Selectivity in a protection relay system refers to:

Question

Question

This question was previously asked in

RRB JE ECE 22 Apr 2025 Shift 2 CBT 2 Official Paper

the ability of the relay to operate only for specific fault types
the ability of the relay to discriminate between fault and normal conditions
the ability of the relay to clear faults without affecting the rest of the system
the ability of the relay to operate with a minimal time delay for all fault conditions

Answer 1

The correct answer is 3) the ability of the relay to clear faults without affecting the rest of the system.

Here's why:

Selectivity in a protection relay system ensures that only the circuit element directly affected by a fault is isolated, while the rest of the healthy system continues to operate normally. This means the relay should "select" the faulty section and initiate tripping of the appropriate circuit breaker(s) to isolate it.

Let's look at why the other options are not the primary definition of selectivity:

1) the ability of the relay to operate only for specific fault types: This describes the sensitivity or characteristic of the relay to respond to certain types of faults (e.g., overcurrent, undervoltage, earth fault). While related to proper operation, it's not the core meaning of selectivity.
2) the ability of the relay to discriminate between fault and normal conditions: This describes the fundamental reliability and security of the relay to distinguish abnormal fault conditions from normal operating conditions or transient disturbances. It's a prerequisite for any protection system, but not specifically selectivity.
4) the ability of the relay to operate with a minimal time delay for all fault conditions: This describes the speed of operation of the relay. While fast fault clearing is desirable, selectivity might sometimes involve intentional time delays in coordination with other relays to ensure the correct sequence of tripping and isolation of the faulty section.