Ceramic insulator used on electrified railways. The ridged shape increases the surface distance along which electrical leakage could occur, thereby enhancing the insulator’s effectiveness, especially in wet or polluted conditions.Three-core copper wire power cable, each core with an individual colour-coded insulating sheath, all contained within an outer protective sheath
Anelectrical insulator is a material in whichelectric current does not flow freely. The atoms of the insulator have tightly bound electrons which cannot readily move. Other materials—semiconductors andconductors—conduct electric current more easily. The property that distinguishes an insulator is itsresistivity; insulators have higher resistivity than semiconductors or conductors. The most common examples arenon-metals.
A perfect insulator does not exist because even the materials used as insulators contain small numbers of mobile charges (charge carriers) which can carry current. In addition, all insulators becomeelectrically conductive when a sufficiently large voltage is applied that the electric field tearselectrons away from the atoms. This is known aselectrical breakdown, and the voltage at which it occurs is called thebreakdown voltage of an insulator. Some materials such asglass,paper andPTFE, which have highresistivity, are very good electrical insulators. A much larger class of materials, even though they may have lower bulk resistivity, are still good enough to prevent significant current from flowing at normally used voltages, and thus are employed as insulation forelectrical wiring andcables. Examples include rubber-likepolymers and mostplastics which can bethermoset orthermoplastic in nature.
Insulators are used in electrical equipment to support and separate electricalconductors without allowing current through themselves. An insulating material used in bulk to wrap electrical cables or other equipment is calledinsulation. The terminsulator is also used more specifically to refer to insulating supports used to attachelectric power distribution ortransmission lines toutility poles andtransmission towers. They support the weight of the suspended wires without allowing the current to flow through the tower to ground.
Electrical insulation is the absence ofelectrical conduction. Electronicband theory (a branch of physics) explains that electric charge flows when quantum states of matter are available into which electrons can be excited. This allows electrons to gain energy and thereby move through a conductor, such as ametal, if an electric potential difference is applied to the material. If no such states are available, the material is an insulator.
Most insulators have a largeband gap. This occurs because the "valence" band containing the highest energy electrons is full, and a large energy gap separates this band from the next band above it. There is always some voltage (called thebreakdown voltage) that gives electrons enough energy to be excited into this band. Once this voltage is exceeded, electrical breakdown occurs, and the material ceases being an insulator, passing charge. This is usually accompanied by physical or chemical changes that permanently degrade the material and its insulating properties.
When the electric field applied across an insulating substance exceeds in any location the threshold breakdown field for that substance, the insulator suddenly becomes a conductor, causing a large increase in current, anelectric arc through the substance. Electrical breakdown occurs when theelectric field in the material is strong enough to accelerate freecharge carriers (electrons and ions, which are always present at low concentrations) to a high enough velocity to knock electrons from atoms when they strike them,ionizing the atoms. These freed electrons and ions are in turn accelerated and strike other atoms, creating more charge carriers, in achain reaction. Rapidly the insulator becomes filled with mobile charge carriers, and itsresistance drops to a low level. In a solid, the breakdown voltage is proportional to theband gap energy. Whencorona discharge occurs, the air in a region around a high-voltage conductor can break down and ionise without a catastrophic increase in current. However, if the region of air breakdown extends to another conductor at a different voltage it creates a conductive path between them, and a large current flows through the air, creating anelectric arc. Even a vacuum can suffer a sort of breakdown, but in this case the breakdown orvacuum arc involves charges ejected from the surface of metal electrodes rather than produced by the vacuum itself.
In addition, all insulators become conductors at very high temperatures as the thermal energy of the valence electrons is sufficient to put them in the conduction band.[1][2]
In certain capacitors, shorts between electrodes formed due to dielectric breakdown can disappear when the applied electric field is reduced.[3][4][5][relevant?]
A flexible coating of an insulator is often applied to electric wire and cable; this assembly is calledinsulated wire. Wires sometimes don't use an insulating coating, just air, when a solid (e.g. plastic) coating may be impractical. Wires that touch each other produce cross connections,short circuits, and fire hazards. Incoaxial cable the center conductor must be supported precisely in the middle of the hollow shield to prevent electro-magnetic wave reflections. Wires that expose high voltages can cause human shock andelectrocution hazards.
Most insulated wire and cable products have maximum ratings for voltage and conductor temperature. The product may not have anampacity (current-carrying capacity) rating, since this is dependent on the surrounding environment (e.g. ambient temperature).
In electronic systems,printed circuit boards are made from epoxy plastic and fibreglass. The nonconductive boards support layers of copper foil conductors. In electronic devices, the tiny and delicate active components are embedded within nonconductiveepoxy orphenolic plastics, or within baked glass or ceramic coatings.
Inmicroelectronic components such astransistors andICs, the silicon material is normally a conductor because of doping, but it can easily be selectively transformed into a good insulator by the application of heat and oxygen. Oxidised silicon isquartz, i.e.silicon dioxide, the primary component of glass.
Inhigh voltage systems containingtransformers andcapacitors, liquid insulator oil is the typical method used for preventing arcs. The oil replaces air in spaces that must support significant voltage withoutelectrical breakdown. Other high voltage system insulation materials include ceramic or glass wire holders, gas, vacuum, and simply placing wires far enough apart to use air as insulation.
The most important insulation material is air. A variety of solid, liquid, andgaseous insulators are also used in electrical apparatus. In smallertransformers,generators, andelectric motors, insulation on the wire coils consists of up to four thin layers of polymer varnish film. Film-insulatedmagnet wire permits a manufacturer to obtain the maximum number of turns within the available space. Windings that use thicker conductors are often wrapped with supplemental fiberglassinsulating tape. Windings may also be impregnated with insulating varnishes to preventelectrical corona and reduce magnetically induced wire vibration. Large power transformer windings are still mostly insulated withpaper, wood, varnish, andmineral oil; although these materials have been used for more than 100 years, they still provide a good balance of economy and adequate performance.Busbars andcircuit breakers inswitchgear may be insulated with glass-reinforced plastic insulation, treated to have low flame spread and to prevent tracking of current across the material.
In older apparatus made up to the early 1970s, boards made of compressedasbestos may be found; while this is an adequate insulator at power frequencies, handling or repairs to asbestos material can release dangerous fibers into the air and must be carried out cautiously. Wire insulated with felted asbestos was used in high-temperature and rugged applications from the 1920s. Wire of this type was sold byGeneral Electric under the trade name "Deltabeston."[6]
Live-front switchboards up to the early part of the 20th century were made of slate or marble. Some high voltage equipment is designed to operate within a high pressureinsulating gas such assulfur hexafluoride. Insulation materials that perform well at power and low frequencies may be unsatisfactory atradio frequency, due to heating from excessive dielectric dissipation.
Flexible insulating materials such asPVC (polyvinyl chloride) are used to insulate the circuit and prevent human contact with a 'live' wire – one having voltage of 600 volts or less. Alternative materials are likely to become increasingly used due to EU safety and environmental legislation making PVC less economic.
In electrical apparatus such as motors, generators, and transformers, variousinsulation systems are used, classified by their maximum recommended working temperature to achieve acceptable operating life. Materials range from upgraded types of paper to inorganic compounds.
All portable or hand-held electrical devices are insulated to protect their user from harmful shock.
Class I insulation requires that the metal body and other exposed metal parts of the device be connected to earth via agrounding wire that isearthed at the main service panel—but only needs basic insulation on the conductors. This equipment needs an extra pin on thepower plug for the grounding connection.
Class II insulation means that the device isdouble insulated. This is used on some appliances such as electric shavers, hair dryers and portable power tools. Double insulation requires that the devices have both basic and supplementary insulation, each of which is sufficient to preventelectric shock. All internal electrically energized components are totally enclosed within an insulated body that prevents any contact with "live" parts. In theEU, double insulated appliances all are marked with a symbol of two squares, one inside the other.[7]
Pin-type glass insulator for long-distanceopen-wire transmission for telephone communication
Conductors for overhead high-voltageelectric power transmission are bare, and are insulated by the surrounding air. Conductors for lower voltages indistribution may have some insulation but are often bare as well. Insulating supports are required at the points where they are supported byutility poles ortransmission towers. Insulators are also required where wire enters buildings or electrical devices, such astransformers orcircuit breakers, for insulation from the case. Often these arebushings, which are hollow insulators with the conductor inside them.
Insulators used for high-voltage power transmission are made fromglass,porcelain orcomposite polymer materials. Porcelain insulators are made fromclay,quartz oralumina andfeldspar, and are covered with a smooth glaze to shed water. Insulators made from porcelain rich in alumina are used where high mechanical strength is a criterion. Porcelain has a dielectric strength of about 4–10 kV/mm.[8] Glass has a higher dielectric strength, but it attracts condensation and the thick irregular shapes needed for insulators are difficult to cast without internal strains.[9] Some insulator manufacturers stopped making glass insulators in the late 1960s, switching to ceramic materials.
Some electric utilities use polymercomposite materials for some types of insulators. These are typically composed of a central rod made offibre reinforced plastic and an outer weathershed made ofsilicone rubber or ethylene propylene diene monomer rubber (EPDM). Composite insulators are less costly, lighter in weight, and have excellenthydrophobic properties. This combination makes them ideal for service in polluted areas.[10] However, these materials do not yet have the long-term proven service life of glass and porcelain.
Power lines supported by ceramic pin-type insulators inCalifornia, USA
High voltage ceramic bushing during manufacture, beforeglazing (1977)
The electricalbreakdown of an insulator due to excessive voltage can occur in one of two ways:
Apuncture arc is a breakdown and conduction of the material of the insulator, causing anelectric arc through the interior of the insulator. The heat resulting from the arc usually damages the insulator irreparably.Puncture voltage is the voltage across the insulator (when installed in its normal manner) that causes a puncture arc.
Aflashover arc is a breakdown and conduction of the air around or along the surface of the insulator, causing an arc along the outside of the insulator. Insulators are usually designed to withstand flashover without damage.Flashover voltage is the voltage that causes a flash-over arc.
Most high voltage insulators are designed with a lower flashover voltage than puncture voltage, so they flash over before they puncture, to avoid damage.
Dirt, pollution, salt, and particularly water on the surface of a high voltage insulator can create a conductive path across it, causing leakage currents and flashovers. The flashover voltage can be reduced by more than 50% when the insulator is wet. High voltage insulators for outdoor use are shaped to maximise the length of the leakage path along the surface from one end to the other, called the creepage length, to minimise these leakage currents.[11] To accomplish this the surface is moulded into a series of corrugations or concentric disc shapes. These usually include one or moresheds; downward facing cup-shaped surfaces that act as umbrellas to ensure that the part of the surface leakage path under the 'cup' stays dry in wet weather. Minimum creepage distances are 20–25 mm/kV, but must be increased in high pollution or airborne sea-salt areas.
A three-phase insulator used on distribution lines, typically 13.8 kV phase to phase. The lines are held in a diamond pattern, multiple insulators used between poles.
Insulators are characterized in several common classes:
Pin insulator - The pin-type insulator is mounted on a pin affixed on the cross-arm of the pole. The insulator has a groove near the top just below the crown. The conductor passes through this groove and is tied to the insulator withannealed wire of the same material as the conductor. Pin-type insulators are used for transmission and distribution of communication signals, and electric power at voltages up to 33 kV. Insulators made for operating voltages between 33 kV and 69 kV tend to be bulky and have become uneconomical.
Post insulator - A type of insulator in the 1930s that is more compact than traditional pin-type insulators and which has rapidly replaced many pin-type insulators on lines up to 69 kV and in some configurations, can be made for operation at up to 115 kV.
Suspension insulator - For voltages greater than 33 kV, it is a usual practice to use suspension type insulators, consisting of a number of glass or porcelain discs connected in series by metal links in the form of a string. The conductor is suspended at the bottom end of this string while the top end is secured to the cross-arm of the tower. The number of disc units used depends on the voltage.
Strain insulator - Adead end oranchor pole or tower is used where a straight section of line ends, or angles off in another direction. These poles must withstand the lateral (horizontal) tension of the long straight section of wire. To support this lateral load, strain insulators are used. For low voltage lines (less than 11 kV), shackle insulators are used as strain insulators. However, for high voltage transmission lines, strings of cap-and-pin (suspension) insulators are used, attached to thecrossarm in a horizontal direction. When the tension load in lines is exceedingly high, such as at long river spans, two or more strings are used in parallel.
Shackle insulator - In early days, the shackle insulators were used as strain insulators. But nowaday, they are frequently used for low voltage distribution lines. Such insulators can be used either in a horizontal position or in a vertical position. They can be directly fixed to the pole with a bolt or to the cross arm.
Bushing - enables one or several conductors to pass through a partition such as a wall or a tank, and insulates the conductors from it.[12]
Sheath insulators are insulators that protect a full-length of bottom-contactthird rail. Sheath insulators must protect the full length to prevent accidental conduction or contact. Sheath insulators also protect the third rail physically.
Sheath insulators are commonly made out of epoxy, due to its longevity and low conductivity. Ceramic is another choice, and is often used in addition to plastic.
This sectionneeds expansion. You can help byadding to it.(April 2021)
Typical number of disc insulator units for standard line voltages[13]
Line voltage (kV)
Discs
34.5
3
69
4
115
6
138
8
161
11
230
14
287
15
345
18
360
23
400
24
500
34
600
44
750
59
765
60
Pin-type insulators are unsuitable for voltages greater than about 69 kV line-to-line. Higher voltagetransmission lines usually use modular suspension insulator designs. The wires are suspended from a 'string' of identical disc-shaped insulators that attach to each other with metalclevis pin or ball-and-socket links. The advantage of this design is that insulator strings with differentbreakdown voltages, for use with different line voltages, can be constructed by using different numbers of the basic units. String insulators can be made for any practical transmission voltage by adding insulator elements to the string.[14] Also, if one of the insulator units in the string breaks, it can be replaced without discarding the entire string.
Each unit is constructed of a ceramic or glass disc with a metal cap and pin cemented to opposite sides. To make defective units obvious, glass units are designed so that an overvoltage causes a puncture arc through the glass instead of a flashover. The glass is heat-treated so it shatters, making the damaged unit visible. However the mechanical strength of the unit is unchanged, so the insulator string stays together.
Standard suspension disc insulator units are 25 centimetres (9.8 in) in diameter and 15 cm (6 in) long, can support a load of 80–120kilonewtons (18,000–27,000 lbf), have a dry flashover voltage of about 72 kV, and are rated at an operating voltage of 10–12 kV.[15] However, the flashover voltage of a string is less than the sum of its component discs, because the electric field is not distributed evenly across the string but is strongest at the disc nearest to the conductor, which flashes over first. Metalgrading rings are sometimes added around the disc at the high voltage end, to reduce the electric field across that disc and improve flashover voltage.
In very high voltage lines the insulator may be surrounded bycorona rings.[16] These typically consist oftoruses of aluminium (most commonly) or copper tubing attached to the line. They are designed to reduce the electric field at the point where the insulator is attached to the line, to preventcorona discharge, which results in power losses.
Suspension insulator string (the vertical string of discs) on a 275 kV suspension pylon
Suspended glass disc insulator unit used in suspension insulator strings for high voltage transmission lines
The first electrical systems to make use of insulators weretelegraph lines; direct attachment of wires to wooden poles was found to give very poor results, especially during damp weather.
The first glass insulators used in large quantities had an unthreaded pinhole. These pieces of glass were positioned on a tapered wooden pin, vertically extending upwards from the pole's crossarm (commonly only two insulators to a pole and maybe one on top of the pole itself). Natural contraction and expansion of the wires tied to these "threadless insulators" resulted in insulators unseating from their pins, requiring manual reseating.
Amongst the first to produce ceramic insulators were companies in the United Kingdom, with Stiff andDoulton usingstoneware from the mid-1840s, Joseph Bourne (later renamedDenby) producing them from around 1860 and Bullers from 1868.Utility patent number48,906 was granted to Louis A. Cauvet on 25 July 1865 for a process to produce insulators with a threaded pinhole: pin-type insulators still have threaded pinholes.
The invention of suspension-type insulators made high-voltage power transmission possible. As transmission line voltages reached and passed 60,000 volts, the insulators required become very large and heavy, with insulators made for a safety margin of 88,000 volts being about the practical limit for manufacturing and installation. Suspension insulators, on the other hand, can be connected into strings as long as required for the line's voltage.
A large variety of telephone, telegraph and power insulators have been made; some people collect them, both for their historic interest and for the aesthetic quality of many insulator designs and finishes. One collectors organisation is the US National Insulator Association, which has over 9,000 members.[17]
Often abroadcastingradio antenna is built as amast radiator, which means that the entire mast structure is energised with high voltage and must be insulated from the ground.Steatite mountings are used. They have to withstand not only the voltage of the mast radiator to ground, which can reach values up to 400 kV at some antennas, but also the weight of the mast construction and dynamic forces.Arcing horns andlightning arresters are necessary because lightning strikes to the mast are common.
Guy wires supporting antenna masts usually havestrain insulators inserted in the cable run, to keep the high voltages on the antenna from short circuiting to ground or creating a shock hazard. Often guy cables have several insulators, placed to break up the cable into lengths that prevent unwanted electricalresonances in the guy. These insulators are usually ceramic and cylindrical or egg-shaped (see picture). This construction has the advantage that the ceramic is under compression rather than tension, so it can withstand greater load, and that if the insulator breaks, the cable ends are still linked.
These insulators also have to be equipped with overvoltage protection equipment. For the dimensions of the guy insulation, static charges on guys have to be considered. For high masts, these can be much higher than the voltage caused by the transmitter, requiring guys divided by insulators in multiple sections on the highest masts. In this case, guys which are grounded at the anchor basements via a coil - or if possible, directly - are the better choice.
Feedlines attaching antennas to radio equipment, particularlytwin-lead type, often must be kept at a distance from metal structures. The insulated supports used for this purpose are calledstandoff insulators.
^Hu, Yi; Liu, Kai (2017). "Transmission lines detection technology".Inspection and Monitoring Technologies of Transmission Lines with Remote Sensing:205–279.doi:10.1016/B978-0-12-812644-8.00004-7.ISBN978-0-12-812644-8.Composite insulators can take wind and rain and have good self-cleaning performance under wind and rain, so need checking for pollution only once every 4–5 years, and requiring less time for the repair and power interruption.
^Donald G. Fink, H. Wayne Beaty (ed).,Standard Handbook for Electrical Engineers, 11th Edition, McGraw-Hill, 1978,ISBN0-07-020974-X, pages 14-153, 14-154
M. Shakiba, F.; M, S. Azizi; M., Zhou (Oct 2022). "A Transfer Learning-Based Method to Detect Insulator Faults of High-Voltage Transmission Lines via Aerial Images: Distinguishing Intact and Broken Insulator Images".IEEE Systems, Man, and Cybernetics Magazine.8 (4):15–25.doi:10.1109/MSMC.2022.3198027.ISSN2333-942X.S2CID252999399.
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