The three rates of relay protection are instantaneous, definite time, and inverse time.

1. Instantaneous Relay Protection

Instantaneous relays operate immediately when the actuating quantity (current or voltage) exceeds the preset pickup value, without any intentional time delay. They are used in situations where rapid disconnection of a fault is critical, such as in high-speed fault clearing zones, to prevent equipment damage and cascading failures. The operating time typically ranges from a fraction of a cycle to a few cycles of the power frequency, depending on the fault magnitude and relay design .

2. Definite Time Relay Protection

Definite time relays operate after a fixed, predetermined time delay, regardless of the magnitude of the fault current. This type of relay ensures that the relay will trip only after the set time has elapsed, providing coordination with other relays in the system. It is commonly used in backup protection schemes where selectivity and coordination are important .

3. Inverse Time Relay Protection

Inverse time relays have an operating time that varies inversely with the magnitude of the fault current: the higher the current, the faster the relay operates. This characteristic allows for graded protection, where relays closer to the fault operate faster than those further away, ensuring proper coordination and minimizing unnecessary disconnections. Inverse time relays are widely used in overcurrent protection schemes for feeders, transformers, and busbars .

Summary

These three rates—instantaneous, definite time, and inverse time—form the basis of relay protection strategies in power systems. By combining these types, engineers can design protection schemes that are fast, selective, and reliable, ensuring both equipment safety and system stability .