Insulators

We know electricity as the most dangerous and powerful source of energy existing in nature. So to protect humans from it's contact, we need some material. A kind of material which opposes the flow of heat, light, sound and electricity is known as an insulator.
In other words, these are the material which have a high value of resistance beyond few, 100 mega Ohms. So practically no flow of current occurs through these types of materials. Stephen Gray was the one to introduce conduction of electrons to the world. he also stated the different materials have different property towards conduction. When he used the metal as conductor, on the other side used silk as an insulator because it showed high resistance towards electric conduction.
Later it was David Brook who altered the old style of insulator and used it for his telegraph lines during 1864 to 1867. His insulator had a thick metal casing around blown glass, the assembly was held together with molten sulfur. Out of the tube extends a "ramshorn" rod that held the telegraph wires. It was mounted into holes drilled into the bottom of wooden cross-arms attached to poles.

                                 The picture shows insulators used by railways for their power transmission.

The term insulator is also used more specifically to refer to insulating supports used to attach electric power distribution or transmission lines to utility poles and transmission towers. They support the weight of the suspended wires without allowing the current to flow through the tower to ground.

Physics defines electrical insulation as the absence of electrical conduction. Electronic band theory (a branch of physics) says that a charge flows if, states(energy bands) are available into which electrons can be excited. This allows electrons to gain energy and thereby move through a conductor such as a metal. If no such states are available, the material is an insulator. Most insulators have a large band 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 the breakdown voltage) that gives electrons enough energy to be excited into this band. If the energy exceeds breakdown voltage, then physical or chemical changes happen that result in the permanent damage of the material's insulating properties.

                                                                            Breakdown of a pin type insulator.

A very flexible coating of an insulator is often applied to electric wire and cable, this is called insulated wire. Wires sometimes don't use an insulating coating, just air, since a solid coating may be impractical(OHE lines). Wires that expose voltages higher than 50 V can cause human shock and electrocution hazards, so insulating coatings help to prevent these problems.

In electronic systems, printed circuit boards are made from epoxy plastic and fiber-glass. The non-conductive boards support layers of copper foil conductors. In electronic devices, the tiny and delicate active components are embedded within non-conductive epoxy or phenolic plastics, or within baked glass or ceramic coatings.

In microelectronic components such as transistors and IC's, 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 is quartz, i.e. silicon dioxide, the primary component of glass.

In high voltage systems containing transformers and capacitors, liquid insulator oil is the typical method used for preventing arcs. The oil replaces air in spaces that must support significant voltage without electrical 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.

Insulation prevents the wire’s current from coming into contact with other conductors, and preserves the material integrity of the wire by protecting against environmental threats such as water and heat. Both the safety and effectiveness of the wire depend on its insulation. Classification based of material used for insulation around cables;

Plastic

• Polyvinyl Chloride(PVC) - Is a relatively inexpensive and easy-to-use material, with the potential to be used in diverse applications. The maximum temperature range is -55°C to 105°C and is flame, moisture, and abrasion resistant. It also holds up against gasoline, ozone, acids, and solvents. PVC jackets display high attenuation and capacitance loss, meaning that power is lost when used in an electrical system.
• Semi Rigid PVC(SR-PVC) - This is mainly used as a primary insulation and is very abrasion resistant. It is also heat, water, acid, and alkali resistant, as well as flame retardant.
• Plenum Polyvinyl Chloride(Plenum PVC) -  Plenum PVC is suitable for use in building spaces behind dropped ceilings or raised floors which are left open to allow for air circulation. Standard PVC is considered a non-plenum insulation option because it does not exhibit the qualities necessary for safe usage in plenum areas. To be plenum-rated the insulation must meet more stringent fire safety regulations.
• Polyethylene (PE) - This compound is used most in coaxial and low capacitance cables because of its exemplary electric qualities. Many times it is used in these applications because it is affordable and can be foamed to reduce the dielectric constant to 1.50, making it an attractive option for cables requiring high-speed transmission. Polyethylene can also be cross-linked to produce high resistance to cracking, cut-through, soldering, and solvents. Polyethylene can be used in temperatures ranging from -65°C to 80°C. All densities of Polyethylene are stiff, hard, and inflexible. The material is also flammable. Additives can be used to make it flame retardant, but this will sacrifice the dielectric constant and increase power loss.
• Polypropylene (PP) - This material is very similar to Polyethylene, but has a wider temperature range of -30°C to 105°C. It is used primarily for thin wall primary insulation. Polypropylene can be foamed to improve its electrical properties.
• Polyurethane (PUR) - Is known for its extreme toughness, flexibility, and flex life, even in low temperatures. It also has excellent ratings for chemical, water, and abrasion resistance. This material works well in retractile cord applications and can be a good option for salt-spray and low-temperature military purposes. Polyurethane is a flammable material. The flame retardant version sacrifices strength and surface finish. Polyurethane’s main disadvantage though, is its poor electrical properties, making it suitable for jackets only.
• Chlorinated Polyethylene (CPE) - CPE displays very good heat, oil, and weather resistance. Many times CPE serves as a lower cost, more environmentally friendly alternative to CSPE. It's reliable performance when exposed to fire also makes it a favorable alternative to PVC insulation. Chlorinated Polyethylene is commonly found in power and control cables and industrial power plant applications.
• Nylon - Nylon is usually extruded over softer insulation compounds. It serves as a tough jacket, exhibiting strong abrasion, cut-through, and chemical resistance, especially in thin wall applications. It is also extremely flexible. One disadvantage of Nylon is its absorption of moisture which degrades some of its electrical properties.

Rubber

• Thermoplastic Rubber  (TPR) - Is used to replace true thermoset rubber. It has improved colorability, higher processing speeds, and a wider usable temperature range. It also displays excellent heat, weather, and age resistance without curing. TPR is not cut-through resistant, but can be used in applications where other properties of rubber are preferred.
• Chlorosulfonated Polyethylene (CSPE) - Sometimes referred to as Hypalon, is resistant to chemicals and UV rays. It works well as a low-voltage insulation and performs at a wide range of temperatures. This insulation material can be found in appliance wire, lead wire, coil leads, transformer leads, and motor lead wire.
• Neoprene (Polychloroprene) - This is a synthetic thermoset rubber that must be vulcanized to obtain it's desired qualities. It exhibits supreme abrasion, cut-through, oil, and solvent resistance. Neoprene is also known for its long service life and wide ranges of temperature and usability. It is remarkably flame retardant and self-extinguishing. Military products often incorporate Neoprene. This material is especially desirable for hand-held cordsets.
• Styrene Butadiene Rubber (SBR) - This is a thermoset compound with qualities similar to Neoprene. It has a temperature range of -55°C to 90°C. 
• Silicone - This material is extremely heat resistant and flame retardant and can be used in temperatures up to 180°C. It is moderately abrasion resistant. Silicone is also extremely flexible. Benefits include a long storage life and good bonding properties necessary in many electrical applications.
• Fiberglass - Fiberglass is the most widely used glass insulation. It can be used continuously in temperatures up to 482°C. This material is moisture and chemical resistant, but only fairly abrasion resistant. Its common applications include heat treating, glass and ceramic kilns, foundries, and extensive applications in aluminum processing.
• Rubber - Rubber insulation generally refers to both natural rubber and SBR compounds, each available in a variety of formulas for use in a wide range of applications. Because formulas vary, so do temperature ranges and some other basic characteristics. While this type of insulation has poor oil, and ozone resistance, it exhibits good low-temperature flexibility, good water and alcohol resistance, good electrical properties, and excellent abrasion resistance.
• Ethylene Propylene Diene Monomer (EPDM) - This synthetic rubber insulation displays outstanding heat, ozone, weather, and abrasion resistance. EPDM also exhibits excellent electrical properties. Further benefits include excellent flexibility at both high and low temperatures, from -55°C to 150°C, as well as good dielectric strength. EPDM replaces silicone rubber in some applications.
• Ethylene Propylene Rubber (EPR) - EPR is known for its excellent thermal characteristics and electrical properties, allowing a smaller cross-sectional area for the same load carrying capacity of other cables. It is commonly used in high-voltage cables. The flexibility of this material also makes it appropriate for temporary installations and applications in the mining industry. These rubbers are also valuable for their heat, oxidation, weathering, water, acid, alcohol, and alkali resistance. EPR can be used in the temperature range of -50°C to 160°C. EPR is not as tear resistant as other insulation options. It is also relatively soft and may require more care during installation to avoid damage.

Fluropolymers

• Polytetrafluoroethylene (PTFE) - PTFE is a thermoplastic material that can be used across a wide temperature range of -73°C to 204°C. It is extremely flexible, as well as, water, oil, chemical, and heat resistant. The mechanical properties of PTFE are low compared to other plastics.
• Fluorinated Ethylene Propylene (FEP) - This material is widely used due to its processing characteristics and wide range of applications. It is also highly flame resistant. Improved data transmission can also be achieved when FEP is used. FEP is commonly used in plenum cable and military applications.
• Polyvinylidene Fluoride (PVDF) - PVDF is flexible, lightweight, and thermally stable, as well as chemical, heat, weather, abrasion, and fire resistant. It is also a relatively low cost insulation option. This insulation is used in a wide range of industries and applications.
• Thermoplastic Elastomers (TPE) - Thermoplastic elastomers consist of a mix of polymers, typically a plastic and a rubber, to combine the benefits of each material into one insulating product. TPE can be molded, extruded, and reused, similar to a plastic, while maintaining the flexibility and stretch of rubber. TPE is commonly used in applications where conventional elastomers are unable to provide the necessary range of physical properties. They are found increasingly in automotive applications and household appliances. Disadvantages of TPE include poor chemical and heat resistance, low thermal stability, and higher cost than other types of insulation.  

Common classification of insulators in power system(OHE lines) are:

• Pin type insulator - As the name suggests, the pin type insulator is mounted on a pin on the cross-arm on the pole. There is a groove on the upper end of the insulator. The conductor passes through this groove and is tied to the insulator with annealed wire of the same material as the conductor. Pin type insulators are used for transmission and distribution of communications, and electric power at voltages up to 33 kV. Insulators made for operating voltages between 33kV and 69kV tend to be very bulky and have become uneconomical in recent years.
  
  
  
  
  
  
  

• 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 69kV and in some configurations, can be made for operation at up to 115kV.
  

• 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 - A dead end or anchor 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 the cross-arm 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 now-a-day, 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.
  

• Line post insulator

• Station post insulator

• Cut-out

Two class of insulation for household equipment and are:

Class 1: Insulation requires that the metal body and other exposed metal parts of the device be connected to earth via a grounding wire that is earthed at the main service panel. This equipment needs an extra pin on the power plug for the grounding connection.

Class 2: Insulation means that the device is double 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 prevent electric shock. All internal electrically energized components are totally enclosed within an insulated body that prevents any contact with "live" parts.