According to NFPA 70, which of the following grounding methods is not permitted?

Delta center grounding is considered not permitted according to NFPA 70, often because it does not provide an effective fault current path, which is essential for ensuring safety in electrical systems. When grounding methods are implemented, they should facilitate the safe operation of electrical installations and ensure that under fault conditions, sufficient current flows to operate overcurrent devices, such as circuit breakers.

In a delta grounding system, particularly when considering the center point, challenges arise as it may not allow for proper grounding and could result in increased hazards, such as unbalanced loads and difficulty in clearing ground faults. The method may also leave phases of the system inadequately protected in case of faults, leading to potential equipment damage or electric shock hazards.

In contrast, methods like solid neutral grounding provide a clear path for fault currents, ensuring effective overcurrent protection, while both high resistance neutral and delta corner possess characteristics conducive to safety and system stability within the guidelines provided by NFPA 70. Thus, the selection of a grounding method must prioritize safety and regulatory compliance, ruling out delta center grounding due to its inadequacies in these areas.