Electrical Insulator | Insulating Material | Porcelain Glass Polymer Insulator

What is an Electrical Insulator?

An electrical insulator supports or separates conductive parts while limiting unwanted current between them and earth. An insulator has high resistance and enough dielectric strength for its rated electrical stress. A small leakage current can still flow across its surface or through its material.

In transmission and distribution systems, overhead conductors need mechanical support and electrical clearance from towers, poles and crossarms. Line insulators carry the conductor load while separating the energised hardware from an earthed or earth-referenced structure.

Insulating Material

Flashover is one overhead-insulator failure mode. It can follow an overvoltage or a conductive wet contamination layer across the surface. The arc may burn glaze or damage hardware, although a surface flashover does not always puncture the insulating body. Mechanical damage, ageing, water ingress and material defects can also cause failure.

Properties of Insulating Material

An insulating material for an overhead line must meet electrical, mechanical and environmental requirements. The frozen list gives general qualities; a real insulator is selected by its rated tests, dimensions, service environment and applicable standard.

1. It must be mechanically strong enough to carry the tension and weight of conductors.
2. It must have a very high dielectric strength to withstand the voltage stresses in High Voltage transmission systems.
3. It must possess high Insulation Resistance to prevent leakage current to the earth.
4. The insulating material must be free from unwanted impurities.
5. It should not be porous.
6. There must not be any entrance on the surface of the electrical insulator so that the moisture or gases can enter it.
7. There physical as well as electrical properties must be less effected by changing temperature.

Porcelain Insulator

Porcelain remains a widely used overhead-insulator material alongside toughened glass and composites. Electrical porcelain is a fired ceramic formulated from clay or kaolin with feldspar, quartz and other controlled ingredients. Firing produces a dense body, and the exposed surface is normally glazed.

The glaze provides a smooth surface that limits moisture and contamination retention. Manufacturing controls and acceptance tests address cracks, porosity and other defects that can reduce mechanical or dielectric performance. The table below contains historical approximate material values, not guaranteed ratings for a finished insulator.

Properties of Porcelain Insulator

Property	Value(Approximate)
Dielectric Strength	60 kV / cm
Compressive Strength	70,000 Kg / cm2
Tensile Strength	500 Kg / cm2

Glass Insulator

Overhead glass insulators use toughened glass rather than ordinary annealed glass. Toughening creates a controlled stress pattern and makes severe damage easier to see because a damaged unit shatters. The frozen lists below state broad comparisons, but product geometry and standard tests determine actual performance.

Advantages of Glass Insulator

1. It has a very high dielectric strength compared to porcelain.
2. Its resistivity is also very high.
3. It has a low coefficient of thermal expansion.
4. It has a higher tensile strength compared to porcelain insulator.
5. As it is transparent in nature the is not heated up in sunlight as porcelain.
6. The impurities and air bubbles can be easily detected inside the glass insulator body because of its transparency.
7. Glass has a very long service life because the mechanical and electrical properties of glass do not be affected by aging.
8. After all, glass is cheaper than porcelain.

Disadvantages of Glass Insulator

1. Moisture can easily be condensed on the glass surface and hence air dust will be deposited on the wed glass surface which will provide a path to the leakage current of the system.
2. For higher voltage glass can not be cast in irregular shapes since due to irregular cooling internal strains are caused.

Properties of Glass Insulator

Property	Value(Approximate)
Dielectric Strength	140 kV / cm
Compressive Strength	10,000 Kg / cm2
Tensile Strength	35,000 Kg / cm2

Polymer Insulator

A polymer insulator normally has a glass-fibre reinforced resin core, an elastomer housing with weather sheds and metal end fittings. Silicone rubber and EPDM are common housing materials. The housing protects the load-bearing core from moisture and environmental exposure. Because several materials work together, a polymer insulator is also called a composite insulator. The end fittings transfer mechanical load into the core and may use hot-dip galvanised steel.

Advantages of Polymer Insulator

1. It is very lightweight compared to porcelain and glass insulator.
2. As the composite insulator is flexible the chance of breakage becomes minimum.
3. Because of lighter in weight and smaller in size, this insulator has lower installation costs.
4. It has a higher tensile strength compared to a porcelain insulator.
5. Its performance is better, particularly in polluted areas.
6. Due to lighter weight polymer insulator imposes less load to the supporting structure.
7. Less cleaning is required due to the hydrophobic nature of the insulator.

Disadvantages of Polymer Insulator

1. Moisture may enter in the core if there is any unwanted gap between core and weather sheds. This may cause the electrical failure of the insulator.
2. Over crimping in end fittings may result in cracks in the core which leads to mechanical failure of polymer insulator.

The following Victoria, Australia account was contributed as field experience. It does not identify the utility, insulator design, sample size or investigation method, so it cannot establish general polymer-insulator performance.

The account reports bird damage to some 22 kV polymeric strain insulators by cockatoos, galahs and parrots. It says some affected units were replaced with glass discs as a local response. No comparison test between material types is provided.

The same account reports heat deformation of polymeric post insulators after bushfire exposure while nearby concrete and steel remained in place. Fire performance depends on heat flux, duration, product construction and prior damage. Glass and porcelain assemblies also need inspection after severe thermal or arc exposure.

It also reports failures near a salt-exposed coast. Salt contamination can increase leakage current on glass, porcelain and composite insulators. Material choice, creepage distance, housing condition, washing practice and local pollution severity determine performance.

1. Subject to bird attack by Parrots, Cockatoos, and Galahs.
2. Not resilient to bushfire temperatures.
3. Not recommended for a location near surf beaches due to salt spray.

The information is contributed by Robert Lancaster of Australian Electricity Supply Industry

Types of Insulators

Overhead insulators can be classified by how they support the conductor. The frozen list below is incomplete, and `Stray Insulator` is a likely spelling error for strain insulator.

1. Pin Insulator
2. Suspension Insulator
3. Stray Insulator

Other common names include strain, line-post, stay and shackle insulators. The voltage range and application depend on the product rather than the category name alone. Read more about the various types of insulators used in transmission lines.