Which of the following is NOT an acceptable grounding electrode for a service grounding electrode system?

The selection of an acceptable grounding electrode is crucial for ensuring the safety and effectiveness of a service grounding system. Each of the grounding electrode options listed serves a purpose in providing a path to ground, but the characteristics of these materials and their method of installation matter significantly.

The first option, a 1/2-inch rebar 20 feet long encased in concrete, is not recognized as an acceptable grounding electrode on its own. While rebar can contribute to grounding when it is part of a larger system (like reinforcing concrete), merely using a single piece of rebar encased in concrete without establishing it as part of a grounding electrode system does not meet the criteria for reliability and effectiveness in grounding. Grounding electrodes need to ensure low resistance paths to the earth, and while concrete may provide some shielding, the rebar alone doesn't create an effective grounding path unless it's properly connected and utilized in conjunction with other elements designed for grounding.

In contrast, ground rods, building steel, and concrete-encased electrodes are codified options recognized for their effectiveness in establishing a reliable grounding system. Ground rods are driven into the earth to provide a direct connection to the ground, while building steel, when properly bonded, conducts electricity effectively to ground, and concrete-encased