Which of the following is not permitted to be used as a grounding electrode?

The use of grounding electrodes is governed by specific codes and standards to ensure electrical safety. Among the options listed, metal underground gas piping is not permitted to be used as a grounding electrode. This is primarily due to safety concerns. If a fault occurs and electricity flows through the gas piping, it could create a dangerous situation by energizing the gas system, which poses a significant risk of fire or explosion.

In contrast, concrete foundations, metal water piping, and ground rods are all recognized and accepted methods for grounding. Concrete foundations can provide an effective grounding mechanism when they contain reinforcing steel, which can serve as a grounding electrode. Metal water piping systems, when adequately bonded and accessible, can also serve as grounding electrodes, as they are usually in direct contact with the earth and provide a low-resistance path for fault currents. Ground rods are specifically designed to be driven into the ground to provide electrical grounding, making them a key acceptable option for grounding.

Understanding the roles and limitations of these grounding systems is crucial for ensuring electrical installation meets safety codes and reduces the likelihood of hazardous situations.