A Strain or tension-type insulator is a type of electrical insulator designed to withstand mechanical tension or stress in addition to providing electrical insulation. These insulators are used in power transmission and distribution lines where conductors (wires or cables) need to be supported while being kept electrically isolated from supporting structures, like poles or towers.
What is a Strain Insulator?
A strain insulator, also known as a strain-type insulator or tension insulator, is a specialized type of electrical insulator designed to withstand mechanical stress and electrical loads in overhead power transmission and distribution lines.
Strain Insulator exam point-
When there is a dead end of the line or there is corner or sharp curve, the line is subjected to greater tension.
1- In order to relieve the line from excessive tension, strain insulators are used.
2- For low voltage lines (< 11 kV), shackle insulators are used as strain insulators.
3-However, for high voltage transmission lines, strain insulator consists of an assembly of suspension insulators as shown in Figure.
4- The discs of strain insulators are used in the vertical plane.
5- When the tension in overhead lines becomes extremely high especially at long river spans, two or more strings are used in
parallel.
Strain or tension-type insulator Applications-
Strain or tension-type insulators are specifically used in high-tension environments to support conductors under mechanical stress while ensuring electrical insulation. Below are the key applications:
- Dead-end poles: Where the transmission line terminates.
- Angle towers: At points where the line changes direction.
- Long spans: Where conductors need to remain under tension across extended distances, like rivers or valleys.
Advantages of Strain Insulators-
- High Mechanical Strength
- Excellent Electrical Insulation
- Corrosion and Weather Resistance
- Vibration and Shock Absorption
- Long Service Life and Low Maintenance
Disadvantages of Strain Insulators-
- High Initial Cost
- Fragility of Porcelain and Glass Insulators
- Complex Installation
- Weight and Bulkiness
- Temperature Sensitivity
Multiple-Choice Questions (MCQs) on Strain or Tension-Type Insulators-
1. What is the primary purpose of a strain insulator?
a) To support vertical loads on the conductor
b) To prevent sagging of conductors
c) To withstand mechanical tension in conductors
d) To conduct electricity from one line to another
Answer: c) To withstand mechanical tension in conductors
Explanation: Strain insulators are specifically designed to handle the mechanical stress or tension in the line, especially at points like dead ends, corners, or long spans, while also providing electrical insulation.
2. Which material is commonly used for traditional strain insulators?
a) Copper
b) Porcelain
c) Plastic
d) Aluminum
Answer: b) Porcelain
Explanation: Porcelain is widely used for strain insulators due to its high strength, durability, and excellent electrical insulation properties.
3. Where are strain insulators most likely to be installed?
a) On live electrical panels
b) At dead-end poles or towers
c) In underground cable joints
d) On distribution transformers
Answer: b) At dead-end poles or towers
Explanation: Strain insulators are typically installed at dead-end poles, corners, or where the line changes direction to resist mechanical tension from the conductors.
4. What type of strain insulator is used in low-voltage lines?
a) Disc insulator
b) Shackle insulator
c) Pin insulator
d) Stay insulator
Answer: b) Shackle insulator
Explanation: Shackle insulators are used in low-voltage distribution systems to handle light mechanical tension, especially on poles.
5. Which of the following is an advantage of polymer strain insulators over porcelain insulators?
a) Heavier weight
b) More brittle and fragile
c) Higher resistance to vandalism
d) Less weather-resistant
Answer: c) Higher resistance to vandalism
Explanation: Polymer strain insulators are more resistant to vandalism and impact damage compared to porcelain or glass, making them a good choice in areas prone to physical interference.
6. Which environmental condition can affect the performance of strain insulators?
a) High humidity
b) Extreme pollution
c) Heavy winds
d) All of the above
Answer: d) All of the above
Explanation: Strain insulators can be affected by high humidity (leading to moisture contamination), pollution (causing flashovers), and heavy winds (introducing additional mechanical stress).
7. How can multiple disc insulators be used together in high-voltage lines?
a) By placing them side by side horizontally
b) By arranging them in a vertical string
c) By connecting them in a parallel electrical circuit
d) By attaching them directly to the conductor
Answer: b) By arranging them in a vertical string
Explanation: In high-voltage lines, disc insulators are arranged in a string to provide higher mechanical and electrical capacity.
8. What is a stay (guy) wire insulator used for?
a) To connect conductors at different poles
b) To reduce tension on transformers
c) To prevent electrical leakage through support wires
d) To transmit power between circuits
Answer: c) To prevent electrical leakage through support wires
Explanation: Stay insulators are installed on guy wires to ensure that electricity does not leak through the supporting wires and cause faults.
9. What is the primary disadvantage of porcelain strain insulators?
a) High cost of installation
b) Susceptibility to impact damage
c) Limited electrical insulation capacity
d) Poor performance in dry conditions
Answer: b) Susceptibility to impact damage
Explanation: Porcelain strain insulators are brittle and can easily break due to impact, which is one of their key disadvantages.
10. What is the purpose of using strain insulators in angle towers?
a) To reduce electrical losses
b) To absorb conductor vibrations
c) To hold the conductor tension at turning points
d) To allow the conductor to expand freely
Answer: c) To hold the conductor tension at turning points
Explanation: Angle towers are subjected to lateral tension, and strain insulators are used to maintain conductor stability at these points.
11. Which of the following is a key reason for using strain insulators in transmission lines with long spans?
a) To reduce the number of conductors required
b) To allow lines to carry higher currents
c) To prevent sagging and maintain line tension
d) To convert AC voltage to DC voltage
Answer: c) To prevent sagging and maintain line tension
Explanation: Strain insulators are used to hold the conductor tightly in long spans, preventing sagging and maintaining the proper clearance between lines and the ground.
12. What happens if a strain insulator fails in a high-voltage transmission line?
a) The entire system switches to backup power automatically
b) The line becomes energized, but transmission continues
c) The conductor may fall or sag, leading to outages or safety hazards
d) The insulator repairs itself over time
Answer: c) The conductor may fall or sag, leading to outages or safety hazards
Explanation: A failed strain insulator can cause the conductor to drop or sag, resulting in potential power outages, safety hazards, or line damage.
