Objective Questions on Electrical Transmission | 4

  1. Maximum temperature limit of class F insulator in (degree centigrade) is

    Maximum temperature limit for Class F Insulator is 155 degree centigrade.

  2. Bakelite is

    Bakelite whose chemical name is polyoxybenzylmethylenglycolanhydride, an early form of plastic. It is incombustible.Bakelite is a plastic made from synthetic components. Bakelite was used for its electrical non-conductivity and heat-resistant properties in electrical insulators, radio and telephone casings, and also in kitchenware, jewellery, pipe stems and children\'s toys.

  3. Insulating material used in spark plugs is

    Porcelain is a ceramic material hence less affected by fire

  4. Which of the following materials can be used for temperature up to 500°C?

    Mica - because of its electrical properties. It has a high dielectric breakdown, is thermally stable to 500°C, and is resistant to corona discharge. In this category another material phlogopite mica remains stable at higher temperatures up to 900°C. Mica sheets are used to provide structure for heating wire (such as in Kanthal or Nichrome) in heating elements and can withstand up to 900°C (1,650 °F).

  5. Which material can be used at temperatures above 100°C ?

    Teflon - because of its electrical properties. Its melting point 327 °C, Resistivity 1018 ω - cm
    Dielectric strength 60 MV/m

  6. The property of materials by which they can be drawn into wires is known as

    Ductility is a the ability of solid material to deform under tensile stress. This is often characterized by the material\'s ability to be stretched into a wire. Malleability, is the material\'s ability to deform under compressive stress, this is often characterized by the material\'s ability to form a thin sheet by hammering or rolling.

  7. What is the maximum working temperature (in degrees) for class Y insulator?

    Maximum working temperature for class Y insulator is 90 degrees. This class consists of materials such as cotton silk & paper without impregnation.

  8. Composition of insulator porcelain is

    Composition of insulator porcelein is clay, quartz, feldspar. Insulator porcelein –ceramic prepared by high temperature treatment of natural clay with certain inorganic matter.

  9. Which class of insulators has high maximum working temperature?

    By experimental analysis the classification of insulators is done in flowing way

    Insulator’s class Temperature (max)
    Y 90°C
    A 105°C

    E 120°C
    B 130°C
    F 155°C
    H 180°C
    C 180°C

  10. The string efficiency of an insulator can be increased by

    String efficiency = voltage across the string/(n x voltage across the disc nearer to conductor). Normally , the disc nearer to the conductor is having high voltage and gradually decreases the voltage as going from down to up of insulators. Since V is inversely proportional capacitance. In capacitance grading, different capacitance of insulators chosen in such way that voltage across all insulators are equal. Then sting efficiency increases.

  11. The insulator should have

    the insulators for overhead lines provide insulation to the conductor from ground. The insulators are connected to the cross arm of tower and conductor passes through clamp of insulators.
    Insulators should have following properties:
    High mechanical strength to with stand the load.
    High insulating resistance in order to avoid leakage currents.
    High relative permittivity in order to have dielectric strength.
    High puncture strength to flash over voltage.
    It should be free from impurities.

  12. Which of following material is uses for insulators?

    Porcelain is using for manufacturing of insulators in present technology. Porcelain is a mixture of quartz, kaolin and feldspar. It has high mechanical strength. The surface will never affect by dirt.it is having dielectric strength of 60 kv/cm.

  13. Glass has dielectric strength of

    Glass is cheaper than porcelain and having dielectric strength of 140kv/cm. it has high resistivity. Compressive strength is high than porcelain but coefficient of thermal expansion is less. Hence it is employed up to 25kv under natural temperatures.

  14. Compressive strength of porcelain is

    The ability of material with which it can with stand during compress of material is called compressive strength. The compressive strength of porcelain is 7000 kg/cm2.

  15. Insulators are required to withstand in

    Insulators should withstand in both electrical stress and mechanical stress. The electrical stress are flash over and punctures. Mechanical stress are mechanical strength in order to with stand the weight of conductor and forces acting due to wind pressure, Ice.

  16. The safety factor of insulators is defined as ratio of

    safety factor = puncture voltage / flash over voltage.
    Flash over is the common type of electrical breakdown that can occur in an insulator. Whenever the voltage across the insulator exceeds the flash over voltage then an arcing occurs between insulators and conductor. The arc jumps to surrounding air and gets heat. The heat due to flash over will break insulators. Puncture is severe electrical breakdown. Whenever the voltage across insulator exceeds the puncture voltage then arc will struck between conductor and insulator and passes through insulator and causes severe breakdown of insulators.

  17. Which of following insulators will be chosen for higher voltages transmission lines ?

    For higher voltages suspension type or disc type insulators are prefer. It is applicable for voltages beyond 33 kv. The insulators are used in the conductors is hung below the point of support or tower is called disc type insulators.

  18. For 66 kV lines the no. of insulator discs used:

    Each insulator can with stand up to 11kv. Hence 66kv/11kv= 6 i.e., no.of insulators= 6.

  19. Voltage across the string of a string insulator assembly is 132 kV, no. of insulators are 8 and voltage across insulator near to conductor is 20 kV. Then string efficiency is

    String efficiency in % = Voltage across the string ⁄ (n x voltage across the disc nearer to the conductor) x 100 % [n = no. of discs in the string]
    = 132 KV ⁄ (8 x 20 KV) x 100 %
    = 0.825 x 100 %
    = 82.5%

  20. Suspension Insulator is mainly used for voltages beyond

    In case of suspension insulator are used for generally voltages beyond 33KV.