Introduction:
Overhead line insulators are used to support and insulate overhead power transmission and distribution lines. They prevent the flow of current from the conductor to the supporting structure and the ground. Insulators are typically made of non-conductive materials that can withstand high voltage and environmental conditions.

Explanation:
The material that is not used for overhead line insulators is PVC (Polyvinyl Chloride).

PVC (Polyvinyl Chloride):
- PVC is a synthetic plastic polymer that is widely used in various applications due to its electrical insulation properties.
- However, PVC is not suitable for overhead line insulators because it has poor mechanical strength and low resistance to ultraviolet (UV) radiation.
- Over time, UV radiation can degrade the PVC material, making it brittle and prone to cracking. This can lead to the failure of the insulator, causing power outages and safety hazards.
- PVC is also not able to withstand high temperatures and can deform under load, which further reduces its mechanical strength.
- Additionally, PVC is not as effective as other materials, such as porcelain and glass, in providing the necessary electrical insulation required for overhead lines.

Other materials used for overhead line insulators:
a) Porcelain:
- Porcelain is one of the most commonly used materials for overhead line insulators.
- It is a ceramic material made by heating clay and other materials at high temperatures.
- Porcelain insulators have excellent mechanical strength, high resistance to UV radiation, and good electrical insulation properties.
- They are capable of withstanding high voltages and are widely used in overhead power transmission and distribution systems.

b) Glass:
- Glass insulators are also commonly used for overhead lines.
- They are made of tempered glass and have similar properties to porcelain insulators.
- Glass insulators provide good electrical insulation and are resistant to UV radiation.
- However, they are more susceptible to breakage compared to porcelain insulators.

c) Steatite:
- Steatite is a type of ceramic material that is sometimes used for overhead line insulators.
- It has good mechanical strength and electrical insulation properties.
- Steatite insulators are resistant to UV radiation and can withstand high temperatures.

Conclusion:
In summary, PVC is not used for overhead line insulators due to its poor mechanical strength, low resistance to UV radiation, and inability to withstand high temperatures. Porcelain, glass, and steatite are the commonly used materials for overhead line insulators due to their superior electrical insulation properties and mechanical strength.