What is Function of Top Most Wire in Overhead Transmission Lines

The top most wire in overhead transmission lines is the ground wire. Ground wires are bare conductors supported at top of transmission lines.

Ground wires serve to shield the power line and intercept lightning strokes before it hits current carrying conductors below. Ground wires are not intended to carry current under normal circumstances.

Also see:

1. What are Stringing Charts in Electrical Power Transmission Lines
2. Application of Impedance Diagrams to Characterize Faults
3. 5 methods to reduce power losses in transmission lines
4. 5 Types of Conductors used in transmission lines
5. Explain ACSR in Electrical Power Transmission Lines