Arc root commutation from the contact region in low contact velocity circuit breakers
Circuit breakers are widely used in commercial and industrial installations for automatically switching off a short circuit current. Arc motion in low voltage (240VAC) high current (103-104 A.) circuit breakers is dominated by arc root mobility. This study provides a vital step in the development of a consistent arc structure and motion in circuit breakers operating at low contact opening velocity. The experimental investigations focus on the influence of the contact material, arc chamber material, contact velocity, power supply polarity, the gap behind the moving contact and the peak short circuit current level on the arc motion. The gas flows and gas composition effects are inspected. This thesis concentrates on the arc commutation at reduced contact opening velocity down to 1 m/s from a moving contact and in particular the point at which the arc root moves off from the contact region To investigate the arc root motion, the Flexible Test Apparatus (FTA) is used to simulate circuit breaker operation. The Arc Imaging System (AIS) is used to record series of the arc motion. New pressure transducers and a spectrometer are used to observe the gas flow and gas composition in the contact region. This data combination provides a new insight into the arc motion. The studies have observed that the gas flow in the region of the moving contact has been shown to affect the arc root mobility. The spectral emission from the arc gases confirms the presence of electronegative species. Experimental results are presented which include the measurements of pressures in the arc chamber, arc spectrum, and arc root motion. The interrelation of gas dynamic and magnetic forces is investigated further by developing relationships between electromagnetic forces and high temperature gas dynamic flows. New semi-empirical modelling is presented in terms of electrical and thermal energy transfer between the arc and the gas flow.