Electrical Insulator

By R.W. Hurst, Editor

An electrical insulator is an indispensable component in electrical systems, providing safety and reliability. However, it comes in various materials and types, each with advantages and disadvantages. Understanding the insulating properties of dielectric materials, electrical resistance, and breakdown voltage is essential for selecting the most suitable insulator for a specific application. By making informed choices, engineers can ensure the longevity and safety of electrical installations and equipment.


An electrical insulator is a material that restricts the flow of electric current, ensuring that electric charge does not easily pass through it. They are essential components in electrical systems, as they help protect equipment, structures, and people from electric shocks and short circuits. Good electrical insulators possess a high resistivity, which means they can withstand high voltage without allowing electrical current to flow.

High Voltage Transmission Uses

High-voltage power transmission insulators are typically made from glass, porcelain, or composite polymer materials. Porcelain insulators consist of clay, quartz or alumina, and feldspar and feature a smooth glaze that allows water to run off easily. When high mechanical strength is required, porcelain rich in alumina is utilized. Porcelain insulators have a dielectric strength of around 4–10 kV/mm. Glass insulators possess a higher dielectric strength; however, they tend to attract condensation, casting them into the thick, irregular shapes necessary for insulators can result in internal strains. 
Consequently, some insulator manufacturers ceased producing glass insulators in the late 1960s, opting for ceramic materials instead.

Electric utilities sometimes employ polymer composite materials for certain insulator types. These insulators usually comprise a central rod of fiber-reinforced plastic and an outer weathershed of silicone rubber or ethylene propylene diene monomer (EPDM) rubber. Composite insulators are more cost-effective and lightweight and exhibit exceptional hydrophobic properties. This combination makes them ideal for use in areas with high pollution levels. However, these materials have not demonstrated the same long-term service life as their glass and porcelain counterparts.

Different Materials

Different types of electrical insulators cater to various applications and environments. They are classified based on the material used, such as ceramic insulators, glass insulators, and polymer insulators. Each type of insulator has specific insulating properties that make it suitable for certain uses.

Dielectric materials are an essential aspect of electrical insulators. They function by inhibiting the electric field within their structure, preventing the flow of electrical charge. A dielectric material's insulating properties are primarily determined by its dielectric constant, which measures its ability to store electrical energy without conducting it.

Ceramic insulators, such as porcelain insulators, have been used for many years due to their excellent insulating properties, mechanical strength, and resistance to high temperatures. They are typically used in high voltage applications, including power transmission and distribution systems. However, they can be heavy and brittle, reducing durability and increasing maintenance costs.

Materials such as glass paper, on the other hand, offer excellent transparency and a smooth surface that helps prevent dirt accumulation. They also have high dielectric strength, meaning they can withstand high voltage without breaking down. However, like ceramic insulators, they are fragile and prone to breakage.

Polymer insulators are a recent innovation made from silicone rubber or epoxy resins. They are lightweight, durable, and have good insulating properties. In addition, polymer insulators exhibit increased resistance to environmental factors such as UV radiation and pollution. However, their long-term performance still needs to be studied, and they may be more expensive than traditional ceramic or glass insulators.

Performance is affected by its electrical resistance and breakdown voltage. Electrical resistance measures a material's ability to prevent the flow of electric current. A higher resistance means that the insulator is more effective at blocking the flow of electricity. On the other hand, the breakdown voltage is the maximum voltage an insulator can withstand before it fails and allows electric current to flow through it. Therefore, a higher breakdown voltage indicates better insulating capabilities.

Electrical insulators play a crucial role in power transmission and distribution systems. They support and separate electrical conductors, ensuring that the electric field and current remain confined within the conductors. Insulators also help maintain the electrical wiring's integrity and prevent short circuits or leakage currents that may cause equipment damage or pose safety risks.

Several factors should be considered when selecting an insulator for a specific application, including the operating voltage, environmental conditions, and mechanical stresses. The insulator's material should possess a high dielectric constant, good resistance to temperature changes, and adequate mechanical strength. Additionally, it should resist environmental factors such as moisture, pollution, and UV radiation.

Various Types

Pin Insulator - This type of insulator is attached to a pin mounted on the cross-arm of a utility pole. It features a groove near its top, just below the crown, through which the conductor runs and is fastened using an annealed wire made of the same material as the conductor. Pin insulators are commonly employed to transmit communication signals and electric power at up to 33 kV voltages. However, they can become bulky and uneconomical for operating voltages between 33 kV and 69 kV.


Post Insulator - Introduced in the 1930s, post insulators are more compact than traditional pin-type insulators. They have rapidly replaced many pin-type insulators in lines with voltages up to 69 kV and, in some configurations, can be designed for operation at up to 115 kV.


Suspension Insulator - Suspension Insulators are typically utilized for voltages exceeding 33 kV. They consist of a series of glass or porcelain discs linked together with metal connectors, forming a string. The conductor is suspended from the bottom of this string, while the top is secured to the tower's cross-arm. The number of disc units required depends on the voltage.


Strain Insulator - When a straight section of a transmission line ends or changes direction, a dead-end or anchor pole or tower is employed. These structures must withstand the lateral (horizontal) tension from the long straight section of wire. Strain insulators are used to support this load. For low-voltage lines (under 11 kV), shackle insulators are strain insulators. For high-voltage transmission lines, cap-and-pin (suspension) insulator strings are used, mounted horizontally to the crossarm. In cases of extremely high tension, such as long river spans, two or more parallel strings may be necessary.


Shackle Insulator - Initially, shackle insulators were employed as strain insulators. Nowadays, they are predominantly used for low-voltage distribution lines. These insulators can be installed in horizontal or vertical orientation and can be directly fastened to the pole with a bolt or to the crossarm.