What are the uses of relay as a protection device? Describe working of protective relays and circuit breaker with neat circuit diagram.

Relays serve as vital protection devices in electrical systems, ensuring the safety and integrity of the equipment and personnel. Here are the primary uses of relays in protection:

1. Overcurrent Protection: Relays detect excessive currents in the circuit and trip the circuit breaker to prevent damage to equipment and wiring caused by overloads or short circuits.

2. Undercurrent Protection: Relays monitor the continuity of circuits and trip the breaker if the current falls below a specified threshold, indicating a fault or equipment failure.

3. Earth Fault Protection: Relays detect earth faults by monitoring the current imbalance between the phase conductors and the ground. They trip the breaker to isolate the faulty section and prevent electric shock hazards.

4. Overvoltage and Undervoltage Protection: Relays monitor voltage levels and trip the breaker if the voltage exceeds or falls below predetermined thresholds, protecting equipment from damage due to excessive or insufficient voltage.

The working principle of protective relays and circuit breakers involves coordination between the relay sensing the fault condition and the circuit breaker interrupting the current flow. Here's a simplified explanation along with a circuit diagram:

Working of Protective Relays and Circuit Breakers:

1. Sensing the Fault: Protective relays continuously monitor the electrical parameters such as current, voltage, and frequency. When a fault occurs (e.g., overcurrent, earth fault), the relay detects the abnormal condition based on predefined settings.

2. Relay Operation: Upon sensing the fault, the relay energizes its trip coil, causing its contacts to change state. In the case of an overcurrent relay, for example, the contacts close when the current exceeds the set threshold.

3. Circuit Breaker Tripping: The change in relay contacts signals the circuit breaker to trip. The trip coil of the circuit breaker is energized, causing the breaker contacts to open, thereby interrupting the flow of current in the circuit.

4. Isolation of Fault: Once the circuit breaker trips, the faulty section of the circuit is isolated from the rest of the system, preventing further damage.

Diagrammatically, the relay and circuit breaker arrangement involves the relay connected in series with the trip coil of the circuit breaker, such that when the relay operates, it energizes the trip coil, causing the breaker to trip and open the circuit. This setup ensures swift and effective protection against faults.