Instructables

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.

Good Ground
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.

Ground Conductors
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
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.