🔍 Key Concepts

• Ring-type current transformer surrounding all three phase conductors together, not each phase separately • For a three‑phase, three‑wire motor with no ground fault, how the three line currents relate vectorially (their phasor sum) when the load is normal and balanced • What changes in the sum of currents when some current leaves the three conductors through a ground fault path

💭 Think About

• Look at figure B and notice that all three phase leads pass through the same magnetic core. If the load is healthy, what is the algebraic (vector) sum of the three phase currents inside that core? • If a ground fault occurs on the motor, part of the return current flows through the equipment grounding path instead of back through the three phase conductors. How does that affect the net current passing through the CT window? • Which answer choice correctly describes the condition (ground fault or no ground fault) AND the state of the three line currents (balanced or unbalanced) that makes the CT output go to zero?

✅ Before You Answer

• Verify that when there is no ground fault on a balanced three‑phase load, the instantaneous sum of the three phase currents is zero, so the CT sees no net magnetizing ampere‑turns. • Confirm that a ground fault causes a residual (unbalanced) current that does not cancel in the CT, producing an output that operates the ground‑fault relay. • Match the correct option by asking: under normal, fault‑free conditions, are the three phase currents expected to be balanced or unbalanced, and should the CT output then be zero or non‑zero?