Abstract
A computer program has been developed to synthesise voltage waveforms with predetermined contents of harmonics up to the twentieth order. Currents of fundamental frequency having various proportions of a single harmonic were then derived using D/A card and an amplifier connecting to a residual-current circuit breaker (RCCB) and load. It was discovered that there was a critical proportion corresponding to each harmonic frequency at which the circuit breaker will be most susceptible to tripping. The harmonics synthesiser was also programmed to generate waveforms representing those resulting from nonlinear circuits and loads. These distorted waveforms were applied to an RCCB to study the changes in its tripping current. Finally, filter circuits were designed for connection across the tripping coil to maintain the consistency of the RCCB tripping characteristics when subjected to distorted current waveforms and voltage spikes.