Which of the following three-phase electrical distribution systems is grounded?

The correct answer is based on the configurations of grounded electrical distribution systems. In three-phase electrical distribution, grounding is essential for safety, protecting equipment, and ensuring system stability.

The 120/208-volt, three-phase, four-wire system is a common low-voltage distribution system that provides grounding through its neutral conductor, making it a grounded system. Similarly, the 277/480-volt, three-phase, four-wire system also includes a grounded neutral which provides similar safety measures and operational benefits.

Both of these systems utilize the fourth wire, which is the neutral, to serve as a grounding reference, providing protection against electrical faults and allowing for proper function of overcurrent devices. This grounding improves safety conditions and reduces the risk of electrical shock or fire hazards associated with ungrounded systems.

On the other hand, the 480-volt, three-phase, three-wire system does not include a neutral wire, and therefore it is ungrounded, meaning it does not provide similar safety and operational benefits. Systems like this may be used in specific applications where grounding is managed through alternative means, but they generally do not offer the benefits of a grounded system that the other options do.

Thus, when identifying grounded systems in the context of the question