficient to induce respiratory arrest and/or cardiac arrest. Thermal
heating of tissues increases with the square of the current (I^2 x R), so
high current levels can cause severe burns, which may be internal.
Resistance
A heavily callused dry palm may have a resistance of 1 megohm. A
thin, wet palm may register 100 ohms of resistance. Resistance is lower in
children. Different body tissues exhibit a range of resistances. Nerves,
arteries and muscle are low in resistance. Bone, fat and tendon are
relatively high in resistance. Across the chest of an average adult, the
resistance is about 70-100 ohms. Thermal burns due to I^2 x R losses in the
body can be significant, resulting in the loss of life or limb long after
the initial incident. Limb diameter determines the approximate "cross
section" which the current will flow through, (for moderate voltages and low
frequencies). As a result, a current passing through the arm generates more
temperature rise and causes more thermal damage than when passing through
the abdomen.
Frequency
The "skin effect" also applies to a human conductor, and as the
frequency gets above about 500 kHz or so, little energy passes through the
internal organs. (I unfortunately have little data in the 50-250 kHz range,
where we operate most tesla coils. I'll check another reference I have at
home.) At a given voltage, 50-60 A.C. current has a much greater ability to
cause ventricular fibrillation than D.C. current. In addition, at 50-60 Hz,
involuntary muscle contractions may be so severe that the individual cannot
let go of the power source. Higher frequencies are less able to cause these
involuntary contractions.
Duration
Obviously, the longer the duration, the more severe the internal
heating of tissues. Duration is particularly a problem when working with
110-240 volts A.C., which can render the individual incapable of letting go.
Pathway
If the current passes through the brain or heart, the likelihood of
a lethal dose increases significantly. For example, hand to hand current
flow carries a 60% mortality, whereas hand to foot current flow results in
20% overall mortality. Be aware that foot to foot conduction can also
occur, if a high voltage lead is inadvertantly stepped on or if grounding is
inadequate.
Precautions
Obviously, the A.C. line voltage, the high voltage transformer and
the high voltage R.F. generated by a tesla coil are all potentially lethal
in their own unique way. 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 one or more mind altering
drugs. Watch a tesla video instead!
Comments/flames welcomed,
Mark S. Rzeszotarski, Ph.D.