Final answer:
If the only available grounding electrode is the building steel, then a supplemental grounding electrode is required according to NEC® Section 250.53(D)(2). Concrete-encased grounding electrodes and metal underground water pipes are typically sufficient on their own. The NEC® emphasizes the importance of ensuring a reliable grounding system for safety.
Explanation:
Understanding Grounding Electrode Requirements in the NEC®
Section 250.53(D)(2) of the NEC® specifies conditions under which a supplemental grounding electrode must be installed. This requirement is particularly relevant when the only existing grounding electrode is not considered sufficient for safe operation.
The question lists three potential scenarios:
- a. a concrete-encased grounding electrode
- b. the building steel
- c. a metal underground water pipe
According to the NEC®, if the only available grounding electrode is the building steel, a supplemental grounding electrode is required. This is because, while building steel can provide grounding, it may not be reliable under all circumstances, especially in certain types of structures or when there is a failure in the connection to the grounding system.
In contrast, a concrete-encased grounding electrode and a metal underground water pipe are generally acceptable grounding electrodes that do not necessitate a supplemental electrode if they meet the necessary requirements outlined in the NEC®. Therefore, the correct choice is b. the building steel.
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