Select insulating materials for following parts :
(i) Insulation between heating element and base plate of electric iron.
(ii) Insulation used over copper or aluminium conductor used for making coils.
(iii) Transformer bushings. (iv) Insulation between transmission line and pole.
1 Answer
(i) Insulation between heating element and base plate of electric iron.
Selected Material: Mica
Reasoning:
The insulation in an electric iron has a unique set of requirements:
1. High-Temperature Resistance: The heating element reaches very high temperatures (often >200°C). The insulator must not melt, burn, or degrade. Mica is a natural mineral that is stable at very high temperatures.
2. Excellent Electrical Insulation: It must prevent any electric current from flowing from the live heating element to the metal base plate (soleplate). This is a critical safety feature to prevent electric shock. Mica has very high dielectric strength.
3. Good Thermal Conductivity: This is the most crucial and often misunderstood property for this application. While it's an electrical insulator, it must be a good thermal conductor. The heat generated by the element needs to be efficiently transferred through the insulator to the base plate to heat it up. Mica, when used as a thin sheet, allows for effective heat transfer while maintaining electrical isolation.
(ii) Insulation used over copper or aluminium conductor used for making coils.
Selected Material: Enamel Varnish (forming what is commonly known as "magnet wire")
Reasoning:
Coils for motors, transformers, and solenoids require a very specific type of insulation:
1. Thin and Durable: The insulation must be extremely thin to allow the maximum number of turns to be wound in a given space, maximizing the magnetic field. It must also be tough enough to withstand the abrasion and stress of the winding process.
2. High Dielectric Strength: It must prevent short-circuits between adjacent turns of the tightly wound wire.
3. Flexibility: The wire is bent into shape, so the insulation must be flexible and not crack or peel off.
4. Heat Resistance: Coils generate heat due to electrical resistance ($I^2R$ losses). The enamel coating (often a polymer like polyurethane or polyester-imide) is designed to withstand these operating temperatures without failing.
For higher voltage or more demanding applications, this enameled wire might be further insulated with wrappings of insulating paper, cotton, or glass fibre, which are then impregnated with oil or varnish.
(iii) Transformer Bushings
Selected Material: Porcelain or Polymer/Composite Insulators
Reasoning:
A transformer bushing safely conducts a high-voltage line through the grounded metal tank of the transformer.
1. Very High Dielectric Strength: Bushings handle very high voltages (from kilovolts to hundreds of kilovolts) and must reliably prevent electricity from flashing over to the grounded tank.
2. Weather and UV Resistance: They are exposed to outdoor elements like rain, sun (UV radiation), and pollution for decades. Porcelain is a ceramic that is extremely durable, waterproof, and unaffected by UV light. Modern polymer insulators (often silicone rubber over a fiberglass core) are also used and offer advantages like being lightweight and having a water-repellent (hydrophobic) surface that resists contamination buildup.
3. Mechanical Strength: It must be strong enough to support the attached conductor.
4. Long Creepage Distance: The "sheds" or "skirts" (rib-like rings) on the bushing increase the surface distance from the high-voltage conductor to the grounded tank. This prevents a conductive path from forming along the surface, especially when it's wet or dirty.
(iv) Insulation between transmission line and pole.
Selected Material: Porcelain, Toughened Glass, or Polymer/Composite Insulators
Reasoning:
These insulators (often called line insulators) support the high-voltage conductors from the transmission tower or pole.
1. High Mechanical Strength: This is a primary requirement. The insulator must bear the immense weight and tension of the conductor, as well as additional loads from wind and ice.
2. Excellent Electrical Insulation: They must insulate the line, which can carry hundreds of kilovolts, from the grounded support structure.
3. Durability and Weather Resistance: Like bushings, they must endure decades of outdoor exposure.
* **Porcelain:** Traditionally used for pin-type insulators (on distribution poles) and disc-type insulators (stacked in strings for high-voltage transmission lines). It is strong and highly reliable.
* **Toughened Glass:** Often used for disc insulators as an alternative to porcelain. A key advantage is that if a glass disc fails, it shatters, making it easy to spot for maintenance from the ground.
* **Polymer/Composite:** A modern alternative that is very lightweight, has a high strength-to-weight ratio, and performs exceptionally well in polluted environments.