Relay protection devices are not considered primary equipment; they are control and decision-making devices that operate primary equipment like circuit breakers.

Role of Protective Relays

Protective relays are designed to monitor electrical quantities such as current, voltage, frequency, and impedance, and to detect abnormal conditions or faults in power systems . When a fault is detected, the relay sends a trip signal to a circuit breaker or other interrupting device, which then isolates the faulted section . Relays themselves do not interrupt current directly; they rely on primary equipment like breakers to perform the physical disconnection .

Primary vs. Secondary Equipment

• Primary equipment refers to devices that directly carry or interrupt electrical power, such as transformers, generators, transmission lines, and circuit breakers .
• Secondary or auxiliary equipment, which includes protective relays, controls, and measurement devices, supports the operation and protection of primary equipment . Relays are part of the protection system, ensuring that primary equipment operates safely and faults are cleared promptly.

Primary and Backup Relays

Within the protection system, relays can be classified as primary or backup based on their role in fault detection . A primary relay is the first line of defense for a specific section of the system, while a backup relay operates only if the primary relay fails. Despite this terminology, both types are still secondary devices, as they control the operation of primary equipment rather than interrupting power themselves.

Conclusion

Protective relays are essential for system reliability and safety, but they are not primary equipment. They function as decision-making devices that detect faults and command primary equipment, such as circuit breakers, to isolate the faulted section, thereby protecting the power system and minimizing damage .