A. Pin insulators
B. Suspension insulators
C. Shackle insulator
D. None of these
Answer: B. Suspension insulators
Explanation:
Suspension insulators:
- Suspension insulators consist of a number of 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 other end of the string is secured to the cross- arm of the tower.
- For high voltage (>33KV), it is a usual practice to use suspension-type insulators.
- In higher voltage, beyond 33KV, it becomes uneconomical to use pin insulator because size, weight of the insulator become more. Handling and replacing bigger size single unit insulator are quite difficult task. For overcoming these difficulties, suspension insulator was developed.
- In suspension insulator numbers of insulators are connected in series to form a string and the line conductor is carried by the bottom most insulator. Each insulator of a suspension string is called disc insulator because of their disc-like shape.
Advantages of Suspension Insulator
- Each suspension disc is designed for a normal voltage rating of 11KV (Higher voltage rating of 15KV), so by using different numbers of discs, a suspension string can be made suitable for any voltage level.
- If any one of the disc insulators in a suspension string is damaged, it can be replaced much easily.
- Mechanical stresses on the suspension insulator is less since the line hanged on a flexible suspension string.
- As the current-carrying conductors are suspended from the supporting structure by suspension string, the height of the conductor position is always less than the total height of the supporting structure. Therefore, the conductors may be safe from lightning.
- suspension string
Disadvantages of Suspension Insulator
- Suspension insulator string is costlier than pin and post type insulator.
- Suspension string requires more height of supporting structure than that for pin or post insulator to maintain same ground clearance of current conductor.
- The amplitude of free swing of conductors is larger in the suspension insulator system, hence, more spacing between conductors should be provided.
Important Notes:
- Pin insulators are used for holding the line conductors on the straight running of poles.
- These are commonly used in power networks up to 33 kV system.
- When there is a dead-end of the line or there is a corner or sharp curve, the line is subjected to greater tension. In order to relieve the line of excessive tension, strain insulators are used.
- For low voltage lines (<11 kV) shackle insulators are used as strain insulators.
- Stay insulators are also known as strain insulators and are generally used up to 33 kV line. These insulators should not be fixed below three meters from the ground level.
- These insulators are also used where the lines are strained.
- Pin insulators are commonly used in power networks up to 33 kV system
- Post insulator is suitable for lower voltage and also for high voltage
- Each suspension disc is designed for normal voltage rating 11 kV (Higher voltage rating 15 kV), so by using different numbers of discs, a suspension string can be made suitable for any voltage level; Hence suspension insulator is suitable for high voltage transmission lines
- The shackle insulator is usually used in a low voltage distribution network
Important Points
- In a suspension insulator number of insulators is connected in series to form a string and the line conductor is carried by the bottom-most insulator.
- Each insulator of a suspension string is called a disc insulator because of its disc-like shape.
- Each suspension disc is designed for a normal voltage rating of 11 kV, so by using different numbers of discs, a suspension string can be made suitable for any voltage level.
Important Notes:
INSULATORS VS USED FOR
- Suspension type------interconnected porcelain discs capable of transmitting higher voltages
- Pin type----Used on straight running transmission lines to transmit voltages upto 33 kV.
- Strain type-----horizontally suspended suspension insulators that can handle large mechanical stresses such as a river crossing.
- Post type-----used in areas with heavy pollution and risk of vandalism exhibit high resistance to puncture