Introduction: Work Safely With High Voltage
There are a bunch of great instructables and other projects on the interwebs that involve high voltage power supplies. Most include a disclaimer that says some variant of "if you have any sense, don't do this project." But rarely is there a reasonable and rational discussion of the risks of working with high voltage, and best practices for managing those risks. This instuctable aims to change all of that.
High voltage electrical systems can be challenging and fun projects to experiment with. In addition, they are very important in the history of the scientific endeavor. As you work with high voltage systems, you are treading the footsteps of Ernest Lawrence, John Cockroft and Ernest Walton, Nicola Tesla, and Irving Langmuir. Many modern systems require or are informed by the technologies and practices of high voltage engineering, from CRT displays to plasma displays, microwave ovens to high power radar, neon lamps to the Sandia Z-machine to pulsed high power lasers.
I know I said no Disclaimers, but...
While high voltage systems are interesting experimentally, they can be dangerous, and if not treated with care, respect and intelligence, they can result in fatal injuring. Low energy versions of these systems are no more dangerous than what William Gurstelle calls "The Golden Third," however, and with proper vigilance, they can yield to amateur experimentation and study. That said, DO NOT BEGIN WITH HIGH VOLTAGE EXPERIMENTS IF:
1. You are not comfortable with basic circuit concepts like current, voltage, resistance, capacitance, inductance, ground, current return, Kirchoff's voltage law, short and open circuit, etc. A good rule is that if you can't calculate the energy stored in a capacitor or inductor, or the power dissipated in a resistance for a given voltage or current, or identify a circuit ground, or calculate what will happen if a circuit component fails short or open, you should acquire that knowledge working on low voltage circuits first. There are many, MANY 3.3V or 5V circuit projects on the internet and in texts that will give you that knowledge. Then, come back to high voltage experimentation.
2. You or your family and friends cannot accept that YOU AND YOU ALONE ARE RESPONSIBLE for your safety and safety of others in any endeavor in which you engage. While this material is provided in hopes that it will keep some amateur scientists and inventors experimenting and inventing, YOU ARE RESPONSIBLE for verifying its accuracy and applicability to your project. YOU ARE RESPONSIBLE for knowing your limitations of knowledge and experience. You agree not to hold me responsible for your actions, nor for errors or omissions in this brief overview.
Step 1: Know the Risks
Some excellent experiments on the effects of electricity on the human body were performed in the 1960s by Charles Dalziel at UC-Berkeley . Dalziel performed tests on men (150 lbs) and women (115 lbs) at differing levels of electrical current, for direct current and alternating current at 60 and 10,000 Hz.
Damaging effects which can be induced in the body by electric shock or contact with live electrical systems include:
1. Ventricular fibrillation - defined by the New Oxford American Dictionary as "(of a muscle, esp. in the heart) make a quivering movement due to uncoordinated contraction of the individual fibrils." This is a potentially fatal condition where the heart muscle quivers rather than beats, eliminating blood flow and causing death.
2. Cardiac asystole - where the heart stops beating. Combined with ventricular fibrillation this constitutes cardiac arrest.
3. Respiratory arrest.
4. Burns from arc flash and resistive heating of body tissues.
5. Radio frequency burns if radio or microwave frequencies are used.
The onset of these effects is dependent on the electric current magnitude, its character (whether DC, AC 60 Hz or higher AC frequency), and the amount of time the electric current is applied. At 60 Hz, studies have shown that ventricular fibrillation occurs at I=100 mA*s / T[s], where T is the time applied in seconds over a range of 0.2-2 seconds . So, at 0.5 seconds of 60 Hz applied, 200 mA of current produces VF, but for a 2 second application, only 50 mA of current is required. The amount of voltage needed to produce these effects depends on the contact resistance between the human and the circuit, which is often in the range of 1000-2000 ohms, but can be higher with protective equipment, or lower if the skin is broken, or wet, etc.
Back to Dalziel, his work showed that the amount of current required to produce pain and muscle contraction depends strongly on weight and frequency, with 60 Hz producing effects at the lowest currents . So, at 60 Hz, only 10 mA are required to produce strong muscle contraction in a 115 lb person, but about 50 mA are required for DC or 10 kHz current.
There are some risks that are particular to high voltage and pulsed shocks . They may cause cardiac asystole rather than ventricular fibrillation, at currents above 1 A. Pulsed shocks with energies above 50 Joules are potentially hazardous. Even at 0.25 Joules, the shocks are painful. Pulsed shocks (as used with pulsed electrical incapacitation devices) can have lasting effects that incapacitate a victim. An additional hazard with high voltage shocks is that the victim need not always contact an energized circuit. Air will breakdown about about 30 kV/cm or 75 kV/inch, potentially connecting a victim to a circuit at high voltage without direct contact.
Bottom line and Risk Mitigation
1. Work on un-energized circuits if at all possible.
2. Be very careful around live 60 Hz electricity, since it requires very little current to injure. Your power supply can kill you!
3. Limit the current and energy to the lowest values possible. Lots of interesting experimentation can be done with low stored electrical energy and low currents of a few mA or even microamperes. Make it a habit to ask yourself if you really need this current or energy.
4. Keep your distance from live high voltage circuits. Since high voltages can breakdown air to connect you to a circuit, keep high voltage circuits in enclosures and behind barricades when in operation.
5. Be sure to properly ground your experiment and your enclosure. Take special care to safely de-energize and ground a circuit before working on it. Know when and how you can end up in the ground path in a circuit and put safeguards in place to eliminate this eventuality. This will be discussed more in step 4.
6. Never work alone, always have a partner who knows your equipment and the risks and hazards involved. That way, you have a second set of eyes to insure safety, and someone who can shut off the power and get help if you are injured.
 C. F. Dalziel, "Deleterious effects of electric shock," in Handbook of Laboratory Saftety, 2nd Ed., N.V. Steere, Ed. Cleveland, OH: Chemical Rubber Co. 1971, pp. 521-27.
 T. Bernstein, "Electrical Shock Hazards and Safety Standards," IEEE T. Educ. vol. 34, no. 3 (1991): 216-22.
Step 2: Know What High Voltage Is
The U.S. National Electrical Code or NEC, defines high voltage as any voltage over 600V. High voltage can be AC (alternating current - usually sinusoidally varying) or DC (direct current - a fixed and steady positive or negative voltage) or pulsed (a non-periodic positive or negative pulse of voltage). The dangers associated with high voltage can vary depending on voltage, the amount of current that can be supplied, the frequency if the source is AC, or the energy stored in a pulse.
AC High Voltage - The NEC deals extensively with this type of high voltage, particularly at the 60 Hz frequency of electrical transmission systems in the US. High voltage in these cases typically occurs at transmission substations, high voltage transmission lines, power generating facilities and industrial or commercial facilities. These systems are very energetic and very dangerous, but are not typically of use or interest to the experimenter.
Also included in this category are high voltage RF systems, like Tesla coils, neon lamp power supplies, or high power radio and microwave systems. Voltage from a low voltage, high frequency AC source is "stepped-up", or brought to high voltage at reduced current and constant power, by a transformer. The low voltage source could be an RF AC generator like a function generator, or it could be a pulsed AC source like a spark gap Tesla coil primary. Pulsed AC usually involves a resonant LC circuit, which is made to "ring" or oscillate by repeatedly energizing with voltage pulses from a switch like a spark gap. In the case of radiating systems, an amplifier of some sort is normally used to produce the high voltage.
Critical things to know about AC high voltage systems include the voltage, frequency, and maximum power of the system. If the system is pulsed, the pulse length is also important.
DC High Voltage - DC high voltage systems typically consist of an AC source and a rectifier, a more complicated AC to DC converter, or a DC to DC converter, but can involve storing electrostatic charge. Circuits of this type include high voltage rectifiers, boost circuits, Cockroft-Walton charge pumps, or Van De Graaff generators. DC systems produces a (reasonably) steady voltage, of positive or negative polarity. They are reasonably steady, in that in almost all cases (except for the Van De Graaff) some remnant AC rides on top of the DC producing ripple.
DC high voltage is very hard to make at extremely high voltages due to corona discharging. At voltage above about 10 kV, sharp metallic points can produce ionization in the air, resulting in bleeding current out of the DC supply. Depending on the supply, this may enhance the ripple, or significantly reduce the DC voltage achievable.
Critical things to know about DC high voltage systems include, voltage magnitude and polarity, as well as the maximum current deliverable by the system.
Pulsed High Voltage - To overcome the limits of corona discharge in DC high voltage, pulsed high voltage is used. Usually pulsed high voltage is a relatively flat voltage that is zero, switches to steady high voltage of positive or negative polarity, and then switches off. Pulsed high voltage circuit types include Marx generators, pulse forming networks, and transmission line pulsers.
Critical things to know about pulsed high voltage systems include voltage magnitude and polarity, as well as stored energy. Internal impedances and current limits are also good things to know.
Step 3: Know the Components
Most high voltage circuits are fairly simple, using primarily simple, passive components and some command-triggered or self-triggered switches. Hence, the component types encountered are usually resistors, capacitors, inductors, transformers, transmission lines, diodes, and spark-gap or possibly solid state switches. There are also field shaping components that reduce the electric field when the circuit is energized to reduce corona discharge and increase breakdown voltage. The most critical components to understand from a safety perspective are those components that can store energy (capacitors, inductors, transformers, and transmission lines) and surfaces that might be at high voltage, like field shaping components.
Capacitors can store substantial energy when charged to high voltage. The energy stored in a capacitor is given by E = QV/2 = (1/2)CV^2, where V is voltage and C is capacitance. Given their ability to store charge, capacitors can have significant electric shock potential even when a circuit is de-energized.
Hazards of capacitors include the following:
+ capacitors can exhibit a property known as dielectric absorption, where charges are absorbed into the dielectric after an extended time at voltage. The effect is that after a capacitor is discharged briefly the dielectric material recharges after the short circuit is removed.
+ previously discharged capacitors that are capable of storing significant energies can re-accumulate charge from the air.
+ high currents may cause capacitors to heat-up and/or explode. This can be a real problem when one capacitor in a bank fails short, and the remaining capacitors discharge through it.
+ the high currents generated when a capacitor is suddenly grounded can produce large voltages across grounding conductors and arcs to the conductor.
Safeguards for using capacitors include the following:
+ if a capacitor can store more than a few joules, it should be stored short circuited, and it should be short circuited as work is being performed.
+ capacitors should be installed with current limiting, and they should have bleed down resistors across the capacitor terminals if at all possible. The bleed down time (~3*R*C) for the capacitor should be as fast as possible, and not longer than five minutes. The bleed down resistance should be connected between the capacitors, and not across long runs of series capacitors.
+ work with capacitors should always be accompanied by a grounding hook, which is discussed in more detail in step 4. Ground each capacitor individually, rather than several capacitors in series.
Transmission lines are essentially distributed runs of capacitance and inductance. Hence, they can also store charge like capacitors. The hazards and safeguards for transmission lines are essentially the same as for capacitors.
Inductors can store significant energy in magnetic fields when current if flowing through them. If that current is suddenly removed, voltages across an inductor or across the inductance in a transformer can rise quickly, resulting in breakdown or arcs to other surfaces. Often, this will result in an arc across the switch that interrupted the current in the first place, which may destroy the switch.
A high current on an inductor is less common for small experiments. If a high current inductor is used, and switches in place are likely to fail when current is interrupted, use a resistance across the inductance to limit the voltage to a safe level while the energy dissipates.
Diodes and Switches
The chief concern here is rupture if currents and voltages are excessive. Hence, these devices (and all high voltage systems) should reside in an enclosure.
Step 4: Know Your Ground
Grounding is a blessing and a curse in high voltage experimentation. It is a blessing in that it can keep us safe, providing a ground to a circuit so that the circuit doesn't use us as its ground path. It can be a curse, particularly for pulsed and RF systems, as it can be a source of coupling between power source and instrumentation, making measurements difficult or impossible. The issues of instrumentation grounding are beyond this Instructable (though maybe a good choice for another) but this step will cover some issues on grounding for safe operation.
The idea of a "good ground" is a connection to ground that has the least possible inductance, capacitance, or resistance along its path to the earth. Also, a good ground path should be able to tolerate the full short circuit current of the electrical source without failing.
Legal grounding requirements for homes and buildings can be found in the National Electrical Code (NEC). A link to a full text online version can be found on the wikipedia page for the NEC. This Instructable is NOT intended to provide instruction on how to properly ground electric service for a structure or a piece of electrical equipment. Consult an electrician for that sort of work. However, many of the ideas surrounding grounding in the NEC apply for experimental practice as well.
The best possible ground is a very short connection to a lot of copper buried in the earth. Homes may have copper grounding rods driven into the earth near the circuit box and electric meter. Ham radio operators will create good ground planes by burying radial copper arms several inches below the ground and bringing a connection point for their antenna out of the ground. This will provide a very good connection point to earth ground, but it is often impractical for the experimenter.
Another reasonably good connection to earth ground is a connection to building steel that either enters the earth or is in concrete that enters the earth (if in a large building) or cold water plumbing near where it enters the earth. Care should be taken to insure that the building steel or plumbing extends directly into the earth. This is particularly important for plumbing, where plastic sections may have been inserted, or hot water heaters may provide electrical isolation from earth ground.
A final source of ground connect is the ground in grounded electrical outlets. This is often the least desirable source of ground for the experimenter for a few reasons. First, instrumentation almost surely will share this ground source, which can introduce coupling between the instrument and circuit under test. Second, connecting securely to this ground may require entering an electrical outlet box, which is not recommended. Third, it is difficult to tell how long this run of ground wire is, if it is broken, how much current it can stand, etc., since these wires are usually deep in walls.
Again, the NEC has a great deal of information on proper grounding, and should be consulted for ground connections. The information here is not intended as instruction on proper grounding for structure electrical service, but for experimental practice.
An important point about ground conductors, which applies primarily to pulsed high voltage systems, is that they should have a large surface area not just cross sectional area. The reason for this is that pulsed systems are typically limited to conduction on the surface of a material, or to its skin depth. Skin depth is dependent on the square root of pulse rise time (or 1/frequency if AC) and the square root of conductivity. This means that a fast pulse will conduct only in the first few microns or at best millimeters of a conductor. And, if the stored energy is high, the current could cause heating, or the resistance of the ground connection could be high resulting in a large voltage drop across the ground conductor.
What to Ground
Any metallic enclosure of an electrical source should be connected to a good ground. Thus, a fault to the case will not cause an exposed metal surface to be raised to high potential. For a piece of commercial equipment this requirement is typically provided by the case and electrical plug. For a piece of experimental equipment, care should be taken to connect the case to earth ground with a solid connection, a large flat conductor, with the shortest possible run. Generally, it is good practice to connect the circuit to this ground, though there are exceptions.
Grounding sticks should also be provided. These are long, non-conductive sticks with metallic ends. The ends have a solid connection to large flat copper braid or large gauge wire, with a short run to good earth ground. The grounding stick can be used to ground the leads of capacitors and other conductors in an experimental circuit, after the circuit is de-energized, and before work begins. These "portable" ground connections can be used to tie the circuit components to ground while work is being performed on a de-energized circuit as well.
Step 5: Know When to Work, and When Not to Work
There are some key things to remember when experimenting with high voltage systems. Here is a list of good practices based on the information presented here. Additionally, I have listed a few good references that are available on the web, which I have used in compiling this Instructable.
+ Always work on a circuit that is de-energized if at all possible. De-energized includes discharging any potential stored energy. Check multiple times that the circuit is de-energized, and insure that no one can apply power to the circuit without you knowing it. This can be achieved with Lock-out Tag-out procedures.
+ Always work with at least one other person who is familiar with the equipment, its hazards, and emergency procedures. Make sure everyone knows how to call 911.
+ Use test instruments and components only at their rated conditions.
+ Wear proper laboratory attire near the circuit. This means rubber soled, closed toe shoes and long pants. Jewelry, especially metal jewelry that can accidentally contact a circuit should be removed.
+ Always discharge capacitors with a grounding stick before working on a circuit. Insure the capacitors remain discharged by shorting them and tying them directly to ground.
+ Don't perform work if you are mentally compromised. If you are tired, upset, or otherwise cannot concentrate fully, do not work with high voltage. Your intelligence is your primary safety tool, and if it cannot be employed fully, you should not work until you are mentally prepared to do so.
+ Keep stored energies, currents and voltage as low as possible at the design stage.
+ Never assume a circuit is safe just because it is powered off. Energy can be stored in capacitors and long cable runs. There can be interior short and open circuit faults. Check, check, and check again that a safe, de-energized state has been achieved.
Here are some links to information I found on the web that may be helpful in keeping you safe and experimenting for a long time.