| A word of caution: This article is about the typical commercially-available tabletop Van De Graaff generators, in general rated at no more than 500,000 volts (spheres or domes no larger than a basketball). Larger custom-built models with much higher voltage capacities exist, mostly at museums and private residences. The large custom-built generators are capable of much longer and powerful sparks than what I describe here, and therefore I cannot vouch for their safety.
It is also important to note the distinction between Van De Graaff generators and Tesla coils. Unlike the VDG, Tesla coils can be very dangerous to operate. Tesla coils produce a continuous arc by 'stepping up' household AC voltage via a transformer. Consequently, a Tesla coil arc can electrocute the user should the arcs simultaneously connect to a 110v component and the person using it. A VDG spark is not connected electrically in any way to the 110v power source, and furthermore is an instantaneous discharge incapable of carrying sustained household AC current.
Van De Graaff generators: high voltages, but safe
Although the voltages involved in Van De Graaff generator sphere charges and sparks are high - upwards of 400,000 volts for larger commercially-available models - they are generally very safe to use, even with young children. These machines have been in use in classrooms, universities, museums and laboratories since the 1930s, with no known cases of injury from their use to either children or adults. Remember, the human body can hold upwards of 20,000 volts of charge itself just by scuffing one's feet on a carpeted floor! VDG sparks are simply 'souped up' versions of those you see on a dry winter day when touching a doorknob. With that said, there are a few items of precaution that anyone using or demonstrating these machines should keep in mind.
Large VDG sparks can hurt!
Anyone who gets close to an operating Van De Graaff machine will get hit with a few sparks, so be ready for it! For most people, getting 'struck' by the sparks is part of the fun of operating these machines. The larger sparks from bigger generators can be somewhat painful, especially if they strike bare skin. At worst, it feels like being slapped with a ruler. The pain is only momentary and goes away quickly. Numerous repeated sparks to the same area of skin can cause redness, again, similar to if you've been slapped hard with a ruler. Bigger sparks also can cause a single small muscle twitch near the site of the spark. The thumb side of your palm and your forearm is generally the least painful place to take a VDG spark.
You can produce the largest sparks by holding a large metal sphere up to the VDG sphere, such as a mixing bowl. These discharges will produce a shock sensation in your arm as the charge flows through your body, even though the spark is not directly contacting your skin.
Limit VDG use to healthy individuals
Although there have been no known cases of VDG sparks causing problems to people with medical issues (nor is there any known way that these sparks could cause medical problems), I would strongly recommend that anyone with a pacemaker, insulin pump, hearing aid or any medical-related electronic device, as well as anyone prone to heart problems, nerve disorders or seizures, avoid an operating Van De Graaff generator.
Electronic device safety
The only real hazard a VDG presents is to electronic devices - this includes anything with electronic chips or circuit boards: cell phones, cameras, watches, computers, clocks, hearing aids, headsets, thumb drives, and so on. Even a small spark can destroy an electronic component. For this reason, you should ensure that anyone getting close to the machine has left behind at a safe distance any devices they may have. Even though it may seem excessive, I usually recommend that all electronic devices be kept at least 20 feet away just to be safe.
The other issue of concern is wired electronic devices, such as computers, TVs, modems, routers or phones plugged in to the same circuit that the generator's motor is plugged into. Sparks and discharges will affect the circuit, particularly if it is not properly grounded. I have had some of my computers, routers and cable modems (in a separate room) power cycle/reboot (but thankfully not damaged) due to the effects of operating a large VDG on the same household circuit. Consequently, I would be very wary of running a VDG on the same electrical curcuit that computers, network equipment or any other sensitive devices were on. Spark overvoltages can travel long distances, and in essence, a large VDG spark in direct contact with an electrical circuit would be similar to the pulse from a nearby lightning strike.
Unfortunately I don't know of any solid way to mitigate this hazard other than using a long heavy-duty extension cord to a dedicated outdoor gasoline generator to power the VDG, keeping the power cord at least a couple feet away from other cords and wires. You would not need a large generator to power a VDG, the smallest ones would be more than adequate. Again, I know of no cases where a VDG was implicated in damage to nearby devices, but due to my experiences at home, I believe it is a valid concern. Even with the added cost and hassle, employing a gas generator and long extension cord to operate your VDG might be good insurance against liability. For instance, I would not want to be responsible for frying a corporate network's routers and switches!
Avoiding VDG sparks; the 'hair raising' trick
Using a grounded discharge wand to produce sparks (connected to the terminal on the VDG base, present on most larger models) will keep the user from feeling the discharges. When doing the famous 'hair raising' trick with a more timid audience member, you should shut off the generator and discharge the sphere before allowing the volunteer to touch it. Once they have their hand on the sphere, you can turn the generator on and off in short bursts to slowly build up the charge. Once the subject's hair is fully extended, you can shut the generator off, though the charge will slowly drain away as long as the generator is off. If you leave the unit running, the effect is much more pronounced - but sparks may begin jumping from the volunteer's body to nearby objects and/or the floor. These will generally not be very painful, but the person will feel them. If the volunteer pulls their hand away from the generator sphere while it is running full tilt, they will get hit with sparks! Before the subject pulls away from the sphere, you can shut the generator off and discharge the sphere with the wand to prevent any sparks from affecting the volunteer.
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