Under what restrictive condition can the grounded conductor continue to be used for grounding at separate buildings according to existing wiring systems?

The grounding conductor can be used for grounding at separate buildings under restrictive conditions, which often involves specific scenarios in existing wiring systems. The correct choice indicates that the use of the grounded conductor is permitted under multiple conditions, reflecting a more comprehensive understanding of wiring systems.

In the context of electrical installations, a grounded conductor can indeed be used to provide grounding for lighting systems—this encompasses one aspect of its permissibility. Additionally, the option to use it for ungrounded systems acknowledges situations where connections do not involve a direct path back to the ground, thus necessitating grounding for safety reasons in those systems as well.

By stating "All of the above," the correct answer captures the various scenarios in which the grounded conductor may still serve a grounding function at separate buildings, validating its use not just in one specific application but across multiple applicable conditions. This reflects a broader application of grounding principles, adhering to safety standards and enabling proper equipment operation in diverse settings.

It's essential to understand that such permissibility comes with caveats regarding safety and operational standards, which must always be considered in the application of electrical grounding practices.