What Are The Types Of Insulators In Overhead Lines? . Insulators play a vital role in overhead transmission lines, preventing the flow of electric current from the conductors to the supporting structures. These specialized materials not only ensure electrical safety but also enhance the efficiency and longevity of power systems. Without proper insulators, electrical leakage, short circuits, and system failures could become frequent, compromising the reliability of power supply networks.
This article dives deep into the types of insulators in overhead lines, their key features, applications, and why they are indispensable in the power sector.
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What is an Electrical Insulator?
An electrical insulator is a hardware accessory applied to low voltage, medium voltage, and high voltage electrical systems operating in public services, such as overhead transmission and railway lines. They help to enable the smooth and safe functioning of the electrical system.
Electrical insulators are crafted from a range of non-conductive, durable materials such as glass, porcelain, and composite polymer to ensure that the insulator can consistently maintain a high level of resistivity (voltage resistance).
Types of Electrical Insulators
Electrical insulators come in different sizes and designs to suit the varying voltage and fixture needs of different power systems. As they are installed outdoors in all seasons, most electrical insulators incorporate shells or ‘petticoats’ into their designs to ensure that they stay dry and non-conductive during wet conditions.
The most common types of insulators used to assist overhead power lines are: i) pin insulators; ii) post insulators; iii) spool/shackle insulators; iv) stay insulators; and v) suspension insulators.
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Pin Insulators
Definition
Believed the earliest type of insulator developed for overhead transmission lines, pin insulators isolate a conducting wire or cable from the rest of the network. They are shaped to enable the secure fastening of the insulator to a conductor wire, making it easier to contain it.
Pin insulators are primarily used in electrical systems with a voltage load up to 33 kV. In contrast to some other insulator types, pin insulators are fixed directly on the structure.
Features
Pins insulators are of a single-layered petticoated design arranged on a pin, which allows their tip to connect easily to a conductor element. Depending on requirements, they can be of a single or multi-pin variety and are usually crafted from either glass or porcelain.
Hardware
Pole top pins are used to support the connection of the pin insulator directly onto the pole. They are sometimes further reinforced with bolts.
Top ties and side ties secure conductors to insulators. Top ties secure conductors at the top of insulators, and come in different sizes to accommodate different shaped insulator heads. Side ties are designed to fasten conductors to the side of insulators by slotting into a built-in groove in the insulator.
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Spool or Shackle Insulator
Definition
Also known as shackle insulators, spool insulators are used on lines with a low voltage load. They are versatile enough to offer insulation to both primary and secondary conductors on a power line, and can be mounted horizontally or vertically.
Spool insulators work with a number of other hardware components including secondary racks, secondary clevises, spool ties, and upset bolts.
Features
Spool insulators have a central hole to facilitate the mounting of the insulator. The hole is also tapered to help reinforce the insulator and minimise the risk of it becoming overloaded with voltage.
Hardware
Spool Tie is used with spool insulators to provide protection to bare conductor cables against motion and damage in harsh climates, especially within wider line angles. Tie tubes can help to further protect fragile lines from the spool bind.
Secondary Racks are steel-based cables which are mounted with spool insulators to provide structural reinforcement. They are forged in a U-shape to mold to the contours of the pole easily during the mounting process.
Secondary Clevises (also known as D irons) are flat steel brackets which help to secure spool insulators to poles in awkward locations, such as dead ends and corners. Their slotted design enables mounting without disturbing any insulators already installed.
Upset Bolts are a type of fixing which help to support connectors by stabilising the position of the insulator. Upset bolts come in single or double versions to support more than one connector at a time.
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Stay Insulator
Definition
Stay insulators (also known as egg insulators owing to their shape) are insulators designed to protect and contain low voltage stay wire by acting as a counterbalance against dislodging or a sudden voltage change.
Particularly useful for dead-end poles, stay insulators prevent the loose or overloaded wire from coming into direct contact with the ground or other surfaces where it could become hazardous.
Features
Stay insulators are a type of strain insulator, designed to withstand extra line pull tension. They are oval-rectangular in shape, with a central hole for threading. Due to their protective function, stay insulators are always installed between the conductor and the earth.
Hardware
Stay wire is used in partnership with stay insulators on pole lines to help to stabilise any extra electrical and mechanical tension on the poles.
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Suspension Insulators
Definition
Suspension insulators are modular insulators that help to protect a power line from overloading using a conductor suspension method.
Primarily applied to networks with a voltage load of 33 Kv and above, a conductor is attached to the bottom end of the insulator and suspended from it, separating that conductor from others on the line.
Features
Suspension insulators are designed similarly to a pearl necklace, with a number of insulator discs assembled onto a flexible insulator rod which allows the voltage load to be safely and evenly distributed across the discs. The insulator discs can also be removed and added to as needed without dismounting the insulator.
Hardware
Thimble Clevises are used as a reinforcement link between the suspension insulator and the line. Threaded through the insulator the clevis bears the stress load of the suspension, protecting the connector loop from damage.
Armour Grip Suspension is a form of suspension specifically designed to minimise the effects of movement, stress, and static load at the support point of the suspension. This minimises the risk of structural damage from kinking or flash over.
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Post Insulators
Definition
Post Insulators have been used in overhead transmission lines for almost a century. Larger than pin types, they are used as both an insulator and to add reinforcement to vulnerable areas of an electrical system.
In modern times, post insulators are often used in place of pin insulators on voltage loads up to 69 kV, with solid core versions able to withstand up to 1100 kV.
Features
Post insulators have a clamp at both their top and bottom end, and can be mounted either vertically or horizontally as needed. Due to their size, they have a greater number of petticoats than pin types in order to maintain their insulating capacity in wet climates.
Hardware
Insulator brackets are often used to securely mount post Insulators between 15 Kv – 34.5Kv directly onto the pole itself. They act as a sturdy brace for the insulator and remove the requirement to involve cross arms in the installation.
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Strain Insulators
A suspension string used to handle significant tensile loads is called a strain insulator. It is used where there is a dead end or sharp corner in the transmission line, requiring the line to bear a heavy tensile load. A strain insulator must have considerable mechanical strength as well as the necessary electrical insulating properties.
| Rated System Voltage | Number of disc insulator used in strain type tension insulator string | Number of disc insulator used in suspension insulator string |
| 33KV | 3 | 3 |
| 66KV | 5 | 4 |
| 132KV | 9 | 8 |
| 220KV | 15 | 14 |
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Special Types of Insulators
Areas where the air conditions are not favorable or normal, for example where there is smoke, chemicals, salts or humidity in the environment etc. then special types of insulators are used instead of normal insulators e.g.:
- Pin Type Anti-dirt insulators: It has more creepage distance (490mm) than the normal insulator which is generally 325 mm. The rain water can easily go inside the shades of these insulators which helps to clean the dirt.
- Inclined Disk Insulators: Looks like disk type suspension insulator but not in complete horizontal plane but bend-over at a specific angle.
- Deep Shade Insulators: It has deep and large shades as compared to prevent the puncturing problems of Inclined Disk Insulators in the rainy seasons.
- Solid Core Insulators: It has a solid core without a slot which is used to fit the pin inside it. The pin is connected to the lower portion of the insulator using cement.
- Aerodynamic Insulators: It has small shades as compared to the deep shade insulators hence, it prevents the dirt and resist even in the strong wind. They are used in sandy areas and desert landscapes.
Except all of the above mentioned types, there are some additional types of insulators used in substations as follows:.
Substation Insulators:
- Post Insulators for Support
- Sectionalizing Insulators
- Bracket Insulators
- Tensile Load Insulators for Driving / Pulling Live parts
- Operating Rod Insulators
- Stray Arm Insulators
- Shaft Insulators
- Roofline Insulators
- Connecting Rod Insulators
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Materials Used in Overhead Line Insulators
Porcelain
Porcelain is a widely-used insulating material, known for its excellent dielectric properties and mechanical strength. It is produced by heating a mixture of clay, kaolin, feldspar, and other minerals at high temperatures, resulting in a dense, glassy, and non-porous material.
Advantages:
- High electrical resistance and dielectric strength
- Good mechanical strength and durability
- Resistance to corrosion and environmental factors
Disadvantages:
- Brittle and prone to breakage under excessive mechanical stress
- Heavier than other insulating materials, which may increase the weight of the supporting structure
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Glass
Glass insulators are made from toughened or tempered glass, which undergoes a thermal treatment process to improve its strength and durability. This material has been used for insulators since the 19th century, offering several benefits over porcelain.
Advantages:
- High electrical resistance and dielectric strength
- Good mechanical strength and durability
- Transparency, allowing for easy visual inspection of the insulator’s condition
Disadvantages:
- Susceptible to surface damage, which may reduce its electrical resistance
- Heavier than polymer insulators
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Polymer (Silicone Rubber)
Polymer insulators, made from silicone rubber or other elastomeric materials, are a modern alternative to traditional porcelain and glass insulators. These insulators consist of a fiberglass core covered by a weather-resistant polymer housing.
Advantages:
- High electrical resistance and dielectric strength
- Lightweight, making them easier to transport and install
- Excellent resistance to environmental factors, including UV radiation, pollution, and temperature variations
- High flexibility, allowing them to withstand mechanical stresses without breaking
Disadvantages:
- Susceptible to aging and degradation, which may reduce their lifespan compared to porcelain and glass insulators
- Higher initial cost compared to traditional insulating materials
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Insulator properties
Insulators are materials with specific properties that make them effective in impeding the flow of electrical current. The key properties of insulators contribute to their role in maintaining electrical isolation and preventing unintentional electrical conduction:
- High Resistivity: Insulators possess high electrical resistivity, meaning they offer significant opposition to the flow of electric current. This property helps in blocking the movement of electrons and prevents the transmission of electrical energy through the material.
- Dielectric Strength: Dielectric strength is the maximum electric field a material can withstand without breaking down and allowing a current to flow. Insulators exhibit high dielectric strength, ensuring that they can endure the applied voltage without undergoing electrical breakdown.
- Mechanical Strength: Insulators must have sufficient mechanical strength to support the weight of conductors and withstand external forces such as wind, ice, and vibrations. This property ensures the structural integrity of insulators in various applications.
- Thermal Stability: Insulators need to maintain their properties over a range of temperatures. Thermal stability ensures that insulators can withstand the heat generated during electrical operation without undergoing significant changes in their physical or electrical characteristics.
- Chemical Inertness: Many insulators are designed to be chemically inert to resist corrosion or degradation when exposed to environmental factors such as moisture, pollutants, or chemicals.
These properties collectively enable insulators to perform their critical role in electrical systems by providing a barrier to the flow of electric current and ensuring the safety and reliability of the associated infrastructure.
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Key Selection Criteria for Insulators
When selecting insulators for overhead power lines, consider the following factors:
- Voltage level: Insulators must be designed to withstand the voltage level of the transmission line.
- Mechanical strength: Insulators should have adequate mechanical strength to support conductors and resist environmental stresses.
- Insulation properties: Insulators must provide sufficient electrical insulation to prevent flashovers and punctures.
- Environmental factors: Insulators should be resistant to pollution, UV radiation, and temperature variations.
- Cost and maintenance: The chosen insulator should be cost-effective and require minimal maintenance.
Causes of Failure in Insulators
There are some common causes of failure in the insulator listed below.
Cracking of Insulator:
Due to unequal expansion, there is a chance of a crack in the porcelain insulator. Also, by the seasonal effect, there is a chance to crack in an insulator. There are many improvements needed to avoid such cracking. Sometimes, the cushions are used between shells and steel pins to allow some expansion.
Defective material:
Generally, the insulator is manufactured from good materials. But there is a chance of defective material. And it causes puncture that allows to flow of the current. These defective insulators are not used in electrical systems.
Porosity of material:
Generally, the insulators are manufactured from porcelain. If the porcelain is manufactured at a lower temperature, it will make it porose. Hence, it will absorb moisture from the air and it may allow flowing leakage current. Therefore, this is unfit to use in electrical networks.
Improper glazing on the surface:
The outer surface of the insulator is glazed properly to free the draining of water. But, if the glazing is not properly done, there is a chance to stick moisture on it and depositing dust on the surface of an insulator. It causes a conducting path and it results in a reduction in a flash over distance. Hence, there is more chance of flashover in an insulator.
Flashover
If the flashover occurs between the two metallic ends of the insulator and generates high temperature and heat which leads to breaking the insulator.
Short circuit
If the distance between the conductors is short, there is a chance of arcing due to the bird strikes on line conductors. This cause of short circuit may also lead to failure of the insulator.
Mechanical stress:
The mechanical stress on the insulator is continuously applied by the conductor. If the insulator is manufactured from defective material, it may break down from a weak point and causes the failure of an insulator.
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FAQs About Types of Insulators
What is an Electrical Insulator?
An electrical insulator is a hardware accessory applied to low voltage, medium voltage, and high voltage electrical systems operating in public services, such as overhead transmission and railway lines. They help to enable the smooth and safe functioning of the electrical system.
Electrical insulators are crafted from a range of non-conductive, durable materials such as glass, porcelain, and composite polymer to ensure that the insulator can consistently maintain a high level of resistivity (voltage resistance).
Types of Insulators in Overhead Lines
Electrical insulators come in different sizes and designs to suit the varying voltage and fixture needs of different power systems. As they are installed outdoors in all seasons, most electrical insulators incorporate shells or ‘petticoats’ into their designs to ensure that they stay dry and non-conductive during wet conditions.
The most common types of insulators used to assist overhead power lines are; i) pin insulators; ii) post insulators; iii) spool/shackle insulators; iv) stay insulators; and v) suspension insulators.
Why electrical insulators are important?
Insulators play a significant role in providing our engineers and equipment with that paramount barrier of safety and support, ensuring that the key balance between essential protection and expansive progress is always kept in equilibrium.
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