Low Voltage circuit-breaker breaking techniques - Schneider Electric

6 The low voltage circuit-breakerA circuit-breaker (see fig. 22 ) is a connectiondevice able to close and break a circuitirrespective of current up to its ultimatebreaking capacity: Icu (refer to standard IEC60947-2).Although its main function is to break shortcircuitand overload currents by self-energized«reflex» action, it also breaks «normal» currentsand overload currents by voluntary action fromexternal sources.Moreover, after opening, it provides voltageinsulation of the broken circuit.The circuit-breaker's design enabling it tohouse all these functions in the same casehas led to the adoption of specific solutionsregarding:c closing/opening mechanisms;c trip units;c pole circuits ;c breaking elements (contacts, arc chutes...).The purpose of this chapter is to analyse itsfunctions, technologies and performances.Fig. 22: cross-section of an industrial 400 A LV circuit-breaker.6.1 Its functionsCircuit closingBy action on the closing mechanism, currentflows to supply the load as soon as the slightestcontact is established. When energized, someloads absorb currents far greater than ratedcurrent In (e.g. motor 7 to 8 In for a fewseconds). To prevent these overcurrentsresulting in dangerous phenomena for thecontact zone (erosion by arcs), closing must beprompt, especially for values ≥ 100A.Cahier Technique Schneider Electric no. 154 / p.19