What fastening device is required when one conductor on a flat roof crosses another?

The correct answer is that a cross splicer is the fastening device required when one conductor on a flat roof crosses another.

A cross splicer is specifically designed for connecting two conductors securely while maintaining proper electrical integrity. It prevents any potential interference or damage caused by direct contact between crossing conductors. This is particularly important in lightning protection systems to ensure that all conductors function properly without any risk associated with crossover points, such as electrical shorts or failure in conducting lightning energy safely to the ground.

While other options like cable ties can be useful for bundling conductors together, they do not serve the purpose of securely connecting two conductors that intersect. Similarly, cross connectors and insulators may have roles in other aspects of electrical installations but are not specifically designed for the task of connecting crossing conductors in a manner that ensures reliability and safety in the context of lightning protection systems.