Electrical Insulators

Types of insulators and history

Insulators are used in all electrical devices from tiny capacitors to giant generators. Engineers need to understand how to design devices with the proper insulator. Unfortunately insulator failure is a primary reason why electrical devices stop working.

1.) Basics
2.) List of Common Insulating Materials
3.) Insulating electric wires
3.a) Guide to wire insulation
4.) High Voltage "Insulators"
4.a) Wood and Glass
4.b) Ceramic
5.) Timeline
5.a) List of manufacturers and innovators
6.) Comments on insulation from R. DeLair
Thomas Edison and Charles Steinmetz inspect damaged insulators after testing


What is resistance and conductivity?

Three major categories of materials include conductors, semiconductors and insulators. Insulators have a high resistance, we can also describe them as having "low electrical conduction". Conductivity: Most metals are conductive, which means that electrons can freely flow to different atoms in a given direction. Metals have loosely bound, or free electrons which allow this to happen. Insulative materials on the other hand have tightly bound valence electrons. Understanding conductivity at an atomic level requires some basic understanding of chemistry.
More on electrical current flow (wiki) >

Insulators come in solid, liquid and gas forms. See the more detailed list in the next section for examples.

Dielectrics - these are materials that are do not allow electricity to flow through them (are insulators) however in the presence of an electrical field the material becomes polarized. This phenomena is useful in electronics. For example we use dielectric materials in the middle of capacitors because it acts as insulator, blocking DC current flow, but the changing polarity allows energy to pass through. We use the measurement of Electric Susceptibility to understand how easy it is for a given dielectric to polarize. You can learn more about dielectrics on the wiki page here.

2.) List of Common Insulating Materials


Clay (ceramic)(porcelain) - This is the standard material for high voltage and RF insulators.
Plastics - PVC, Cresyl Pthalate, DEHP and other plastics replaced rubber as an insulator for wires and other parts. PVC and nylon are now standard in most types of wire.
Glass (silica, soda ash and limestone) - This material worked fine for telegraph and other low voltage apparatus. It is still used today to some degree.
Paper/Cardboard - paper and cardboard are used as insulators in certain circumstances as these materials are cheap and can work in situations without high heat or high voltages.
Mica - This is a good stable material even when exposed to the elements. It is a good thermal conductor while being an insulator. Sheet mica is easily stamped and shaped for electrical components. Mica is very important for the most common types of
Teflon (PTFE) - (polytetrafluoroethylene) - Slippery, durable and resistant to corrosion this Dupont made material is used in cable jackets. Other forms besides PTFE include FEP and ETFE.
PFA (Perfluoroalkoxy) - This substance is resistant to chemical attack, transparent and better than PTFE when it comes to flexibility. The weakness is that the number of times it can be folded is less than PTFE. It is good for applications near the ocean as it is resistant to salt spray. The dielectric strength of PFA is up to 4 times higher than PTFE.
Rubber - Rubber in its natural and synthetic forms was used as an insulator from before the 1870s until the 1950s. Plastics (especially PVC) replaced rubber.
Wax and oil - in the 1880s Edison used trinidad asphaltum with linseed oil, beeswax and paraffin to insulate copper wires mounted inside of iron pipes. This was used for durable underground power lines. This was used at the famous Pearl Street Station in NYC.

Normally when you separate two high voltage conductors an arc forms in between in open air. In the utility industry we use special non-conductive gas in a compact encapsulated metal container to stop arcs from forming. Gas-insulated switchgear is designed to disconnect very high voltages safely. There are gas insulated transformers as well as other devices.

Insulating Oil (Transformer Oil) - This petroleum product is used as an electrical insulator and thermal conductor. It conducts heat away from hot transformer coils. Some capacitors also use insulator oil.

Left: gas-insulated switchgear using gas and ceramic parts
Insulator failure and design

Insulation must be designed by an engineer for a given purpose. Making it thicker, changing its shape and determining the best material are part of the engineer's job. Insulation can fail in a variety of ways, if voltage gets too high and exceeds the "breakdown voltage" electrons will get excited to the point where they break out of their stable orbit, current will then pass through the material and often destroy the insulator.

3.) Insulation in electric wires

Knob and Tube

Early in-home wiring consisted of both braided sheaths for wires and "Knob and Tube" system to keep wires away from wood beams, and send wires through wood beams (tubes).

See our video demonstrating how it worked here.

Left: Photo of display of knob and tube wiring at the Edison Tech Center. Some historical houses still have knob and tube wiring in use after over 100 years.

Flexible wire insulation

Over the years wire insulation changed from rubber to complex polymers. Materials engineers have been working to reduce the negative properties of older insulator technology, here is a list of properties which they test for.

Negative properties to avoid:

  • Brittle with age - material stiffens and cracks form leading to shorts
  • Conversion of solid to liquid - loses solid characteristics and breaks down to a "goop"
  • Stiffening - loss of flexibility with age, this can effect machines with moving parts
  • Heat resistance - does it change properties over time and with exposure to high temperatures
  • Extreme cold resistance - something to consider when using in cryogenic HVDC applications and extreme cold latitudes
  • UV Light resistance - does the material hold up or break down under sunlight over time

insulated wires for the home
Right side: Modern Romex cabling with polymer insulation, left side has old rubber with cloth braid sheath.

Rubber used to insulate wire:

Many types of rubber were used for electrical wiring and apparatus. Gutta Percha - a natural latex product was used for some time but was replaced by vulcanized rubbers. The end of the rubber era was sealed when chemists developed plastics that could withstand higher heat and resist corrosion better and any rubbers. See Rick DeLair's commentary on rubber at the end of this article for more information.

Modern wire insulation

Modern electric wires commonly use PVC plasticized with linear Phthalates and an over jacket of clear nylon for heat and oil resistance. Oil can dissolve plastics so it is important to have the nylon coating. Linear phthalate chords of today can stand heat up to 105 centigrade. There are many specialized types of wire with unique insulation including Polyimide.

magnet wire with PI coating

guide to wire insulation

3.a) Guide to common wire insulation

Listed top to bottom:

Modern: Black jacket service cable designed for outdoor use. XHHW cross linked polyethylene.

Modern: 3 sizes of Romex. PVC with nylon coating

Historic: Type-SE cable from the 1950s with neoprene insulation, thin mylar color black and read for color coding.

Historic: Early PVC Romex cable from the 1950s. This cable has thermal plastic insulation around the copper. Fiberglass and cotton outer jacket with aluminum paint

Modern: Vinyl speaker chord - thalite plasticized PVC, rated up to 300 volts

Historic: POSJ 32 Early flat chord, rubber insulated cotton braid, overall insulation of rubber (the first zip chord).

Historic: SJTWA - PVC hard service chord, plasticized PVC

Historic: triplex telephone chord, rubber insulated, cotton braid painted.

Historic: type-R rubber covered house wire. Copper tinned with solder to keep rubber insulation from sticking to the copper. Asphaultum outer coat to protect the rubber.

Historic: type-C lamp chord 1880s-1970s. Cotton weave with rubber insulator

Historic: Bell wire, low voltage. Two underwraps of cotton and treated with wax. Wax keeps cotton together and gives some moisture resistance.

4.) "Insulators" used with high voltage power transmission

Insulators in the context of power transmission describe the support structures for wires and other devices. While knob and tube house wiring transitioned into flexible insulation, higher voltages found in HV power transmission remain the same in that there is no other economic way to insulate HV lines except by suspending the wires(using the surrounding air as the insulator) or burial.

Insulators in power transmission are designed to:
1. Separate the wire from the structural support
2. Stop lighting from bridging the gap, so this means insulating more than just for the amount of current in the wire
3. Stop ground shorts by resisting buildup of oils, water and dirt on the insulator (this is the reason for the ridged design)

Ceramic Insulators

Above: Three major sizes of insulators: Low voltages ~120 V, Medium 4k-60kV, and high voltage 60-700kV
4.a) Wood and glass insulators

Dry lumber is an insulator to some degree, although it is not a good one, so early pioneers of electricity started using glass. Telegraph lines came before power utility lines and glass worked effectively for telegraph lines as the voltage was low. Before the age of high voltage power (1870s onward), electricity was used primarily in telegraph and telephone technology. At that time some people including a young Thomas Edison built systems using glass bottles mounted on top of nails stuck into trees and poles.

As telegraph lines expanded around the world glass companies and bottle manufacturers added insulators into their product line. The National Insulator Association has documented this history well on their website.

4.b) Ceramic

High voltages created new problems for insulator design. Glass would crack apart with high temperatures and failed to withstand certain high temperatures, so the solution lay in Czech pottery. Master potters were hired to create porcelain products for the new electrical revolution of the 1880s. Simply changing to existing porcelain products was only a temporary solution, because the needs for even higher voltages came about in the 1890s. Chemists and material engineers helped design higher performance porcelain insulators with special coatings and designs.

cross section of porcelain insulator

William Cermak was a pioneer in insulators at General Electric. He developed the now famous "petticoated" insulator design which has a succession of ridges. One of his designs was the first to handle over 10,000 volts.

William Cermak came from Kasejovice, Czech, a region famous for glass and porcelain pottery. While in NYC Cermak worked at Bergmann and Company developing small ceramic insulators. He developed "Electrical Porcelain" with transmission engineers at GE. His family continued to work at Building 68 in Schenectady for decades. William's son Frank Cermak advanced insulator design into the mid 20th century.

Learn more about his life here.

5.) Compact Timeline and List:

Here is a very short timeline featuring some of the most prominent events in history:
1880 - Volta develops the battery and Davy demonstrates the carbon arc lamp, the early age of electricity begins. Wood, silk and plant products were used to wrap cables or hold wires.
1844 - Samuel F.B. Morse sends the first telegraph message. He like others before him continue to use a flat wood board beneath apparatus as an insulator to hold wires.
1840s-1870s - This was the age of the telegraph, and glass became the primary insulator type at the time.
1883 - It was common for electricians to use old gas light fixtures to hold electrical lines which went to light bulbs. Without proper insulation sparks could be seen during thunder storms. Running wires at that time was quite unprofessional in today's perspective!
1891 - Oerlikon Company makes insulators for the Lauffen to Frankfurt line. This is the first truly long distance demonstration of 3 phase AC power.
Lauffen Frankfurt insulator

1893 - Westinghouse: For the famous
Niagara Falls to Buffalo transmission line porcelain insulators rated at 11,000 volts were used temporarily until insulator technology was developed that could handle 22,000 volts
1930s - PVC insulation was developed and came into mainstream use after the 1940s
1950s - Natural rubber compounds were used in electrical wiring up until the 1950s

5.a) List of Prominent Manufacturers and Innovators of Insulators:

Pittsburgh High Voltage Insulator Company
General Electric Company. The Schenectady Works
Harloe Insulator Co.
National Insulator Company
Prior to 1900 there were many companies producing glass insulators see the NIA website for more details on the many companies involved.

If you like the subject of insulators you may be interested in joining the National Insulator Association. This group collects and discusses communication and electric utility insulators.

6.) Comments on insulation from ETC resident expert Rick DeLair:

About rubber and vinyl insulation

Goodyear developed the carbon-containing black rubber and came up with a process to vulcanize it. Some early wires probably had India Rubber for insulation, but Gutta Percha was common too, I have some GP wire on an early knife switch at the ETC. It had to be thicker than the later rubber compounds because of lower insulation strength (dielectric strength). I have seen wires with red rubber and black rubber and even white rubber, which I believe to be the India rubber actually. Natural rubber compounds were used in wire insulation up to at least in the early 1950's and even later. Neoprene rubber is not really "rubber" at all, it is a plastic. It also gains flexibility from plasticizers, I believe it contains aliphatic oils. They might have used Di-isodecyl Phthalate, (DIDP) as well for neoprene.

PVC vinyl insulation started coming around in the late 1930's and took off after world war 2. The first plasticizers used were Cresyl Phthalate (CP) and Di-Octyl Phthalate (DOP) now known as Di-2-Ethylhexyl Phthalate (DEHP) and is still used today. The Cresyl Phthalate (CP) is seldom if ever used today. Wires plasticized with these plasticizers are good for only 55-60 centigrade and 75 centigrade respectively. Today, modern type THHN (THWN) uses PVC plasticized with linear Phthalates, and over jacketed with clear nylon for both heat and oil resistance. Oil will solvate and remove most plasticizers, and the nylon jacket keeps oil away from the PVC, plus, the linear phthalates are not liquid plasticizers, but are semi solid, paste like, and more resistant to oil and fat solvation. The modern cords, like type SPT-1, SPT 2, SPT 3, as well as type SJTWA hard service cord all use linear phthalates, though some cheapo stuff still uses DEHP! Real linear phthalate cords have a 105 centigrade rating. Chinese made cords can be DEHP or DIDP plasticized, and yet still have the 105 C rating, which is bogus! They should be max 85 C. There are many more insulations as well.

- Rick DeLair

Read more on the history of wire insulation on our page here >

Related Topics

Wires and Cables

Dynamos and Generators

AC Power History

Meters and Voltage Regulators

Lightning and Surge Arrestors

High Voltage DC Transmission

This article by MW and Rick DeLair. Photos by Whelan Communications.

A Life of George Westinghouse, by Henry G. Prout. 1921
American Manufacturer and Iron World. Vol 77, July 6, 1905
PFA vs PTFE. Difference Between.net. 2015
Teflon Insulated Wire. Dacon Systems, Inc. 2015
Edison His Life and Inventions. by Frank Lewis Dyer. 2013
Interview with Ethyl Cermak, Edison Tech Center. 2006
Sandwich Glass Museum. Sandwich, MA
The Subdivision of Light. author not listed.
Men and Volts, the Story of General Electric. by John Winthrop Hammond
Valence Electrons. Bodner Research Web. Purdue.edu
Gas-insulated Switchgear. ABB.com. 2015
National Insulator Association. nia.org. 2015


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