RMS digital trips offer increased accuracy and reliability advances in low voltage circuit breaker trip technology

The function of low voltage circuit breaker trip units, and the need for direct RMS (root mean square) overcurrent sensing of the sinusoidal and nonsinusoidal current wave, including percentage error analysis are reviewed. It is found that the application of digital microprocessor technology to achieve direct RMS sensing allows the true RMS (heating) protection of thermal-electromechanical trip units, along with the precise protection and coordination selectivity of solid-state trip. This direct RMS sensing provides accurate long-time and short-time overcurrent protection in power distribution systems with the increasing occurrence of nonsinusoidal currents