Which of the four characteristics of relay protection must be satisfied first

Relay protection must satisfy reliability, selectivity, sensitivity, and speed to ensure safe and effective fault isolation. 1.

Reliability

Reliability ensures that the relay operates correctly when a fault occurs and does not operate under normal conditions or in the absence of a fault. A reliable relay must neither fail to act during a fault nor cause false trips, guaranteeing the safe removal of faulty equipment without affecting healthy parts of the system. Reliability is achieved through robust design, high-quality components, proper maintenance, and simplicity in construction, such as dust-free enclosures, good contact materials, and well-braced joints.

2.

Selectivity

Selectivity (or discrimination) ensures that only the faulty section of the system is isolated while leaving the rest of the network operational. Absolute selectivity occurs when a relay responds only to faults within its own protection zone, as in unit systems like differential protection. Relative selectivity is achieved by grading settings across multiple zones, typical in non-unit systems such as current time-graded or distance protection.

Proper selectivity prevents unnecessary outages and maintains system stability. 3.

Sensitivity

Sensitivity refers to the relay's ability to detect faults within its designated protection zone, regardless of fault location or type. A sensitive relay responds to the minimum fault current or voltage expected under the most unfavorable operating conditions. Sensitivity is quantified by a sensitivity factor (Klm), which ensures that even small or remote faults trigger the protective action reliably.

Adequate sensitivity prevents undetected faults that could escalate into larger system failures. 4.

Speed

Speed is the requirement for the relay to operate quickly enough to minimize damage to equipment and maintain system stability. The relay must act within the critical clearing time for the fault, avoiding delays that could cause equipment damage or cascading failures. However, it should not operate too fast to avoid false trips.

Speed depends on the relay type, operating mechanism, and coordination with circuit breakers and other protective devices.

Summary

In practice, a well-designed relay protection system balances these four characteristics: reliability ensures correct operation, selectivity isolates only the faulted section, sensitivity detects all relevant faults, and speed minimizes damage and maintains system stability. Meeting these requirements is essential for the safe and efficient operation of modern power systems.