All copper parts used on the top 25 feet of a heavy duty smokestack should be ...

For the top 25 feet of a heavy-duty smokestack, using copper parts that are sheathed with lead is correct because it helps to enhance the conductivity and longevity of the lightning protection system. Lead sheathing serves as a protective layer that can help mitigate environmental factors such as corrosion due to weather elements, which is particularly important given the exposed nature of smokestacks.

In high-altitude applications like smokestacks, where metal components are subjected to harsh conditions, protecting the underlying copper with lead can help ensure that the conductivity of the copper remains intact over time. This is crucial for effective lightning protection, as any deterioration of the conductive material could compromise the entire system.

While using coatings or insulating methods might seem beneficial in terms of protection against corrosion or electrical interference, they do not enhance electrical conductivity the same way that lead sheathing does. Solid copper components are certainly effective for conductivity but would not offer the additional protective benefits that sheathing provides in this specific application.