In what condition should a protective earthing conductor be used?

A protective earthing conductor serves a crucial role in electrical safety by providing a path for fault currents to flow safely to ground, thereby minimizing the risk of electric shock or fire. The most appropriate context for its use is within all final subcircuits that have Residual Current Device (RCD) protection.

RCDs are designed to detect earth faults and disconnection of the supply if a fault is found, thus preventing potential hazards. For these systems to function correctly, those circuits must include a protective earthing conductor, ensuring that any fault current is directed to ground, allowing the RCD to operate efficiently and mitigate risk. This is particularly important in areas where people are likely to use electrical equipment, such as kitchens or outdoor environments, where moisture increases safety hazards.

While the other options may seem relevant in certain situations, they do not encompass the full range of safety requirements that apply to all final subcircuits with RCD protection, making this choice the best response in the context of electrical safety measures.