Which of the following is NOT considered an acceptable grounding electrode for a building or structure?

A concrete-encased electrode, while commonly used for grounding, is not considered acceptable when it is installed with a plastic vapor barrier at the bottom. The grounding effectiveness heavily relies on the electrode maintaining proper contact with the soil to facilitate the dissipation of electrical currents. The presence of a plastic vapor barrier can inhibit this contact, compromising its function as a grounding electrode.

In contrast, options such as a ground ring buried at an appropriate depth, a metal underground water pipe in contact with the earth, and driven ground rods are all conventional grounding methods recognized for their reliability and efficacy in establishing a connection to the ground. These methods promote effective grounding and bonding, integral to electrical safety in buildings.