Low Voltage circuit-breaker breaking techniques - Schneider Electric

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2 The electric arcThe electric arc is no invention, but appeared tothe first physicist who tried to break a circuitthrough which a current flowed.Circuits, always inductive, supply electrons withsufficient energy to cross the distance in theconductor separation zone.The gas present, normally air, is ionised bythese "pioneer" electrons and the resultingplasma will then facilitate current flow.Breaking thus seems somewhat compromised....unless a better understanding of thisphenomenon were to reveal remarkable andeven irreplaceable properties.Luckily this is the case !2.1 Its formation conditionsThe arc appears in gaseous atmospheres:c by dielectric breakdown between twoelectrodes:v beyond an electric field value E/d, dependenton electrode shape and on gas type and density(d = distance between electrodes ( see fig. 2 );v further to moving over insulating materials inthe ambient gas.c as soon as an electric circuit opens throughwhich current flows, even if the circuit is purelyresistive, a certain opening distance is necessaryto prevent dielectric breakdown.EFig. 2 : electric arc and influence of field E/d value.Moreover, any attempt to reduce current rapidlycreates a high L di/dt thus favouring breakdownat any current level.d2.2 Its physical properties (see fig. 3a)As soon as two contacts separate, one of them(cathode) transmits electrons and the other one(anode) receives them. Since electronicemission is by its very nature energy generating,the cathode will be hot. With the arc foot thusbecoming thermoemissive, the electrons aremostly emitted at the hot spot, resulting in arcstagnation which can give rise to metallicvapours. These vapours and the ambient gaswill then be ionised, hence:c more free electrons ;c the creation of positive ions which drop backon the cathode, thus maintaining its hightemperature;c the creation of negative ions which bombardthe anode causing temperature to rise.All this occurs in a high temperature plasmacolumn, from 4 000 to 20 000 K according tocolumn current and confinement.2.3 Its electrical properties (see fig. 3b)c its most striking property is the appearance ofan arcing voltage, which has:v a fixed part, U AC ≈ 20 to 40 V which appearson the slightest separation of the contacts(depending on the materials used),v a variable part, U L = 50 to 100 V/cm, when thearc is stabilized in elongation in pressuretemperaturebalanced conditions.i.e. a total value U a = U AC+ U L.Note that:- the sign of Ua changes at the same time as thearcing current sign,- the arcing current value does not have any realeffect on arcing voltage. This is because the arc"works" with a virtually constant current density(j=i/s), (the anode and cathode spot cross-Cahier Technique Schneider Electric no. 154 / p.6
a: composition of the arc columnb1: arc voltage U AC b2: arc voltage U LAnodeAnodeAnodee←ion -U ACion +e←e←e←e←Nion +CathodeU LCathodeUa = U AC +U LCathodeFig. 3: the electric arc, its physical phenomenon (a) and its characteristic or arcing voltage Ua (b).sections and the arc column cross-section areproportional to the current, hence by analogywith a resistance:U= R i= ρl = ρ.. lj="constant");sc an arcing energy is produced;Wa =∫u a i a dtc if the arc is placed in a magnetic field, it issubjected to Laplace's law forces,F = B i l sin α which causes it to bend if E isperpendicular to i , and then to movetransversely.2.4 Its extinguishing conditionsThe arc is extinguished when the arcing currentvalue becomes and remains zero.Thermal aspectWhen the arcing current is low or drops, below10 A for example, the heat energy exchangesmay exceed internal arcing energy, causing thearc to «die» of cold. This results in an increase inarcing voltage (see fig. 4a).During this voltage rise, the arc may evensuddenly be extinguished if it is "short-circuited"by stray capacities. This is the case when arcingvoltage becomes and remains greater than thecharging voltage of the distributed capacities(see fig. 4b). This phenomenon is known as"pinching off".However there are exceptions:c if the arcing current stabilises against aninsulating wall, its heat exchange areadecreases, and the insulating materialcomponents, locally very hot, may promote arcconduction and holding.c if the arcing current is high, the arc column isextremely exothermal and only the jointevolutions in arcing voltage and mains voltagecan reduce and finally cancel this current.a: extinction of the arcarc grèlei,uUdUaiUrai mi mb: stray capacitors "short-circuit"the arcEL,ri ci aFig. 4 : extinguishing of an electric arc by "pinching off".CtCahier Technique Schneider Electric no. 154 / p.7
Page 3 and 4: no. 154LV circuit-breakerbreaking t
Page 5 and 6: LV circuit-breaker breaking techniq
Page 7: high, but ceases to be of any help
Page 11 and 12: 3 Using the arc to break the curren
Page 13 and 14: c if U a = E then Wa/W L0 = 2 only!
Page 15 and 16: 5 Breaking prospective currents (wi
Page 17 and 18: 5.2 Breaking with limitationThe exp
Page 19 and 20: 5.5 In a three-phase AC systemTwo c
Page 21 and 22: 6 The low voltage circuit-breakerA
Page 23 and 24: ⇒The contactsLV circuit-breaker c
Page 25 and 26: ⇒Arcing voltage when splitting oc
Page 27 and 28: ,,,-./012345:;c the former, reinfor
Page 29: 7 ConclusionThe future of the arc?T

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