In relay protection, disconnection refers to the automatic isolation of a faulty section of an electrical system by tripping circuit breakers to prevent damage and maintain system stability.

Definition and Purpose

Disconnection in relay protection is the process by which a protective relay detects abnormal conditions or faults—such as short circuits, overcurrent, or earth faults—and triggers circuit breakers to open, thereby isolating the affected part of the system from the rest of the network . This ensures that the fault does not propagate, protecting equipment, maintaining safety, and minimizing disruption to other parts of the system .

Automatic Disconnection of Supply (ADS)

Automatic disconnection of supply is a key protective measure where one or more line conductors are interrupted automatically when a fault occurs . It is commonly applied in TN and TT earthing systems, where the protective device must operate within specified disconnection times to prevent electric shock and equipment damage . For example, in TN systems, the disconnection time for a 230 V AC final circuit is typically 0.4 seconds, while in TT systems it is 0.2 seconds .

Selectivity and System Stability

Effective disconnection is not just about isolating the fault but also about selectivity—tripping only the circuit breakers closest to the fault while leaving the rest of the system operational . This minimizes unnecessary outages and helps maintain power system stability, as disconnecting too many elements can lead to voltage drops or loss of transmission paths .

Summary

In essence, disconnection in relay protection is a safety and reliability mechanism. It allows the system to quickly remove faulty sections, protect equipment and personnel, and ensure that the rest of the electrical network continues to operate normally. Proper relay settings, coordination, and timing are critical to achieving effective and selective disconnection .