What is Core Balance Current Transformer(CBCT)? Diagram, Application

A core balance current transformer is a specially designed current transformer constructed by a magnetic coil wound on a ring-type core. This coil is used as a secondary coil. Generally, two or more live conductors are to be passed through the ring-type core and they act as a primary winding. 

Core Balance Current Transformers have a very useful function for the electrical protection system. Basically, it is a current sensing device. When an unbalanced current occurs in a circuit due to any earth fault, earth leakage, or ground fault, it can sense the unbalanced current and produce output. It acts as a primary device of a protection system. It sends the signal to the protective device to break the circuit when any fault occurs.

Core Balance Current Transformer Diagram

Here, you can see the diagram of CBCT. Here, the connection between CBCT and an Earth Leakage Relay is shown.

You can see the output terminal of the CBCT is connected to an earth leakage relay and all three phases and neutral are passed through the core. In normal conditions, all the phases and neutral will conduct a balanced current. But during the fault condition, an unbalanced current will cause to generate a magnetic flux that will produce an emf in the secondary coil that is used to activate the protection system.

How a Core Balance Current Transformer Works?

The working principle is very simple. We know all the three phases are passed through the core. In normal conditions, when the flow of current in  all the three phases are equal there is no unbalanced current. So, the magnetic flux produced by the conductors will be minimized to each other, or the overall magnetic flux will be zero. But, when any earth fault or earth leakage occurs an unbalance current will flow through those conductors. And a resultant magnetic flux will be produced which causes to create an emf in the transformer's secondary winding. The secondary winding will send the signal to the protective device connected to it.

Now, let's understand it with an example.

Suppose, all the three phases passes through the core carrying current 10A each phase(R,Y,B). As, the flow of current in all the phases are equal, there is no resultant or unbalance current, so the resultant magnetic flux will be zero. Now an earth leakage happens with the Y phase, and it carries 10.5A Current. Now the current flow in all phases are - R Phase - 10A, Y Phase - 10.5A, B Phase - 10A.

So, the unbalance current is 0.5A. A resultant magnetic flux will produce due to this unbalance current. This magnetic flux will links with the secondary of the CBCT, and causes to produce an emf in the secondary winding.

Actually, the CBCT works in very similar to the Residual Current Circuit Breaker.

How a Core Balance Current Transformer(CBCT) is Used?

Generally, the CBCT is used with a earth fault relay or earth leakage relay. The earth leakage relay is connected to a circuit breaker through a  NC(Normally Closed) or NO(Normally Open) contact. It may be a MCCB(Moulded Case Circuit Breaker) or any other circuit breaker. In the normal condition, the circuit breaker keep closed the main power circuit to provide power to the load. But when any earth fault or earth leakage fault occurs, the CBCT will sens the fault current and sense signal to the earth leakage relay. Once the earth leakage relay got the signal from the CBCT, it will send signal to the circuit breaker to trip the circuit breaker immediately. So, the load will disconnect from the main power supply.

Core Balance Current Transformer Applications

1. CBCT is used in electrical power control panels.

2. CBCT is used for motor protection circuits.

3. CBCT is used for the protection of heavy electrical machinery.

4. CBCT is used in leakage current measurement system.

5. CBCT is used in large electrical power system installations.

Read Also: 

• What is Ground Fault Circuit Interrupter(GFCI)? Use
• Working and connection diagram of Single Phasing Preventer
• Application, Advantages and Disadvantages of all types of Circuit Breaker
• Main Purpose of Using Neutral Grounding Resistor
• [Actual] Purpose and Function of Earth Wire

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