Porcelain and Ceramic Insulators - Poinsa
Porcelain insulators are primarily composed of high concentrations of minerals such as alumina or clay. This composition allows electricity to pass through without interacting with nearby electrical conductors, ensuring greater safety and maintaining the power charge. The ideal characteristic of porcelain being a non-conductive material, especially when combined with other non-conductors, makes it an excellent choice for insulators.
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Insulator (Electricity)
Material that does not conduct an electric current
This article is about electrical insulation. For insulation of heat, see thermal insulation
An electrical insulator is a material in which electric current does not flow freely. Insulators have tightly bound electrons that cannot move easily, distinguishing them from conductors or semiconductors. This property, known as resistivity, is higher in insulators compared to conductors or semiconductors. Common examples include non-metals.
Even perfect insulators do not exist, as they may contain small numbers of mobile charges. High voltage can cause insulators to become conductive, a phenomenon known as electrical breakdown. Materials like glass, paper, and PTFE are excellent good electrical insulators, whereas materials like rubber-like polymers and thermoplastics, though having lower resistivity, still effectively insulate electrical wiring.
Insulators support and separate electrical conductors without allowing current to pass through them. They are used in electrical equipment and are crucial in wrapping electrical cables, supporting power distribution or transmission lines on utility poles, and preventing current from grounding.
Physics of Conduction in Solids
Electrical insulation prevents electrical conduction. According to electronic band theory, charge flows when available quantum states allow electron excitation, facilitating movement through a conductor when a potential difference is applied.
Insulators usually exhibit a large band gap, separating the highest energy "valence" band from the next band above it. Breakdown voltage can excite electrons to this higher band, causing material breakdown. This breakdown due to high electric fields accelerates free charge carriers, leading to rapid current increase and material degradation. Even a vacuum can experience breakdown, involving ejected surface charges from electrodes.
All insulators become conductive at extremely high temperatures due to valence electron excitation.
Uses
Flexible insulator coatings are often applied to electric wires and cables. Wires that touch can create dangerous cross connections and short circuits. Uninsulated high voltage wires pose shock and electrocution hazards.
Printed circuit boards in electronic systems use epoxy plastic and fiberglass as a base for copper conductors. Components within these devices are enclosed in nonconductive materials like epoxy, phenolic plastics, or baked glass coatings.
High voltage systems often employ liquid insulator oil to prevent arcs, replacing air in spaces needing high voltage support. Materials such as ceramic or glass wire holders, gas, vacuum, and air provide insulation.
Insulation in Electrical Apparatus
Air is the most crucial insulating material, supplemented by a variety of solid, liquid, and gaseous insulators. In smaller transformers, generators, and motors, multilayer polymer varnish film insulates the wire coils. Larger transformers use traditional materials like paper, wood, and mineral oil for economy and adequate performance. Materials like glass-reinforced plastic insulation prevent current tracking across surfaces in busbars and circuit breakers.
Older apparatus may include boards of compressed asbestos, which require careful handling due to fiber release risks. High voltage equipment may operate within high-pressure insulating gases like sulfur hexafluoride. Electrical wires use materials like polyethylene, PVC, rubber-like polymers, oil impregnated paper, Teflon, and silicone for insulation.
Flexible insulators like PVC, used for circuits up to 600 volts, may be replaced by alternatives due to safety and environmental legislation.
Class I and Class II Insulation
Portable electrical devices are insulated to prevent shock. Class I insulation connects exposed metal parts to the earth via a grounding wire, requiring an additional pin on the power plug. Class II insulation, or double insulation, encloses energized components within an insulated body, preventing contact. These devices, such as shavers and power tools, are marked with a symbol of two squares.
Telegraph and Power Transmission Insulators
High-voltage electric power transmission conductors are usually bare and air-insulated. Insulating supports are required at utility poles, towers, and where wires enter buildings or devices. Bushings are hollow insulators containing conductors.
Materials
High-voltage insulators are made from glass, porcelain, or composite polymer materials. Porcelain insulators, rich in alumina, provide high mechanical strength and a dielectric strength of 4-10 kV/mm. Glass insulators, while having higher dielectric strength, are challenging to cast without internal strains. Polymer composite insulators, being cost-effective, lightweight, and hydrophobic, are suitable for polluted areas but lack long-term proven service life compared to glass and porcelain.
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Power lines supported by ceramic pin-type insulators in California, USA
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10 kV ceramic insulator, showing sheds
Design
Electrical breakdown due to excessive voltage occurs via two mechanisms. Puncture arc involves material breakdown causing an interior electric arc, while flashover arc involves surface breakdown causing an exterior arc. Insulators are designed to withstand flashover without damage, ensuring flashover voltage is lower than puncture voltage to avoid damage.
Dirt, pollution, salt, and water on high-voltage insulators can cause leakage currents and flashovers, reducing flashover voltage when wet. Outdoor high-voltage insulators maximize surface leakage paths, called creepage length, through corrugations and concentric disc shapes with sheds to stay dry in wet weather.
Types
Insulators are categorized into several common classes:
- Pin insulator - Mounted on a pin; used for transmission and distribution up to 33 kV.
- Post insulator - More compact than pin-type, used for up to 69 kV and sometimes up to 115 kV.
- Suspension insulator - For voltages over 33 kV, consisting of series-connected discs.
- Strain insulator - Used for supporting horizontal tension in lines, crafted as shackle insulators for low-voltage stralations.
- Shackle insulator - Used for low-voltage lines, fixable in horizontal or vertical positions.
- Bushing - Enables conductors to pass through partitions.
- Line post insulator
- Station post insulator
- Cut-out
An insulator that protects a full length of bottom-contact third rail.
History
The Brookfield Glass Company is known for producing CD145 insulators, or "Beehive" insulators. Early electrical systems found direct wire attachment to wooden poles ineffective, especially in damp conditions. The first mass-produced glass insulators were unthreaded, seated on tapered wooden pins. UK's Stiff and Doulton, Joseph Bourne, and Bullers were early ceramic insulator producers. Patent 48,906 was granted to Louis A. Cauvet in 1865 for threaded pinhole insulators.
Suspension-type insulators enabled high-voltage power transmission, overcoming manufacturing and installation limits of traditional insulators. Suspension insulators, forming long strings by necessity, resist high voltage more efficiently.
Telephone, telegraph, and power insulators are collectible for historical and aesthetic value. The US National Insulator Association has over 9,000 members.
Insulation of Antennas
Broadcasting radio antennas often use mast radiators energized with high voltage, insulated from ground using steatite mountings to withstand high voltage, mast weight, and dynamic forces. Arcing horns and lightning arresters are necessary for protection.
Guy wires supporting antennas have strain insulators to prevent electrical short circuits and shock hazards. Ceramic insulators, under compression, withstand greater loads and maintain cable integrity if broken. Overvoltage protection considers static charges on guys, critical for high masts. Grounded guys via coils or directly are often optimal.
Feedlines attaching antennas to equipment use standoff insulators to maintain separation from metal structures.
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