Low Voltage circuit-breaker breaking techniques - Schneider Electric

3 Using the arc to break the currentThe current established on closing can becalculated by the extended Ohm's law:e - R i - L di/dt = 0.Following a transient closing state, the currentbecomes stable or in steady state I = e R .On the basis of such a law, the current can onlybe cancelled permanently if the voltage "e"becomes zero or if R becomes infinite.In preference to these two extremes which wouldpresent too many operational restrictions, an arcwas introduced in the circuit for use of its Uavoltage properties and extinguishing conditions.As soon as the circuit opens, the equationbecomes:e - R i - L di/dt - u a= 0.The current will thus be forced to zero or willpass through zero, and the arc extinguishingconditions will break the current.The examination of the two different cases belowprovides a progressive approach to the breakingtheory:c the U a arcing voltage was introduced in thecircuit when the current was in steady state(see chapter 4);c the U a arcing voltage was introduced inthe circuit before the current reached thestabilized value of the prospective current (seechapter 5).Cahier Technique Schneider Electric no. 154 / p.9