What is a requirement for alternating-current systems of 50 to 1000 volts that supply premises wiring?

The requirement for alternating-current systems of 50 to 1000 volts that supply premises wiring states that they must be grounded if the maximum voltage to ground does not exceed 150 volts. This is essential for safety and compliance with National Electrical Code (NEC) standards. Grounding helps to mitigate the risk of electric shock and ensures that any fault currents have a safe pathway to the ground, thereby protecting both the system and individuals.

In the context of alternating-current systems, when the maximum voltage to ground is at or below 150 volts, grounding becomes mandatory to enhance safety and reduce hazards associated with electrical faults. This requirement is particularly crucial for premises wiring, where the presence of people and sensitive devices increases the likelihood of potential dangerous situations occurring.

Thus, the grounding requirement aligns with established safety protocols aimed at protecting electrical installations and users. As a result, option A is the correct understanding of the necessary grounding practices for these systems.