Which of the following types of grounding electrodes is prohibited?

Underground plastic water pipes are prohibited as grounding electrodes because they do not conduct electricity. Grounding electrodes are designed to provide a low-resistance path for electrical faults to safely dissipate into the earth, and they must be made of materials that can reliably carry electrical current. Plastic, being a non-conductive material, does not fulfill this requirement.

In contrast, concrete embedded in earth, metallic water piping, and systems that use direct burial conductors can all serve as effective grounding electrodes. The concrete itself can provide a conductive path when combined with essential grounding components, while metallic water piping is often grounded due to its conductive properties. Direct burial conductors are typically designed for grounding applications and can provide adequate electrical connection to the earth. Thus, underground plastic water pipes fall short of the necessary standards for grounding electrodes.