The metal mounting yoke of a replacement switch is not required to be connected to an equipment grounding conductor if the wiring at the existing switch does not contain an equipment grounding conductor, and the what condition applies?

The metal mounting yoke of a replacement switch does not need to be connected to an equipment grounding conductor if the wiring at the existing switch does not contain such a conductor and the switch faceplate is nonmetallic with nonmetallic screws. This is primarily due to the principle of minimizing conductive paths that could carry fault current back to the ground.

Using a nonmetallic faceplate and screws reduces the likelihood of any metallic contact that could inadvertently create a grounding path through the switch, thus mitigating the risks associated with electrical shock. The absence of metallic components in the faceplate and its hardware means there is a lower chance of the switch becoming a grounding point. This establishes a safe condition where the metal parts do not need to be grounded because they are less likely to become energized.

The other conditions relating to GFCI protection, safety system involvement, or the installation's residential context do not directly address the fundamental requirement for grounding based on the conductive properties and potential hazards associated with metallic versus nonmetallic components in the switch installation scenario.