Which option is not an approved method for assuring continuity between cable and metallic tubing with the conductor running inside?

The correct answer is the use of an insulating bushing. In the context of lightning protection systems, assuring continuity between the cable and the metallic tubing is essential to maintain an effective path for electrical currents. An insulating bushing, by its very nature, is designed to prevent electrical conduction between two conductive components. This creates a barrier that would inhibit the continuity necessary for effective grounding or earthing, which is crucial in lightning protection and grounding systems.

In contrast, conductive bushings, metallic connectors, and compression fittings are designed to facilitate electrical continuity. Conductive bushings allow the electrical connection to remain intact while also providing protection against physical stresses. Metallic connectors create direct connections between conductive elements, ensuring that current can flow freely between them. Compression fittings mechanically secure and maintain contact between cables and the metallic conduit, ensuring that no insulation breaks the electrical connection.

Thus, an insulating bushing would be inappropriate for assuring continuity in this setting, as it negates the very requirement of unbroken electrical contact necessary for safety and effectiveness in a lightning protection system.