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Nonconductors or electrical insulators are materials which lack movable electric charges, and which therefore lack a low-resistance path for charge flow. When a difference in electrical potential is placed across a nonconductor, no free charges are exposed to the electric field, so no flow of charges appears, and an electric current cannot arise. Ideally, a nonconductor has infinite resistance and zero conductance.

Nonconductors are commonly used as a flexible coating on electric wire and cable. Since air is a nonconductor, no other substance is needed to "keep the electricity within the wires." However, wires which touch each other will produce cross connections, short circuits, and fire hazards. In coaxial cable the center conductor must be supported exactly in the middle of the hollow shield in order to prevent EM wave reflections. And any wires which present voltages higher than 60V can cause human shock and electrocution hazards. Nonconductive coatings prevent all of these problems.

In electronic systems, printed circuit boards are made from epoxy plastic and fiberglass. The nonconductive boards support layers of copper foil conductors. In electronic devices, the tiny and delicate active components are embedded within nonconductive epoxy or phenolic plastics, or within baked glass or ceramic coatings.

In microelectronic components such as transistors and ICs, the silicon material is normally a conductor because of doping, but it can easily be selectively transformed into a good insulator by the application of heat and oxygen. Oxidized silicon is quartz, i.e. silicon dioxide.

In high voltage systems containing transformers and capacitors, liquid nonconductor oil is the typical method used for preventing sparks. The oil replaces the air in any spaces which must support significant voltage without electrical breakdown.

• Plastics and solidified resins
• Rubber and Silicones
• Glass and ceramics
• Most metal oxides
• Most minerals and crystals
• cold, un-ionized gases (including Air)
• Oil
• Vacuum
• Water, if purified and deionized
• The depletion zone within a semiconductor

Nonconductors suffer from the phenomenon of electrical breakdown. When any voltage applied across a length nonconductor exceeds a threshold breakdown field for that substance, the nonconductor suddenly turns into a resistor, sometimes with catastrophic results. During electrical breakdown, any free charge carrier being accelerated by the strong e-field will have enough velocity to knock electrons from (ionize) any atom it strikes. These freed electrons and ions are in turn accelerated. One carrier creates two, which create four, etc. Rapidly the nonconductor becomes filled with mobile carriers, and its resistance drops to a low level. In air, the outbreak of conductivity is called "corona discharge" or a "spark." Similar breakdown can occur within any nonconductor, even within the bulk solid of a material. Even a vacuum can suffer a sort of break down, but in this case the breakdown or vacuum arc involves charges ejected from the surface of metal electrodes rather than produced by the vacuum itself.

• Dielectric
• Electrical insulators
• Resistance
• Conductivity