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Define RCCB – An interview question

What is RCCB and how does it work?

This is one of the questions I have encountered during my job interview for a sales distribution company. Honestly, I had a hard time answering the question and just relied on the words itself. There is a specific purpose and explanation why they call it RCCB and its working principle. That actually made a way for me to study it and make an article for it.

As the improvement of modern industry, I observed that the electricity is becoming more useful, but it also becoming more hazardous too! Do you know that it takes only few milliseconds for a strong electric current flowing through your heart to harm you? Think about this, if you’re using an electric cutter and you accidentally chop through the cable, If the tool has a metal case, you’re holding on to it, or you’re repairing the circuit and you accidentally touch the damaged wire, and you’re standing on the ground, the current has to flow somewhere. There’s a very high risk that your body will form a “short circuit”—the path of least resistance for the current to flow through. It takes just the blink of an eye for a current that’s doing you a favor to change its mind, zap through your body. So how to prevent electricity to through our body? One way to reduce the risk is to use a protective device called a residual current device (RCD), or residual current circuit breaker (RCCB), is a device that instantly breaks an electric circuit to prevent serious harm from an ongoing electric shock.

In the Europe these are better known by their initials RCD, and a combined RCD+MCB (miniature circuit breaker) is known as a RCBO (residual-current circuit breaker with overcurrent protection). In Australia, they are sometimes known as safety switches or an RCD. An earth leakage circuit breaker (ELCB) may be a residual current device, although an older type of voltage operated earth leakage circuit breaker also exists.

In such situation, in absence of an RCCB in the circuit, an earth fault may occur and the person is at the risk of receiving an electrical shock.

However, if the same circuit is protected with RCCB, it will trip the circuit in fraction of a second thus preventing the person from receiving an electrical shock. Therefore, it is a good and safe practice to install RCCB in your electrical circuit.

How does it protects?

As I have explained above, RCCB is meant for protection from earth faults and associated risk to human life such as electrical shocks.

The underlying fundamental principle behind operation of RCCB is that in ideal situations the current flowing in to the circuit through live (hot) wire should be same as the returning current from the neutral.

In case of an earth fault, the current finds a passage to earth through accidental means (such as accidental contact with an open wire etc.). As a result the returning current from neutral is reduced. This differential in the current is also known as “Residual Current”. See figure 1.1

RCCB is constructed such way that it continuously senses and compares for difference (residual current value) in current values between the live and neutral wires. Any small change in the current value on account of such event would trigger the RCCB to trip off the circuit.

There are different types/brands of RCCB available in the market. It comes with different number of poles depending on your circuit. The old version of RCCB is what we call the ELCB or Earth Leakage Circuit Breaker which some manufacturers made it obsolete or discontinued. ELCBs are voltage operated and not current sensing. It means it only protect against fault or shock to metalworks. Unlike the RCCB which detects current leaving a live wire or running to ground by another path.

Sometimes, you’ll realize that even job interviews can give you so much lessons. And it will make a way for you to study it and be more confident to it. That even we fail an interview, we knew that we will learned from it and you’ll be a better version for the next call.

P.S. – I passed the interview, and I’m so much grateful that they saw my potential.

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