Which of the following is NOT a valid method for connecting the lightning conductor to structural steel?

The assertion that "None of the above" is the correct answer suggests that all listed methods—brazing, mechanically connecting, and welding—are valid techniques for connecting the lightning conductor to structural steel. This means that each method is acceptable within the standards of lightning protection systems.

Brazing, which involves melting a filler metal to join two pieces of metal, is often used for its ability to create strong, conductive joints without compromising the integrity of the metals being joined. Mechanical connections, such as using clamps or bolts, are straightforward and allow for easy disassembly if needed. Welding creates a strong, permanent bond and is beneficial for ensuring excellent electrical conductivity between the conductor and structural steel.

In light of these valid methods and their importance in establishing a robust and reliable lightning protection system, the choice indicating that "None of the above" is valid reflects a comprehensive understanding of accepted practices in the field.