Hello learners. Welcome to the topic on Introduction to Circuit Breakers. At the end of this topic you should be able to : explain the operation of circuit breakers, describe mediums and methods used for arc extinction in circuit breakers. Circuit breakers are mechanical devices that are used to make and break electrical circuits in both normal and abnormal circumstances, such as short circuits. For the most part, circuit breakers automatically disrupt the flow of a current and isolate the faulty parts of a circuit. Capacitive currents, small inductive currents, and loads must also be able to be interrupted by a circuit breaker at the system voltage. Now let us look at how circuit breaker operates in a circuit. A fixed contact, a sliding contact, and a moving contact make up the circuit breaker. As the moving contact approaches the sliding contact, it will be drawn in the sliding contact. A handle is attached to the moving contact, it can be automatically operated using a mechanism that uses the secondary current of the current transformer CT to energize a trip coil. The trip coil cannot be fully energized by the secondary CT coil current under normal working conditions. At fault conditions, the main of CT has an exceptionally high current flow, so the secondary circuits trip coil will be energized, releasing the circuit breakers handle mechanism as a result of the secondary circuits, high secondary current. The fault circuit is isolated from the rest of the system by a circuit breaker. When the trip circuit is turned on, a delay of a few milliseconds occurs before the opening mechanism may be released. Now, let us discuss the opening time, and arcing time of the breaker. When the trip circuit is turned on, a delay of a few milliseconds occurs before the opening mechanism may be released. The opening time of the circuit breaker is defined as the time between the trip coils energization and the point of contact separation. Moving and fixed contacts generate an arc that last for a long time after they separate. The arcing time is the amount of time it takes for the arc to disappear once the two points of contact separate. The sum of the opening and arcing times is the overall break time. As shown, a fault can occur at the load end. A dead short-circuit seems to be at fault. At this point, the breaking mechanism is activated by an external signal and the breaking moving contact begins to move away from the breakers fixation point. Between the two points an arc is created. As long as the arc exists, the fault current is supplied through it. The fault resistance of a dead short circuit is believed to be zero. Consequently, the arc remains even if the contacts are drawn closer together. Even though the current interruption processes is delayed due to this arc, it also causes significant heat, which may cause damage to the circuit breaker itself. The fundamental goal of a circuit breaker is to extinguish the arc as quickly as possible so that the heat created by it does not reach a dangerous level. A circuit breaker's basic design necessitates the separation of contacts in an insulating medium. When the circuit breaker opens, this insulating material extinguishes the arc between the contacts. It also functions as a barrier between contacts and between each contact and the ground. Oil circuit breaker, vacuum circuit breaker, SF_6 circuit breaker and air blast circuit breakers are some of the examples of circuit breaker. Now let us discuss about the methods of arc extinction. There are two methods for arc extinction in circuit breakers, high resistance method and low resistance or current zero method. First, let us have a look at the high resistance method. In this method, the arcs effective resistance increases over time. As a consequence, the arc current has dropped to extremely low levels and the arc can no longer be maintained. As a result, the current flow is disrupted and the arc is extinguished. To improve the arcs resistance, a variety of tactics can be used including cooling the arc, lengthening the arc and reducing the cross section of the arc. In DC circuit breakers, and air break type AC circuit breakers with limited capacities, the high resistance method of arc extinction is frequently used. Now, let us have a look at the low resistance or current zero interruption method. This method is only applicable to AC circuit breakers because the current in an AC system is zero after each half cycle. As soon as the current drops to zero, the arc is allowed to naturally extinguish, and this method prevents it from reigniting. All modern high-power AC circuit breakers employ this method for arc extinction. DC arcs are more difficult to disrupt than AC arcs because they lack a current zero instant. Assume that a defect occurs at the load end. The forward voltage is considered to be zero due to a dead short circuit. As a result, the voltage across the capacitor is nearly equal to the voltage differential between the breakers poles, which is zero, V_b is equal to 0. Figure 2 depicts the wave morphologies of the system voltage, current and voltage across the breaker polls while at up to point O. At current zero, the arc has died out, point O in the figure. The arc is extinguished just as the current reaches zero, while the supply voltage is at its highest point. The voltage between the breakers poles is the voltage across the capacitance C, which should be obvious. As a result, the voltage across the breaker poles is the same as the voltage across the capacitance, which is zero, just at the moment of arc extinction point O. The circuit is depicted in figure with the direction of current shown for the new state, which is after the point of arc extinction, that is after point O in figure C. The voltage across the capacitance, which is the same as the voltage across the breaker terminals, attempts to reach the system voltage by flowing current wire, the inductance and capacitance in this new configuration, neglecting the resistance of line. Because of the development of a series LC circuit, the voltage across the capacitance, which is the voltage across the breaker poles, approaches the system voltage in an oscillating fashion. Now, let us have a look at the ideal features of a circuit breaker. A circuit breaker must have the following features. In the closed position, it must be a good conductor. In the open position, it must behave as a good isolator between systems. It must be able to change from the closed to open position in a very short period of time typically in less than 0.1 second. During switching, it does not cause over voltage. It is reliable in its operation. Let us summarize all that you have learned. Operation of circuit breaker, arc extinction mediums and methods in circuit breakers.
