Connection between circuit breaker and relay protection device

A circuit breaker is connected to a protective relay via current transformers (CTs) and a trip coil circuit, allowing the relay to detect faults and automatically trip the breaker.

Components Required

Protective Relay – Detects abnormal conditions such as overcurrent, overvoltage, or phase faults and sends a trip signal ( ). Circuit Breaker (CB) – Electrically isolates the faulty section when tripped. Current Transformers (CTs) – Step down high line currents to a safe level (typically 5A) for the relay to measure ( ).

Trip Coil Circuit – Connects the relay output to the breaker's trip mechanism. Auxiliary Power Supply – Provides DC or AC power to the relay and trip coil.

Step-by-

Step Connection Procedure

Install CTs on the Line – Place CTs around the conductors feeding the circuit breaker.

Ensure the C

T ratio is slightly higher than the maximum line current to avoid saturation ( ).

Connect C

T Secondary to Relay – Wire the secondary terminals of the CTs to the relay input terminals. Maintain correct polarity (P1 to relay input, S1 to common reference) to ensure proper directional operation ( ). Wire the Trip Coil – Connect the relay's trip output contacts to the breaker's trip coil.

This can be a normally open (NO) contact that closes when the relay detects a fault, energizing the trip coil ( ). Provide Auxiliary Power – Ensure the relay and trip coil have the required voltage supply. Some relays include internal power supplies, while others require external DC/AC sources ( ).

Test the Circuit – Perform functional tests by simulating fault conditions to verify that the relay correctly trips the breaker. Check for proper timing, selectivity, and coordination with other relays in the system ( ).

Important Considerations

Polarity and Phasing – Incorrect CT polarity can cause misoperation or failure to trip. Relay Settings – Configure pickup current, time delays, and protection zones according to system requirements ( ). Selectivity – Ensure the relay only trips the breaker closest to the fault to minimize system outages ( ).

Safety – De-energize circuits when making connections and follow standard electrical safety practices. By following these steps, the protective relay can reliably monitor the circuit and trip the breaker during abnormal conditions, ensuring system stability and minimizing damage to equipment ( ).