A Residual Current Circuit Breaker (RCCB) is an important safety measure when it comes to protection of electrical circuits. It is a current sensing device, which can automatically measure and disconnect the circuit whenever a fault occurs in the connected circuit or the current exceeds the rated sensitivity. The International lEC Standard defines four types of RCD for AC applications. These types, which address the various kinds of residual current waveforms, are known as type AC, type A, type F (this paper does not explain) and type B. All these devices are intended to be used within AC supply systems.