Which of the following statements is true regarding lightning conductor installations?

The statement "Concealed installations can utilize external routing" is correct because lightning conductor systems often require pathways that are both functional and safe. Concealed installations are sometimes necessary due to aesthetic or structural considerations, allowing for the conductors to be hidden from view while still connecting to external routes that provide a direct and effective path for lightning strikes to reach the ground. This method ensures that the conductivity is maintained without compromising the system's performance, as external routing can still safeguard the integrity of the installation and ensure proper grounding.

The other statements have limitations that do not reflect best practices in lightning protection. For example, while it is important for some components of a lightning protection system to be visible for inspections and maintenance, requiring all components to be visible at all times is not practical or necessary. Additionally, embedding lightning conductors into any masonry does not take into consideration the suitability of the materials used in construction and may not provide proper conductive pathways. Lastly, attaching conductors to roof materials can be essential for securing the system and ensuring that it effectively directs lightning strikes away from sensitive structures, hence prohibiting attachment to roof materials isn't universally applicable.