Electrical isolation
An electrical insulator is a material whose internal electric charges cannot move causing a small magnitude of current under the influence of an electric field, unlike conductive and semiconductor materials, which easily conduct a electric current. The fundamental characteristic that distinguishes insulating materials is their high resistivity compared to semiconductors and conductors.
Perfect electrical insulation does not exist; even the best insulation contains small mobile carriers (charge carriers), capable of carrying current. Therefore, any type of insulation becomes conductive when a high enough voltage is applied to it to trigger electrons from the atoms that make up the material. This value is known as the breakdown voltage of an insulation. It is commonly attributed as good Insulating glass, paper and Teflon, which have a high resistivity. Insulating materials have the function of avoiding contact between the different conductive parts (insulation of the installation) and protecting people from electrical voltages (protective insulation). An example of these materials are rubber-like polymers, and most plastics which can be thermosetting or thermoplastic in nature.
Insulation is widely used in electrical equipment to separate electrical conductors and prevent electrical current from flowing between them. Insulation is also used to cover electrical wires. There are different levels of insulation for electrical cables, in low voltage the two most common are 450/750 V and 0.6/1 kV. The term insulator refers specifically to the insulating supports used to fix the lines transmission or distribution lines to poles and transmission towers. Insulators support the weight of the lines and prevent current from flowing through the lines to the supporting structure.
Physics of solid conduction
According to the modern theory of matter (proven by experimental results), the atoms of matter are made up of a positively charged nucleus, around which negative electrical charges revolve at high speed. These negative charges, the electrons, are indivisible and identical for all matter.
In elements called conductors, some of these electrons can pass freely from one atom to another when a potential difference (or electrical voltage) is applied between the ends of the conductor.
This movement of electrons is what is called electric current. Some materials, mainly metals, have large numbers of free electrons that can move through the material. These materials have the ability to transmit charge from one object to another, these are the aforementioned conductors.
Ohm's Law
Voltage causes electricity to flow along the copper wires, while the insulation covering the copper wires exerts a resistance to the flow of current, which is much less along the length of the wire.
By applying Ohm's law to the wire, we will have that the lower the resistance of the wire, the more current will be had with the same voltage. It is important to keep in mind that no insulation is perfect (its resistance is not infinite), so some amount of electricity flows along the insulation through the ground. This current can be in the millionths of amps, but it must be measured with a good insulation test instrument, such as a megger, popularly known as a "Megger."
In summary, a good insulation is the one that does not deteriorate when the voltage and therefore the current increase, obtaining a high resistance, which must be maintained over time. This is visualized by making periodic measurements and studying the trend that causes an insulation to deteriorate.
There are different types of surges:
- Overvoltage on a permanent basis, or on a permanent basis or in its proximity. They are characterized by a steep front of duration between microseconds and milliseconds:
- Slow front: Front of 20 microseconds to 500 microseconds, tail of up to 20 milliseconds.
- Quick front: Front of 0.1 microseconds to 20 microseconds, tail of up to 300 microseconds.
- Very fast front: Minor front (electric contact) The materials most frequently used are plastics and ceramics.
The behavior of insulators is due to the potential barrier that is established between the valence and conduction bands, which hinders the existence of free electrons capable of conducting electricity through the material (for more details see semiconductor).
An electrically insulating material has theoretically infinite resistance. Some materials, such as air or water, are insulators under certain conditions but not for others. Air, for example, is insulating at room temperature, but under conditions of relatively low signal frequency and electrical power, it can become a conductor.
Conducting materials: metals, iron, mercury, gold, silver, copper, platinum, lead, etc.
Insulating materials: plastic, wood, ceramics, rubber.
History
The first electrical systems that used insulators were telegraph lines; direct fixing of cables to wooden posts gave very poor results, especially when there was humidity.
The first glass insulators that were used in large numbers had an unthreaded hole. These pieces of glass were placed on a tapered wooden dowel, which extended vertically upwards from the crosshead of the pole (commonly just two insulators to a pole and perhaps one on top of the pole itself). The natural contraction and expansion of the cables attached to these "wireless insulators" it caused the insulators to come off their pins, requiring manual readjustment.
Among the first to produce ceramic insulators were UK companies, with Stiff and Royal Doulton using stoneware from the mid-1840s, Joseph Bourne (later renamed the Denby Pottery Company) producing them from about 1860 and Bullers since 1868. Patent number 48,906 was granted to Louis A. Cauvet on July 25, 1865 for a process for producing insulators with a threaded hole: pin-type insulators still have threaded holes.
The invention of suspension insulators made high-voltage power transmission possible. As transmission line voltages reached and exceeded 60,000 volts, the insulators required became very large and heavy, and those built to a safety margin of 88,000 volts were the practical limit for their manufacture and installation. Suspension insulators, on the other hand, can be connected in strings as long as the line voltage requires.
A wide variety of telephone, telegraph and electrical insulators have been manufactured; some people collect them, both for their historical interest and for the aesthetic quality of many insulator designs and finishes. There is a collectors organization, the US National Insulator Association, which has more than 9,000 members.
Internal Links
Wikimedia Commons hosts a multimedia category Electrical insulation.