HV materials
From HvWiki
Materials used in high voltage applications have different requirements than those used in typical electric circuits. Because the voltages are typically in the kilovolt range, materials normally considered as 'insulators' become fairly conductive. Wood, paper, and other porous materials are poor insulators for high voltage and should be avoided.
Additional concerns are arcing, corona discharge, as well as normal concerns such as durability and strength. The geometry the materials also comes into play in high voltage applications.
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HV Insulators
- Alumina - Extremely good resistance and very tough, difficult to work, expensive. Preferred for high vaccuum, high voltage equipment.
- PVC pipe - Cheap, readily available, easy to work, but poor heat resistance, can melt.
- Lexan - Somewhat expensive, very tough, and transparent.
- Air - Depends on humidity.
- Oil
- RTV silicone caulk. (RTV is the vinegar-smelling type. Avoid using water-based silicone.)
- Nylon
- Vinyl aquarium tubing
HV Conductors
While the conductivity of the material is of lesser concern, because high voltages can overcome even fairly high resistance values, there are concerns for current heating, as well breakdown of the material from electrical arcing.
A common HV conductor is copper plumbing tubing. It is easily shaped, has a larger diameter than wires, and is readily available.
Metal foil tape may be used in some circumstances.
Shapes
High voltage potentials collect at any sharp point, protrusion, or angle. Because of these higher potentials, arcing and corona discharge tend to start at these sharp protrusions. Sharp points can even radiate streams of ionized air molecules to great distances away from your device.
Smoothly rounded surfaces tend to have more gentle potential gradients and tend to hold charge better than sharp or rough surfaces. Under most circumstances, conductors must be kept as smooth as possible. Large round shapes are preferred. The Van de Graaff generator uses a large metal sphere to collect the charge, which is evenly distributed around the surface.
Conductors in high voltage systems should use large-diameter smooth conductors. Small diameter conductors radiate their charge by rapidly ionizing the air around the conductor. Use of wire-stranded cables is highly discouraged. Charge tends to travel on the outer skin of the conductor, so for these reasons copper tubing is often used instead of cables. Because the current travels on the skin, using hollow or solid metal is irrelevant, and tubing is easier to obtain and work with.
Avoid using extremely thin, bare wire. The thin wire qualifies as a "sharp edge", and corona discharges can appear. These leak charge away to the air. For voltage up to 10,000 volts, bare wire needs to be #12 or greater diameter, like coat-hanger wire. At higher voltages, bare wire must be replaced by copper tubing or pipe.
Sharp angles can be used to intentionally create coronas and sparks, or to transfer charge. In the classic Van de Graaff design, charge is passed to and from the belt via metal combs, the charge is readily transferred to and from the sharp teeth of the comb to the belt without the comb actually touching the belt.
Cleanliness
Keeping your parts clean is absolutely vital. Oils and salt from your skin can contaminate your insulators, creating coronas, allowing charge to travel along the surface and eventually find an arc point. Clean everything with degreaser or alchohol, then dry thoroughly. Use a hair-drier.
