Document v1G6ZpLQw2yozKL4xXpbK0EyR

SEVERAL METHODS USED IN CIRCUIT BREAKERS TO INTERRUPT FLOW OF FAULT CURRENT Oscillogram shows Interruption within ft cycle of . 6450-omp short circuit. Breoker rated SO amp, 120 v 3Stotlonory contoct cells around Insu* loted Iron core thot connects steel plotes (one shown) to form a U-shaped blowout coll 4 0ion ore quencher has o series of metol plates which break up the ore 5 Life of breaker contacts Is Increased through use of the deion are chamber CIRCUIT BREAKERS Incondescerit load draws o heavy current momentarily The circuit breaker Is a device from a remote point. Rliere time of, destined to loterrupt abnormal circuit Interruption must be reduced] current (low In a circuit. It dif fers from the fuee In that it re quires too parte to: (l) detect excessive current and (2) open the main circuit. Here-we will discuss principles of circuit Interruption as veil as types of Interrupters. to a minimum, a reclosing relay'] allova the breaker to close as soon, as ths fault is cleared. Circuit breakers are available with time iag to provide for start log surges, as In Pig. As la' motor starting, large surge currents] flov shop Incandescent loads are] applied. Pig. e shove the momentary, ' Circuit-Breaker Principles. The circuit breaker usually servos a dual function, actlni both as sirltcb and ovoreurrent device. When the breaker trips no replacenent of parts Is necessary, as It is with a fuse, so tloe and cost of circuit Interruption'are reduced. Poraerly circuit breakers sere used mainly in high-current circuits (over 600 asp). Of late they have current that floss then a cold] filament load Is applied to a circuit. Note the Initial surge* current due to tbe loo resistance; of the cold lamp filaments. Ae tht lamps beat, resistance of the fila ments Increases, reducing vlthln ! fev cycles tbe curreot flowing U, the circuit. Pig. 7 also shoos var-j latlon of the circuit breaker trip-' ping characteristic with time, shed also found side application in the used with an Incandescent load. Aij lov-eurrent field. The aajorlty of current decreases, the tripping such breakers have a fault-detect point of the circuit breaker is ing element that operates.on tbe lowered, assuring full circuit thermal principle. Many others protection. combine a thermal and magnetic element for more selective opera tion. Pig. 11. The magnetic princi ple alone os a fault deteotor is used In a limited number of circuit breakers In ths low-current class, Pig. 3. Construction and character istics of thermal and magnetic dotectors sill be described later. When most los-current circuit interrupting tbe Arc. A circuit breaker ' oust break the primary circuit and also extinguish the art that forms between the contact eur-1 faces when tbs breaker opens. Belo*<j oe outline tbe fundamentals of clr-' cult interruption using &n air breaker, In addition to a few comooi breakers trip on overload, the methods of breaking the arc. operating toggle goes to midposition. To reset, simply return 2Mony breakers use ploln breok shown tbe toggle to `off * position then above to Interrupt the current flow. snap to *on". Contacts uSuolly ore silver since the sliver Pig. io and 12 shoe circuit Oxide that forms on the contact surface breakers in the blih-current class. Is o QOOd electrical conductor. Speed In Their fault-deteotor usually Is opening breoker reduces contoct plttlno nsgostlo. This type may be operated Direct Curreot. To Interrupt dc; successfully, the circuit breakei oust open to a point where voltagi required' to-, maintain an are .1* greater than that exlstlog'betwe*? the open contacts. Consider the load's nature when selectlnj circuit breakers for de system* (aeaodescent and resistive loads point where shortclrcult ocoure. uoy be interrupted with relative All eynchronous generatore, ease. Heavy arelog occurs when s synchronous motors and Induction aeries motor circuit la opened motors are sources of short-circuit because of the mognetle effect of current. They contribute on initial .the series field. Ths collapsing heavy current surge thot diminishes aagaetlc field Induces a voltage rapidly, Pig. 9. This is a typloal tlthlo the series field windings, eurvo of a short-circuit current ln which In turn trios to maintain one phase of a low-voltage 3-phase current la the circuit during the system. Ths curve Afi represents the period of circuit-breaker opening. dc component that displaces the Alternating Current. Let' a con total current wave from the hori sider tbe ideal ease If an ac clr- zontal zero axis. After a few cycles colt breaker Interrupts a unity this dc component of current dies poeer factor or resistor load. down and leaves only ths ae compo Circuit breoker* with Initial time log Since current and voltage are in nent. which la symmetrical to the will not trip on Incondescent surge Pbose, tbe arc Is lnterruptedwben zero axis. the voltage reaches its zero value. . Since the maximum interrupting A sufficient gap must exist between* duty of low voltage air circuit the contacts at that time to breakers is assumed to occur one- prevent re-establlshment of the arc half cycle (60-eycle basis) after *heo voltage Increases In the next cycle. Interrupting a reactive or motor lead presents' a greater problem albce voltage and current arc out of phase. Current continues to flov daring tbe tine voltage passes through its zero value. When corrent jecreasee to zero, a voltage exists oetoeen the open contacts, resulting ln r'fstoblicblng .the arc. The loe necessary to Interrupt a re vive load depends mainly on the Peed at which the contacts separate the fault occurs, effect of the dc component on the short-circuit current la considered ln design of circuit breakers for blgh-current . applications. Plain Break. The smaller circuit breaker, often uses a simple break arrangement to Interrupt tbe primary circuit. Successive steps ln inter rupting current flov are Illustrated ln Pig. 2. phen circuit breaker contacta open in air. on oxide of the contact metal la formed on the contact * necessary distance to prevent "Urting the ere. surfaces, after on electrical arc Is established. Silver contacts are At .rt Clrcalt- A Short circuit generally used since sliver oxide hriA PlBt ln a system causes on vernal current to flow. The value clreM*..current ln an ac i--* * depends upon tbe eub-tranchl reactance of tbe rotating ma th* m* ^aasded to the system ot elra-J?*^ th* lRDwdanco of the- between the source and Is a good electrical conductor. Delon Principle. For an are to exiet, tbe gas betvceo open contacts, must be in a conducting state. When the electric-current flow is inter rupted between two surfaces, air betweon tbe contacts beeomes Ionized, making it an electrical 8Whin breoker trips the main contacts shift food to contacts B'B. These in tum should open before ore It broken by carbon tips C'C. Carbon wot used to break are because of its non-pitting characteristic. Most monufocturers now use a silver olloy In place of carbon S4 (408) POWER July -1f4i July 1948