Correct answer: C — Balanced earth fault
How balanced earth-fault protection operates
Balanced earth-fault protection detects earth faults inside a defined generator protection zone. In a typical arrangement, the residual current obtained from the three phase current transformers is compared with the current measured by a neutral current transformer. CT ratios, polarities and connections are selected so that the relay currents balance for healthy operation and faults outside the protected zone.
For normal balanced operation, the phasor sum of the three line currents is approximately zero. During an external earth fault, the corresponding phase-residual and neutral-current signals should balance at the relay. Little operating current therefore flows through its measuring element.
An earth fault within the CT-defined zone disturbs this balance. A difference or spill current flows through the relay, which initiates tripping when its operating threshold is exceeded. The protected zone and sensitivity depend on the actual CT arrangement and generator neutral-earthing system.
The examination associates this relatively economical scheme with generators rated 1 MVA to 5 MVA. The rating range is a textbook selection guide, not a universal design rule. Real generator protection is selected using winding arrangements, earthing, fault-current levels, required coverage and coordination.
