Under what condition must alternating-current systems of less than 50 volts be grounded?

Alternating-current systems of less than 50 volts are required to be grounded when they are supplied by transformers exceeding 150 volts to ground to ensure safety. This is based on the principle that when transformers step down higher voltages, grounding becomes essential for protecting users from accidental contact with higher voltage potentials. Grounding in this scenario ensures that a fault condition can be safely managed, often by allowing an overcurrent protection device to operate and disconnect the circuit, thereby reducing the risk of electric shock or fire hazards.

The other options do not correctly reflect the grounding requirements under the National Electrical Code (NEC). For example, just using the system indoors, having a neutral conductor present, or limiting the installation to residential settings does not appropriately dictate the conditions under which grounding is necessary for systems under 50 volts. The critical factor in the requirement for grounding in this case is the relationship between the system and the higher voltage transformers, which highlights the importance of maintaining safety standards in electrical installations.