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

The correct answer is that underground metal gas piping is not permitted as a grounding electrode. The primary reason for this is that gas piping systems are not considered a reliable or safe means of grounding an electrical system. Grounding electrodes are essential for ensuring safety by providing a path for fault currents to dissipate into the ground, and they must meet specific criteria established by the National Electrical Code (NEC).

Metal gas piping is subject to changes in its integrity due to corrosion, wear, and other factors, which can compromise its effectiveness as a grounding electrode. Additionally, the use of such piping can pose safety hazards, especially in the event of a gas leak or fire, where an electrical fault could create dangerous conditions.

In contrast, metal water piping, concrete-encased electrodes, and ground rods are all recognized and permitted as grounding electrodes because they provide a stable and effective path for electrical currents. They have established performance characteristics that ensure effective grounding and bonding, thereby enhancing safety and compliance with electrical codes.