🔍 Key Concepts

• Residual (zero-sequence) current in a 3‑phase system: the vector sum of the three phase currents under normal, no‑fault conditions • Operation of a window-type current transformer (CT) used as a ground fault sensor • Difference between power conductors through the window and small CT secondary leads on the terminal screws

💭 Think About

• In a healthy 3‑phase, 3‑wire circuit, what must the sum of the three phase currents be for the CT output to be zero? What does that tell you about how many phase conductors must pass through the window? • Are the terminals on the sensor intended to carry the full motor feeder current, or just the small current going to the ground‑fault relay input? How does the illustration support your conclusion? • If only one or two phase conductors passed through the window, what would the sensor see even under normal balanced load?

✅ Before You Answer

• Verify in the illustration whether the three motor feeder conductors appear to pass through the sensor window in figure A. • Confirm that the relay is wired to the small terminals on the sensor, while the power feeders do not land on those terminals. • Make sure the statement you choose clearly distinguishes between conductors passed through the window and connections made to the small sensor terminals.