What are the secondary devices in relay protection

Protective relays are classified as secondary equipment because they monitor electrical quantities and control primary devices without directly interrupting current.

Classification and Role

Protective relays are secondary devices in power systems because they do not directly carry or interrupt the main power flow; instead, they receive signals from instrument transformers such as current transformers (CTs) and voltage transformers (PTs) that scale high voltages and currents to safe levels for measurement and control purposes. The relays analyze these secondary signals to detect abnormal conditions like overcurrent, overvoltage, or frequency deviations and then send a trip signal to a primary device, typically a circuit breaker, to isolate the faulted section.

Operational Principle

The relay acts as a decision-making unit: it continuously monitors electrical parameters, compares them against preset thresholds, and determines whether a fault exists. Once a fault is detected, the relay energizes the trip coil of a circuit breaker or sends a control signal to other protective devices. This indirect action is why relays are considered secondary equipment—they control the primary equipment rather than interrupting the main current themselves.

Importance in Power Systems

Being secondary equipment, protective relays are essential for system reliability and safety. They allow high-voltage and high-current circuits to be monitored safely, provide fast fault detection, and coordinate with backup protection to minimize outages and equipment damage. Modern numerical relays combine multiple protection functions, event recording, and communication capabilities, further enhancing their role as intelligent secondary devices.

In summary, protective relays are secondary equipment because they operate on scaled-down signals from primary devices, make protection decisions, and control circuit breakers or other interrupting devices to safeguard the power system.