Which statement best describes how an impedance relay decides to trip?

• A. It trips when measured impedance is too low or current is too high.
• B. It trips only on voltage anomalies.
• C. It trips based on oil temperature.
• D. It trips when a fault is detected on a bus only.

Answer: (A)

An impedance relay protects a transmission line by treating the measured impedance from the relay to the fault as a distance indicator. It uses the ratio of voltage to current, Z = V/I, to determine how far away a fault is. When a fault occurs, current spikes while voltage drops, so the calculated impedance falls into a pre-set fault zone near the relay. If that measured impedance is sufficiently low (inside the fault zone), the relay trips to clear the fault. The idea that it also trips when current is too high fits because a high fault current reinforces that a fault is present within the protected section and may trigger the relay’s protection criteria.

Voltage anomalies alone aren’t the basis for this distance-type protection, and oil temperature is unrelated to the electrical fault-detection function. A fault on a bus isn’t the defining scenario for impedance protection, which targets faults along the protected line between the relay and potential fault locations.