Why RCCB cannot give protection against Overload and Short Circuit Faults?

To better understand Why RCCB cannot give protection against Overload and Short Circuit Faults, you must know the simple working principle of RCCB. RCCB is mainly used to sense the leakage current. RCCB stands for Residual Current Circuit Breaker. In this article, I have tried to explain the working of RCCB with diagrams, which will help you to understand why RCCB cannot work in overload and short circuit faults.

RCCB Internal Circuit Diagram

Here the simple internal circuit diagram of RCCB is given below.

Here you can see, that three coils are wound on a core. The upper coil is for the incoming line, the lower coil is for the return line, and the middle coil is the sensing coil which is connected to the relay. So when the current in the incoming and return lines is the same, there will be no resultant flux produced because the flux produced by incoming and return will oppose each other.

But if the current in incoming and return wire is unequal, then flux also be unequal hence a resultant flux will be produced for which an emf will be produced in the sensing coil. As the sensing coil is connected to the relay, the relay will trip the RCCB and disconnect the circuit.

So, it is clear that the working of RCCB is based on Kirchoff's Current law that is the incoming current must be equal to the outgoing current in a circuit. So the simple concept is, when the current is unequal in both lines it will trip and disconnect the circuit.

Why RCCB cannot give protection against Overload Fault?

The below diagram shows, there is an overload fault occurs in a circuit where RCCB is connected.

Here, the incoming current and return current are equal even overload fault occurs. A  high current flows through the RCCB due to overload fault but the RCCB does not trip as the current in both lines is equal. Now, if the overload current exceeds the current rating of the RCCB, it will be damaged.

Why RCCB cannot give protection against Short Circuit Faults?

In this case, the concept is also the same as the previous one. During a short circuit fault, a very high current flows through the RCCB, but it does not trip as the current in both lines is equal.

When RCCB will Trip?

Here you can see, the phase wire touches the earth wire for which, Current comes from the phase wire and goes back through the earth wire, not through the neutral wire. So, the current in incoming and return wire in RCCB becomes unequal. According to the working principle, as the current in both lines is unequal it will trip and disconnect the circuit.

Read Also:

• Proper RCCB connection Diagram with MCB.
• MCCB Full Form | MCB and MCCB Difference
• [Explained] Main Differences Between Relay and Contactor
• Why AC needs more Insulation than DC for same Voltage Level?

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