Code Standards and Practices Level 6 Practice Test

Zero-sequence ground-fault protection looks for an imbalance in the currents that return through the same path in a grounded system. When a ground fault occurs, current leaks from a phase conductor to earth, so the sum of currents in the phase conductors and the neutral is no longer zero. A residual current/core-balance transformer is designed to sense this imbalance by encircling the phase conductors and the neutral; the zero-sequence component exists chiefly in those conductors during a fault, triggering the protection. The grounding conductor isn’t part of the sensing path in this arrangement, so it isn’t monitored for the zero-sequence condition.