When may a single conductor be used in a metallic raceway without a ground?

Using a single conductor in a metallic raceway without a ground is permissible specifically when that conductor is a grounding conductor. This is because grounding conductors are intended to safely carry fault currents and ensure the electrical system's protection. They do not need an additional ground since their purpose is to establish a reliable path to ground, aiding in the protection against electrical shock and equipment damage.

In this context, single conductors that are not designated as grounding conductors typically require a grounding means to ensure safety and compliance with electrical code standards. Additionally, isolated applications and temporary installations often come with additional regulations and limitations that may not universally justify the lack of grounding. Therefore, the option emphasizing that a single conductor can be utilized without ground only applies in the context of grounding conductors, adhering closely to safety protocols and electrical standards.