Number of Relays for Inter-Phase Faults

Correct Answer

B — Two

Concept and Reasoning

In a three-wire system, two properly connected phase overcurrent relays are sufficient to detect any inter-phase fault because any line-to-line fault involves at least one of the two monitored phase-current combinations. Therefore option B is correct.

Calculation / Key Relation

Minimum phase-fault OC elements in a 3-wire system: 2

Exam Takeaway

Two phase relays can cover all inter-phase faults in a three-wire system.

Common Mistake

Do not assume one relay per phase is always the minimum required arrangement.