Which protective device calculates the impedance of a line by measuring the voltage and current of that line?

• A. Differential relay.
• B. Special Protection System.
• C. Distance relay.
• D. Overcurrent relay.

Answer: (C)

The correct answer is that a distance relay calculates the impedance of a line by measuring the voltage and current of that line. Distance relays operate by using the principle of calculating the ratio of voltage to current, which provides a measurement of the impedance along the transmission line. This is critical for determining the distance to a fault on the line; if a fault occurs, the impedance decreases, prompting the relay to respond based on preset threshold levels.

In contrast, a differential relay functions by comparing the current entering and leaving a particular section of the system to detect imbalances caused by faults within that section. This does not directly relate to impedance measurements. A special protection system involves numerous components and strategies designed to maintain system reliability and safety but does not specifically focus on calculating impedance from voltage and current directly. An overcurrent relay, meanwhile, responds to excessive current levels but does not measure impedance; it simply activates when a certain pre-defined current threshold is exceeded, reacting to the magnitude of current rather than incorporating the voltage to derive impedance.

Thus, the distance relay stands out as the device specifically designed for calculating the line impedance through voltage and current measurements, making it the best choice in this context.