Electricity can jump from wires to nearby people, metal or other conductors. To prevent this, a nonconductive material must be used to guard against short circuits, shocks and fires.
Insulation types and techniques have changed over the years. Modern insulators are made of a range of materials, some inexpensive and good for household purposes and others that are more expensive but able to tolerate very high heat.
In early 20th century wiring, the actual electrical conductors may be bare. Wire was insulated from the surrounding structure using glass or ceramic insulators nailed to beams or other parts of the house.
Asbestos was used as an early wire insulator. Health and safety issues have made this type of insulation obsolete and very rare.
No modern wiring uses asbestos. It is found only in existing wiring.
Polyvinyl chloride—the ubiquitous PVC of construction sites the world over—has now become the most common type of electrical insulation. It can tolerate both moderately low and moderately high temperatures and is doesn't cost very much.
Higher-temperature installations, however, require different types of insulation.
Teflon can be used as an insulator on electrical wiring. Its protection can range from extruded Teflon, suitable for up to 400 degrees Fahrenheit, to a fused tape-type insulation, good for installations up around 500 degrees, to a high-heat Teflon tape insulation used in situations where wiring is run through areas with temperatures up to 900 degrees.
The next step up in heat tolerance is glass fiber and glass braid insulation. There are many different forms of glass fiber and glass braid insulations that range from plain glass braid insulation that will tolerate up to 900 degrees to a high-temperature glass braid that will tolerate up to 1,300 degrees.
Wiring that runs through very high-temperature areas often must use ceramic fiber insulation. Ceramic fiber insulation can tolerate up to 2,200 degrees Fahrenheit.