A protective relay is an electrical device that detects the electrical fault and initiates the operation of the circuit breaker to isolate the defective components from the power system.
In this article you’ll learn the difference between Instantaneous and Time Overcurrent Relay Protection.
Perhaps the most basic and necessary protective relay function is overcurrent: commanding a circuit breaker to trip when the line current becomes excessive. The purpose of overcurrent protection is to guard against power distribution equipment damage, due to the fact that excessive current in a power system dissipates excessive heat in the metal conductors comprising that system. Overcurrent protection is also applied to machines such as motors and generators for the exact same reason: electric current dissipates heat in the windings’ resistance (P = I^2.R), and excessive heat will damage those winding conductors.
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Contents
Instantaneous overcurrent Relay Protection
Instantaneous overcurrent protection is where a protective relay initiates a breaker trip based on current exceeding a pre-programmed “pickup” value for any length of time. This is the simplest form of overcurrent protection, both in concept and in implementation (relay design). In small, selftripping circuit breakers, this type of protection is best modeled by “magnetic” breakers where the tripping mechanism is actuated by the magnetic field strength of the line conductors: any amount of current greater than the tripping threshold will cause the mechanism to unlatch and open the breaker. In protective relay-based systems, the instantaneous overcurrent protection function is designated by the ANSI/IEEE number code 50.
Time Based overcurrent relay protection
Time overcurrent protection is where a protective relay initiates a breaker trip based on the combination of overcurrent magnitude and overcurrent duration, the relay tripping sooner with greater current magnitude. This is a more sophisticated form of overcurrent protection than instantaneous, expressed as a “time curve” relating overcurrent magnitude to trip time. In small, self-tripping circuit breakers, this type of protection is best modeled by “thermal” breakers where the tripping mechanism is actuated by the force of a bimetallic strip heated by line current: excessive current heats the metal strip, which then forces the mechanism to unlatch and open the breaker. In protective relay-based systems, the time overcurrent protection function is designated by the ANSI/IEEE number code 51. Time overcurrent protection allows for significant overcurrent magnitudes, so long as these overcurrent events are brief enough that the power equipment avoids heat damage.
Summary of the difference between Instantaneous and Time Overcurrent Relay Protection
| Instantaneous Overcurrent Relay Protection | Time Overcurrent relay protection |
|---|---|
| Relay initiates a breaker trip based on current exceeding a pre-programmed “pickup” value for any length of time | Relay initiates a breaker trip based on the combination of overcurrent magnitude and overcurrent duration |
| The simplest form of overcurrent relay protection | Comparatively complex |
| Comparatively cheaper | Is expensive mostly |
| Best modeled by “magnetic” breakers | best modeled by “thermal” breakers |
| Tripping mechanism is actuated by the magnetic field strength of the line conductors: any amount of current greater than the tripping threshold | Relay tripping happens sooner with greater current magnitude. Tripping mechanism is actuated by the force of a bimetallic strip heated by line current |