PSBS20/1 is a Residual Current Circuit Breaker, employs the current balance principle which involves the supply conductors to the load (phase and neutral) being wound onto a common transformer core to form the primary windings. Under healthy circuit conditions, the current in the phase conductor is equal to the current in the neutral and the vector sum of the current is zero. In the event of an earth fault, an amount of current will flow to earth, creating an out of balance situation in the transformer assembly. The RCD trip mechanism will operate at a residual current of between 50–100% of its rating tripping current and incorporate a high level of immunity to tripping when subjected to transient earth leakage current. Type A RCD ensured for residual sinusoidal alternating currents and residual pulsating direct currents, whether suddenly applied or slowly arising.