Overcurrent Relay Protection Circuit Diagram

An overcurrent relay circuit connects a current transformer to the relay coil, which then triggers a trip coil to operate the circuit breaker when current exceeds a preset limit.

Basic Circuit Description

An overcurrent relay (OCR) is a protective device that monitors current in a circuit and trips the breaker if the current exceeds a preset threshold. The main components of a typical OCR circuit include:

• Current Transformer (CT): Steps down the high line current to a safe level for the relay coil.
• Relay Coil: Energized by the CT secondary; its magnetic effect moves the relay armature when current exceeds the pickup setting.
• Contacts: Normally open (NO) contacts close when the relay operates, completing the trip circuit.
• Trip Coil of Circuit Breaker: Receives the signal from the relay contacts to open the breaker.
• Power Supply: Usually a DC battery or auxiliary supply to energize the trip coil.

Working Principle

1. Under normal conditions, the current through the CT is below the relay pickup setting, so the relay contacts remain open.
2. When the line current exceeds the preset value, the CT secondary current energizes the relay coil.
3. The magnetic force overcomes the restraining spring, moving the relay armature.
4. The relay contacts close, completing the trip circuit to the breaker coil, which then opens the breaker to isolate the faulted section.

Types of Overcurrent Relays

• Instantaneous OCR: Trips immediately when current exceeds the pickup value; no intentional time delay.
• Definite Time OCR: Trips after a preset time delay once the pickup current is exceeded.
• Inverse Time OCR: Trip time decreases as the current increases, providing faster operation for higher fault currents.
• Directional OCR: Operates only for fault currents in a specific direction, using both current and voltage inputs for torque generation.

Typical Connection Diagram

Line | |---- CT ----> Relay Coil ----> NO Contact ----> Trip Coil ----> Circuit Breaker | Load

• The CT secondary is connected in series with the relay coil.
• The relay contacts are connected in series with the trip coil of the circuit breaker.
• For directional OCR, an additional voltage coil is connected to the system voltage to provide directional sensitivity.

Notes

• The pickup current and time settings are adjustable depending on system requirements.
• In digital or microprocessor-based OCRs, the CT input is processed electronically, and the relay can provide both overcurrent and earth fault protection.
• Proper coordination with upstream and downstream relays ensures selective tripping and system stability. This configuration ensures that any overcurrent condition is detected promptly, and the circuit breaker is tripped to protect equipment and maintain system safety .