Electrical insulating materials

Electrical insulating materials are materials that used to insulated the live wire,conductor in electrical cable,machine,transformers,motor etc.have high resistance to flow of current,less hygroscopic and long life.The properties of insulating materials have high dielectric strength,low dielectric losses,resistance to moisture ,high mechanical strength and good heat conductivity.According to physical state the insulating materials are gaseous, liquid, solid, fibrous, plastic,rubber,minerals,glass and porcelain.Gaseous insulating materials mostly used in high voltage applications.Air is important gaseous insulating materials used in cooling as well as insulation. Other gaseous insulating materials are hydrogen and sulfur fluoride use in generator cooling and HT circuit breakers respectively. Liquid insulating materials mostly used in transformers,HT circuit breakers and reactors.The insulating oil is used  as a cooling as well as insulation in transformers and HT switchgears for arc quenching at time of operation.Solid insulating materials are mica,paper,cotton and many more used in rewinding of electrical machines also found in liquid state and after solidify convert in to solid insulating materials.Varnish and drying oil are examples of solid insulating materials used to coating of super enamelled wire in rewinding electrical machines.Fibrous insulating materials mostly is textile materials like cotton and jute have mechanically strong,highly hygroscopic and cheap mainly used for cable manufacturing.Rubber,plastic and minerals insulating materials  mainly used for cable manufacturing plastic as a filler,rubber protect from moisture and minerals to provide high mechanical strength to the cable.Porcelain insulating materials are mainly used for overhead line insulation between pole and conductor.According to thermal classification solid insulating materials are
Class Y working temperature 90℃ used for small single phase machines and domestic appliances insulating materials are cotton, paper and silk not impregnated form.
Class A working temperature 105℃ used for rewinding of  AC and DC motors insulating materials are cotton, paper and silk impregnated form.
Class E working temperature 120℃ used for High voltage windings of transformers and induction motors insulating materials are cotton, paper laminated with formaldehyde bonding and epoxy resins.
Class B working temperature 130℃ used for manufacturing of welding machine insulation, DC machines commutator and salient pole alternator insulating materials are mica,asbestos and fiberglass.
Class F working temperature 155℃ used for lamination of machines insulating materials are silicone and heat resistance varnish.
Class H working temperature 180℃ used for high voltage transformer and slot lining of high voltage machines.
Class C working temperature 225℃ used for special purpose machine insulating materials are mica, glass,ceramics and quartz having high thermal stability.

Cotton Insulation

Cotton has low dielectric strength and hygroscopic.The insulating properties of cotton can be improved by applying insulating varnish.The application of cotton is rewinding of chokes,armature of small electrical machine,coils of small transformers and covering of bare wires.Empire cloth and black insulating tape are made when cotton impregnated with linseed oil and bitumen base varnish respectively.Silk is similar to cotton have higher tensile strength needs to impregnation with varnish or insulating oil.When cotton tape impregnated with bituminous compound called black tape used for protective braide of cables.If cotton tape coated with an insulating varnish known as empire tape are in black and yellow colour depends on varnish used.Due to high tensile strength,resistance to moisture and heat resistant used for HT and LT machines.
Wood a bamboo is used as an insulating materials in motor rewinding for supporting the coil in slots.The insulating properties of wood can be improved by impregnation with oil and varnish.

Rubber Insulation

Rubber is an insulating materials used as an insulation of cable.Natural rubber is vulcanised to make it suitable insulating materials, during vulcanisation sulphur is added to improve insulating properties.Rubber insulation tape is used for jointing rubber insulated cable.Hard vulcanised rubber are called ebonite is used for making of bushes,pins and terminal bases.It is available in sheets and rods.Paper is an insulating materials that can be used for providing insulation for power cable,rewinding of motors in slots and telephone cable.Ordinary paper have adequate insulating  properties can be further improved by addition of chemicals and varnishing during manufacturing
process. Chemically treated paper known as insulation board used for slot linear in electrical machine and transformer as spacers,washers and cylinders.

Paper Insulation

Mica insulating materials mostly used for high voltage generator and turbo generator.Mica have high temperature resistance,electric strength,mechanical strength and low dielectric loss.
Bakelite insulating materials used for main switches covers, terminal connectors and transformer cylinders mainly in plastic form.The bakelite sheets are made of cloth treated with phenolic resins and bonded in to homogeneous mass with heat and pressure.Such type of sheets available in black and brown depends upon varnish used.Paper  and fabric based bakelite available.

Glass insulating materials in brittle form used as a glass tape and varnished glass cloth.Glass varnished cloth used for insulating the high voltage machines working high temperature.
Insulating varnish is in liquid form.the main properties of insulating varnish have high chemical stability,air drying and not react with base materials.It is used for coating to fill voids in porous materials and motor windings to withstand the electrical and mechanical stresses better.Insulating varnish protects the windings against moisture,dirt,oils and chemicals,selection of suitable varnish depends on nature of winding wire insulation and thermal classification of insulation.

Mica Insulation

Insulating oil used in transformers as a insulation as well as cooling and oil type circuit breakers arc quenching at the time of making and breaking of contact.The insulating oil should be free
from moisture ,clean and high dielectric strength. The properties of transformers oil should be 40 Kv (r.m.s.)electric strength the gap will be 4mm between testing rods,specific gravity 0.85 to 0.88 and flash point min 145℃.If electric strength of transformer is low due to moisture and harmful

contaminants it is necessary treat in a filter plant before use in electrical equipment.
Polymers are also used as a electrical insulating materials. Some of the main polymers are Polyethylene,Polyisobutylene, Polyvinyl chloride,Polystyrene and Acrylic polymers.

Polyethylene commonly used for insulation of coaxial cables and moisture proofing because of high water resistant.

Polymer Insulation

Polyisobutylene used for cable coating have flexible and elastic nature.
Polyvinyl chloride is tough and plastic nature mostly used for cable jackets,fabric coating and lead wire insulation.
Polystyrene have high resistivity and  moisture resistant used to insulation of instrument panel.

Acrylic polymers mostly used for aircraft shields and having clarity and transparent.

Importance of earthing in electrical technology.

Earthing is most important part of  the electrical technology that provides protection of personnel and equipments by the operation of the protective control gears like (ELCB) earth leakage circuit breakers and isolate the faulty circuit when insulation failure and accidental contact between high and low voltage.The earthing is the electrical connection between neutral or non current carrying electrical equipments part to the earth (Ground).The main part of the earthing is earth pit,earth electrode,connection wires and soil treatment materials like salt, charcoal etc.Earthing is two types system and equipment earthing. In system earthing the electrical connection between neutral and earth in generating station transformer used in power house.The heavy strip earth electrode use in mesh arrangements.The equipment earthing is electrical connection between non current carrying metal parts of electrical equipments and general mass of the earth like breakers,CT and PT etc. used in sub stations.In other word the earthing that restrict the voltage of live conductor with respect to the potential of earth.In practice there are three types of earthing method adopted a direct earthing system,earth neutral system and (ELCB) earth leakage circuit breakers system. In direct earthing system the all installation directly connected  to earth  not through the trip coil of the an earth leakage circuit breaker.In earth neutral earthing system the all installation connected  to earth in addition connected within the installation to the neutral conductor of the supply system.In (ELCB) earth leakage circuit breakers earthing system the part of an installation which are sought to be protected are connected to an earth electrode through the coil of ELCB which control the supply to that part of the installation also in addition to the equipement earth.The value of earth resistance kept low including earth electrode ,connection wire etc.for the operation of protective devices i.e. maintain the voltage between neutral and earth potential should be zero for good earthing, practically it is not possible due to  presence of some resistance.It is must to maintained low earth resistance when a circuit with large current carrying capacity is protected by a high capacity fuse. If the value of earth resistance is high than  the fault current not flowing to the earth cased fire as heating and arcing at point of high resistance which can set any inflammable object in the vicinity on fire.So in dry season to reduced the  resistivity of soil by pouring the water in earth electrode and maintain the moisture. The value of earth electrode resistance depends upon the electrical resistivity of soil in which the electrode is installed.The wet marshy soil,clayey or loamy mixed with small quantity of sand ,closely packed soil with fine grain and vegetation indicates soil with good moisture content is best for installation of earth electrode for getting low resistance path to flow of fault current.If moisture fall below 20% the resistivity of soil increases sharply so it is necessary to keep up the moisture level periodic addition of water.Where the soil is dry the artificial treatment of soil required done by adding of sodium chloride (common salt) calcium chloride and sodium chloride.The salt and charcoal added for the same purpose.The other method of treatment of soil by adding the chemical in soil.The material is suitable for earth electrode should be able to resist corrosion for a long time e.g. Galvanize iron,copper,and cast iron.Based on shape the earth electrode are Rod or Pipe, Strip and Plate. The minimum size of earth Rod electrode 16 mm for steel and 12.5mm for copper rod in 2.5meter in length.In case of pipe earthing electrode minimum inner dia 40mm for galvanize iron and 100mm for cast iron pipe in 2.5meter in length.In case of strip minimum cross sectional area for copper 20X3mm and 25X4mm for galvanize iron. When we used conductor for the earthing diameter 4mm(8 SWG) for copper and 6mm for galvanize iron.The length of buried conductor shall not be less than 15mtr.In case of plate electrode the minimum size of plate 60x60x3mm for copper and 60x60x6mm for iron and steel.Plate earthing is suitable for generating station.The driven electrodes earthing is suitable for soil free from stone , rock and undistributed soil so no excavation is necessary and cost of installation is low.Generally GI pipe are suitable for incomer breaker up to 200 ampere and GI plate earth electrode suitable for incomer breaker more than 200 ampere, neutral earthing of transformer as well as DG set up to 500 KVA.Copper plate earth electrode suitable for neutral earthing of transformer as well as DG set above 500 KVA. The size of earth bus 50mmX5mm of two copper strip in 11KV sub station and neutral of transformer, generator shall not be connected to this earth bus. The earth electrode fixed by welding ,bolting and clamped joints.All the surfaces where connections were made shall be free from grease,paint,dirt or any other foreign materials. All plugs and three pins sockets ,the earth pin connected to the earth wire.It is must to earthing of domestic appliances accepting those double insulation.The special earthing provided for data processing machine known as 'Clean Earth'. For protection against static charges such as hospital and operation theatres the static earthing suitable. The earth tester used for the measurement of earth resistance it contains three electrodes, one under measurement other two is potential electrode and  current electrode.The size of the potential  and current electrode is 15mm diameter  and length is 400mm long bars.The value of earth resistance should not more than two ohms another method to measure the earth resistance by voltmeter and ammeter method the electrode arrangement usually as the earth tester and find the earth resistance V/I reading shown in voltmeter and ammeter. The instrument used for measurement of resistivity contains four spikes two are current electrode and two are potential electrode driven into the ground at equal interval 'A' .The earth resistivity is given byρ=2ㄫAR where R is the steady reading on the instrument. The testing of insulation resistance between an insulated conductor and earth with the help of 500 volts megger. The phase conductor connected Line marked on megger and earth continuity conductor connected to an earth marked on megger.The handle is turned to indicate the steady reading on the instrument.The protection of all building and structure against lighting strokes like power house,large warehouses and monuments is done by installing lighting arresters at the top of the structure and connecting the same to the general mass of earth through a connecting conductor   and earth electrode so that the structure is not subjected to the dangerously high potential of the cloud discharge and system should be designed and maintained with low ground resistance.The conductor laid around the building near the top to the interconnected the various air termination called roof conductors protection against lightning strokes. The earthing system used for lighting protection must be independent of  the equipment earthing system.