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Silicones 3. Problem of Electrical Insulation

ChemCases lets you learn chemistry and enjoy doing it by applying the same chemical principles that the inventors used develop the products you use. And you can join with other learners to debate the fundamental issues that confront these scientists as they make responsible decisions about what they do.

Heat and Chemical Resistant Silicone Rubber

The great systems of electrical generation, transmission and use depend on versatile, high-temperature insulating materials. The expansion of the power grid drove the need for more effective insulating materials.


Thomas A. Edison

An inventor cannot succeed in isolation. When Thomas Edison developed his incandescent lamp, he had a vision. He would roll back the darkness of night.  Cities would be bathed with light from end to end.  Homes, schools, businesses and streets would be bright with the glow from Edison's ingenious globes. But artificial lighting required much more than Edison's creativity. Artificial lighting required a system -- an integrated source of energy, a method for distribution and delivery.

  • It required central generation of the the electricity.
  • It required transmission of the current from the generation station out to the buildings throughout the city.
  • It required wiring within the buildings to bring electricity to the fixtures to heat the filaments inside the evacuated glass bulbs that would glow with light.

 

 

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Used with permission of Douglas C. Brackett, virtualab@earthlink.net

None of these systems existed in any form whatever. Edison and others began to build them.

19th Century Telluride Edison's direct current systems soon disappeared (and not without Edison's protests). George Westinghouse's alternating current became the power distribution of choice from the time in 1891 when L. L. Nunn built a hydroelectric generating station to run equipment at the Gold King mine at 11,000 feet above sea level in the mountains near Telluride, Colorado.

Nunn was up to difficult challenges. He once chased down Butch Cassidy and the Sundance Kid when the outlaws' gang robbed Nunn's Telluride Bank. The gang disarmed Nunn and let him go.


19th Century Telluride

One problem Edison and Westinghouse and L. L. Nunn faced in applying electrical power was the choice of insulation of the current from its surroundings in generators, motors, cable and appliances. Telluride's citizens climbed to the mine one night each spring as the snow began to melt and the stream powering the generator began to run. They came to watch the yearly startup of Gold King. Nunn would throw the switch to bring power to the mine and the high voltage electricity would arc through the building, the mine building would glow spectacularly. This dangerous arcing would drive the power industry to failure if it could not be controlled by effective insulation in daily service.

As installations got larger, as power distribution became more extensive, the rags and pitch and paper and mica that Edison used for insulation became obsolete. Power distribution cable insulation must be flexible and waterproof. Motors and generators required tough insulators, capable of wedging into tight spaces, capable of staying in place at higher and higher rotor speeds.

And as the electrical energy from motors did more and more work, it produced more and more heat. You could impregnate cotton with varnish and use the composite for insulation. You could insulate wire and cable with natural rubber. But if the heat of the system reached above 125 degrees Celsius, these insulating materials broke down; they cracked, became embrittled, shorted out. In the early century Class A (now Class B) insulation with an upper temperature limit of 130 degrees Celsius was the best available for the power industry.

So the manufacturers of power equipment, Westinghouse, General Electric, limited to no more than 130 degrees Celsius, had to build large motors and generators with enough mass of iron and copper in them to dissipate the heat. They had a big financial incentive to find higher temperature insulations which would reduce the size and costs of their equipment. They switched to asbestos impregnated with resins, they raised the service temperature to 155 degrees Celsius. This was Class B (now Class F) insulation. We did not know then the toxicity of asbestos; asbestos-based insulation failed because the fibers became embrittled after long term service near 155 degrees Celsius.


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The rest of the story is this: science and technology, the development of new coatings and fiber substrates, leads to an enormous range of insulating materials, allowing higher and higher temperature use. Silicones enabled us to take the jump to 180 degrees Celsius. This ChemCases unit now begins the silicone story.