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(VandeGraaff) Electrostatic Machine Safety

Tabletop Vandegraaff machines aren't so dangerous. They've been used in classrooms and Science Museums for decades with little problem. I've not encountered any literature on hazards associated with electrostatic machines, so I've put together a bit here.

Note that I'm describing worst-case events, so I might make a VDG machine seem dangerous. They are not. Remember that people are not killed by VDG machines; people are constantly killed by car accidents, by falling off ladders, by slipping in the shower, etc. Don't lose your perspective: riding in a school bus is far more dangerous than playing with a VDG machine.

******* I'M NOT A DOCTOR, NOR A SAFETY EXPERT! ********
NOTE: I am not an expert regarding electrical safety. This webpage is for your information only. It contains common-sense advice, but the information here is neither reliable nor complete. For accurate safety information, consult an expert in electrical shock hazards.
******* I'M NOT A DOCTOR, NOR A SAFETY EXPERT! ********
ELECTRIC VOLTAGE: by themselves, DC electric potential and electrostatic fields cannot harm humans. Rubbing a balloon upon dry fur can produce potentials approaching those of a tabletop VDG generator: 20,000 volts. As far as voltage is concerned, the high voltage produced by a tabletop VandeGraaff generator is about as dangerous as an electrified balloon.

ELECTRIC CURRENT: a tabletop VDG produces an electric current below 100 uA (microamperes), which is about ten times smaller than a human is able to feel. If you lick your fingertips and touch them to the terminals of a 9v radio battery, the level of current in your fingers will be much higher than that produced by a VDG machine. As far as constant currents go, a 9v battery is more dangerous than a VDG.

SPARKS AND IMPULSE CURRENT: here is where there is possible danger.

The peak current during the spark from a VDG machine can be as high as thousands of amperes. However, at this amperage the current lasts for less than a millionth of a second. At a more moderate discharge current of 10mA, the discharge might last up to a millisecond. This is above the threshold of human perception of current, and yes, if you allow the spark to jump to a metal object held in your hand, you sometimes can feel a small twitch in the muscles of your arm. On the other hand, when you feel pain from a "static" spark, the pain comes mostly comes from the high electric current concentrated in a tiny area, and from the microscopic region of burned skin where the spark enters your body. Small sparks might be slightly painful, but there is far more danger from the suprise they can cause if they happen unexpectedly. However, even the smallest spark can trigger a heart attack in a person who has a severe heart condition.

SPARK ENERGY: The danger of a spark can also be rated in terms of spark energy. If you receive a "static" spark from touching a car, the energy can be as high as 500 mJ (millijoules) Typical spark energy E =1/2CV^2, and since human body capacitance might be as high as 400pF, then 50,000 volts on a human body gives an energy discharge of 0.5 Joules. Touching the sphere of a VDG is about as dangerous as touching a car if your own body is electrically charged.

GENUINE HAZARDS

  • AVOID SUPRISES: A person who is suprised by the small shock might stumble and fall, especially if they are standing on a rickety chair during the usual "hair raise" demonstration.
  • NO LEYDEN JARS OR CAPACTIORS: If you use your VDG to electrify a large Leyden Jar, you can store extremely dangerous amounts of energy and create an electrocution hazard. If you touch the wrong parts of the electrified leyden jar, the discharge through your body can violently tense your muscles, causing you to crash into nearby furniture. The discharge might send you into unconsciousness so that you fall and hit your head. It may even stop your heart (by triggering fibrillations). Don't use Leyden Jars in your demonstrations unless you are trained in dealing with hazardous high voltage. PARENTS: never let children experiment with VandeGraaff machines and Leyden jars together.
  • NO HUMAN CHAINS: If you charge up a long chain of people holding hands, the total stored electrical energy can become much larger than that stored on one person. Yes, each individual person does not receive a larger charge than normal. However, if one of the people should touch an electrical ground, they and the people closest to them in the chain can experience a large jolt. The charge of the entire chain of people must flow through the person touching ground. Also, the path for current is through that person's heart (because charge flows in through one arm and out through the other.)
  • NO PACEMAKERS OR IMPLANTS: Medical implants could be damaged either by the large e-fields which surround a VDG, or by the powerful pulses of radio waves created during spark discharge. A person with a cochlear implant in their ear, a portable external defibrillator, or other sensitive device, is often advised to steer clear of radio transmitters, microwave ovens, cellular phones, and sparks from doorknobs. They should also steer FAR clear from playing with VDG machines!!!
  • UNDIAGNOSED HEART DISEASE: The small spark from a VDG could cause a heart attack in a person having a serious undiagnosed heart condition. Needless to say, this possibility gives nightmares to public science demonstrators. It doesn't matter if that person would have had a heart attack within hours anyway. If YOUR device triggered it, guess who will be blamed! On the other hand, if I myself was going to get hit with a heart attack out of the blue, I'd rather have it happen while inside a building and surrounded by a group of quick-thinking people.
  • If a person has flammable liquids or gasses on their skin or clothing, a spark from a VDG might trigger a fire. The classic hazard is the butane gas from a cigarette lighter which is leaking unnoticed in an inside pocket in a jumpsuit. If that person receives a big spark from a doorknob or a VDG machine, the butane could ignite. Yes, this is a rare instance: a smoker wearing a jumpsuit who has a lighter which is leaking.
  • Some older VDG machines contain a high voltage power supply. This supply presents an electrocution hazard if the generator's case is opened or if the charged lower comb is touched. Also, if the generator's belt becomes conductive because of built-up dirt, and if both combs electrically arc to the dirty belt, then the generator's upper sphere will become connected to the internal high-voltage power supply. In that case, touching the VDG sphere is the same as touching the high voltage DC supply directly. If the DC supply in the VDG machine does not have safe current limiting, then there will be an extreme electrocution hazard.
  • And last: If you operate a VDG next to a computer, and the computer circuitry gets trashed, the owner of the computer might kill you! (grin)
******* I'M NOT A DOCTOR, NOR A SAFETY EXPERT! ********
NOTE: I am not an expert regarding electrical safety. This webpage is for your information only. It contains common-sense advice, but the information here is neither reliable nor complete. For accurate safety information, consult an expert in electrical shock hazards.
******* I'M NOT A DOCTOR, NOR A SAFETY EXPERT! ********

 











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