What is an impedance relay and what does it protect?

• A. A relay that protects by measuring impedance; trips when fault impedance is too low or current too high.
• B. A relay that monitors oil temperature.
• C. A relay that only detects earth faults.
• D. A relay that measures phase rotation.

Answer: (A)

An impedance relay is a distance protection device that protects transmission lines by monitoring the impedance seen between the relay and any fault on the line. It uses the relationship V and I to calculate the apparent impedance; under normal load conditions this impedance stays outside the fault region. When a fault occurs, the impedance seen by the relay drops significantly (the fault impedance is low), placing the fault inside the relay’s predefined fault zone, which causes the relay to pick up and trip the circuit to isolate the fault. This is why the correct description emphasizes measuring impedance and tripping when the fault impedance is too low (and the associated fault current is high). The other options describe unrelated protections.