Which of the following AC systems is permitted to be grounded, but not required to be grounded?

The grounding requirements for electrical systems depend on their voltage, configuration, and application. In this case, the 480-volt delta, 3-phase, 3-wire system can be grounded, but it is not required to be grounded under specific conditions outlined by the National Electrical Code (NEC).

A delta system that does not have a neutral point cannot be grounded in the same way that wye systems can. However, grounding may still be implemented for safety and to control transients, but it is not mandated. This flexibility allows for safety practices without enforcing strict grounding requirements, making it an option rather than a necessity.

In contrast, the other systems mentioned each have grounding requirements dictated by their design and intended use. The 120-volt single-phase system is commonly required to be grounded for safety due to its lower voltage and the relative risk of shock. The 240-volt split-phase system generally also has grounding requirements for similar reasons. Meanwhile, the 600-volt three-phase system typically has more stringent grounding requirements due to the higher voltage level and associated safety considerations. Thus, the distinct characteristics of the 480-volt delta system confirm that while grounding is permitted, it isn't an obligatory requirement.