Question

One of the most commonly used materials for insulators of an overhead transmission line, i.e. glass, has the drawback (when used as an insulator material) that:
1. its thermal expansion is high
2. moisture condenses more readily on its surface
3. it has low resistivity when properly annealed
4. material always remains porous

Answer 1

Correct Answer - Option 2 : moisture condenses more readily on its surface

Concept:

• Insulator rings used for electricity delivery poles are generally made of glazed porcelain or toughened glass or ceramic or steatite.
• Insulators are made from clay, quartz or alumina and feldspar, and are covered with a smooth glaze to shed water.
• The materials used for porcelain are silica 20%, feldspar 30%, and clay 50%. Porcelain has a dielectric strength of about 4–10 kV/mm.
• Glass has higher dielectric strength, but it attracts condensation and the thick irregular shapes needed for insulators are difficult to cast without internal strains.
• So that manufacturers stop making glass insulators and switching to ceramic materials.

Properties of overhead line insulators:

• High mechanical strength to withstand the conductor load, wind load, etc.
• High electrical resistance (insulation strength) to minimize the leakage currents.
• The high relative permittivity of insulating material so that the dielectric strength is high.
• High ratio of puncture strength to flashover.
• High thermal expansion.
• Low resistivity when properly annealed.

 

Solution:

One of the most commonly used materials for insulators of an overhead transmission line, i.e. glass, has the drawback (when used as an insulator material) that moisture condenses more readily on its surface.