Fault Analysis MCQ [Free PDF] – 5 Objective Questions with Answers

For a double-line-to-ground fault on phases b and c, the unfaulted phase current Ia is zero and the fault-point sequence voltages satisfy the appropriate equal-voltage relation. Three independent signatures agree: zero sequence present → ground fault; Ia=0 → phase a healthy; negative sequence present → unbalanced two-phase involvement.

Exam focus: Fault ID shortcut: zero-sequence present means the ground path participates. Common trap: Selecting a 3-phase fault despite nonzero I0 and I2.

In a line-to-line-to-ground (LLG) fault, positive-, negative- and zero-sequence networks are all involved and their sequence voltages at the fault point are equal because the three sequence networks meet at a common fault-point voltage.

For an L-L fault there is no zero-sequence current, so the zero-sequence network is not fixed by current flow. At open-circuited generator terminals this can leave the common zero-sequence voltage reference indeterminate even though line-to-line voltages remain uniquely determined. Hence both assertion and reason are true and the reason explains the assertion: option A.

Immediately after a sudden symmetrical fault, damper-winding currents produce the strongest opposing transient flux and therefore the smallest subtransient reactance. The effect associated with the damper winding decays first as those induced currents die out rapidly.

B — Open conductor fault. Breaking one or two phase conductors interrupts the series current path and creates an unbalanced three-phase condition.