🔍 Key Concepts

• Study how the current transformer (CT) in figure A encircles all three phase conductors (L1, L2, L3) and what it measures under normal balanced load versus a ground fault. • Contrast a ground fault relay with an overcurrent (overload) relay: one looks for unbalanced phase currents, the other looks for high total current. • Trace the wiring from the ground fault relay contacts to the breaker shunt trip coil and think about whether the breaker should trip when the relay contact OPENS or when it CLOSES.

💭 Think About

• In the illustration, when there is NO ground fault and the three phase currents are balanced, what is the net current seen by the CT, and should the relay operate in that condition? • During the illustrated fault to the motor frame, which path does the fault current take back to the source, and how does that change the balance of the three line currents through the CT window? • For the breaker shunt trip coil to energize and open the breaker, do you need the relay contacts to make (close) a circuit or break (open) a circuit?

✅ Before You Answer

• Confirm that the relay is responding to imbalance between the individual phase currents rather than simply the magnitude of total motor current. • Verify from the diagram whether the shunt trip coil receives power only when the relay contacts are closed, or only when they are opened. • Make sure the option you choose matches BOTH: (1) operation on unbalanced phase currents due to a ground fault, and (2) the correct normal position (open or closed) of the relay contacts as implied by the trip coil circuit.