What does a potential transformer primarily measure?

A potential transformer primarily measures high voltage. It is designed to step down high voltages to lower, manageable levels that can be used for accurate measurement or control purposes. This allows electrical systems to monitor and analyze high-voltage circuits safely without direct exposure to the dangerous high voltages.

Potential transformers are essential in applications where high voltage measurements are necessary, such as in substations or for protecting equipment and personnel. The measurement output from a potential transformer is in a much lower voltage, typically suitable for standard measuring instruments, which ensures safe operation while still providing accurate readings of the original high-voltage circuit.

Understanding the characteristics of potential transformers emphasizes their importance in power systems, where safety and accuracy in high voltage measurements are critical. This context underscores why the other options do not correctly describe the primary function of a potential transformer: they are not designed for low or medium voltage applications, nor do they measure direct current, as their operational principle is based on alternating current (AC) systems.