Which of these electrodes does not meet the minimum requirements set by NEC?

The answer indicates that a plate electrode buried 18 inches does not meet the minimum requirements set by the National Electrical Code (NEC). According to NEC, the minimum depth for direct burial of grounding electrodes, such as plate electrodes, is generally required to be at least 30 inches. By being buried at only 18 inches, this specific plate electrode fails to satisfy the depth requirement, thereby making it less effective for grounding applications.

In contrast, the other options adhere to the necessary specifications for grounding electrodes as dictated by NEC. A stainless steel rod with a 5/8 inch diameter meets the standards for rod electrodes, which specify that a rod must be at least 8 feet long and exhibit a specific diameter for adequate grounding. A ground ring that is 20 feet long also aligns with NEC rules, as ground rings must have a specified length to effectively dissipate electrical currents. Lastly, a metal underground water pipe electrode buried 30 inches complies with NEC depth guidelines for bonded metallic systems, ensuring safe grounding.