Final answer:
If the supply voltage to the transformer exceeds 150V, systems less than 50V must be grounded.
Step-by-step explanation:
In a power distribution system, systems less than 50V must be grounded if the supply voltage to the transformer exceeds 150V. Transformers are used to change voltages at different points in the power distribution system. Electric power is usually generated at higher voltages and transmitted long distances at even higher voltages to limit energy losses. Local power distribution to neighborhoods or industries goes through a substation and is sent short distances at intermediate voltages before being reduced to 120V, 240V, or 480V for safety at the individual user site.