Know about the phenomena of arc in Circuit Braker

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Introduction

The phenomenon of arc is an important one to take into account in electrical engineering, particularly when talking about circuit breakers. Arc can happen when there is a break in the electrical current and they can seriously affect the operational efficiency and safety of the equipment. Let's examine arc phenomena in circuit breakers in more detail.

What is an Arc?

  • Arc is an essential part of any circuit breaker operation where contact is physically parted. The study of arc and its behavior is necessary for switchgear designers. Here we will explores the various types of arcs,their properties and behavior so as to facilitate an understanding of the complexities of electrical circuit breaking. The basic design of circuit breaker interrupter essentially depends upon how the extinction of arc is managed.
  • The heart of an ac circuit breaker is a switching element having a variable resistance. This variable resistance is a high-pressure arc burning in the corresponding gas in various circuit breaker mediums such as air, oil and SF6. Whereas in vacuum circuit breakers, it is a vacuum arc burning in electrode vapor. In certain other switching devices, which include high-voltage dc converters, is either a low-pressure mercury arc or a 'discharge' in the crystal lattice of a solid- state device.
  • The physics of discharges in circuit breakers is understood only qualitatively. Many of the processes occurring during this phenomenon are too complex to be subjected to analytical treatment. Since our knowledge of gas properties, particularly at high temperatures, is not very accurate,there are several uncertainties. Nevertheless, progress in circuit breaker-arc physics has been considerably rapid in the last 10-15 years, owing mainly to advances in aerodynamics, and computational and experimental techniques.
The study of arc phenomena, though very complex, is of immense importance for understanding the design and operational characteristics of circuit breakers. In this article, an attempt has been made to discuss some of the physical processes, which are vital to an understanding of the phenomena of
electric arcs.

Role of arc in current interruption

  • When two current-carrying contacts open, an arc bridges the contact gap and prevents an abrupt interruption of the current. This arc is useful in a way as it provides a low resistance path for the current after contact separation, thereby preventing current chopping and associated abnormal switching over-voltages in the system.
  • In the case of alternating current (ac), arc is momentarily extinguished at every current zero. To make the interruption complete and successful, re-ignition of the arc between the contacts has to be prevented after a current zero.
  • It is thus evident that the arc plays an important role in the process of current interruption and therefore must not be regarded as an undesirable phenomenon. It must also be realized that, in the absence of the arc, the current flow would be interrupted instantaneously, and due to the rate of collapse of the associated magnetic field, very high voltage would be induced which would severely stress the insulation of the system.
  • On the other hand, the arc provides a gradual, but quick, transition from the current-carrying to the current-breaking states of the contacts. It thus permits the disconnection to take place at zero current without inducing the potentials of dangerous values. The function of an arc-control device in a circuit breaker is therefore clearly to employ the beneficent action of the arc as efficiently as possible.

Arc interruption Theories

  • In the early 1930s, Slepian put forward the concept of a 'race' between the growing dielectric strength of an arc gap after arc extinction and the re-striking voltage of the circuit. However, it was realized later that the two quantities were not independent of each other, which led Cassie in 1939 to put forward the first energy-balance theory of arc extinction, and Slepian and Browne in 1941 to suggest that turbulence in the gas flow was the means by which the rapid dissolution of the arc was obtained.
  • At this time, a considerable amount of work, both experimental and theoretical,commenced around the world, on the manner in which the resistance of the arc changed during the current zero period.
  • In 1939,Cassie proposed a model of arc in which the arc was assumed to have a cylindrical column with a uniform temperature and current density so that its area varies to accommodate changes in the current.The power dissipation was assumed to be proportional to the column cross-section.This model was intended to represent an air-blast arc and was represented by the following differential equation;
`frac{Rd}{dt}left(frac1Rright)=frac1theta{left(frac v{v_0}right)^2-1}`
where R is the arc resistance, V, the arc voltage at any instant, `V_0`, the arc voltage in steady state, and 𝛳 the arc time constant (i.e. the ratio of energy stored per unit volume to the energy loss rate per unit volume).
  • In the steady state, Cassie's equation leads to a constant voltage characteristic V = `V_0` which is qualitatively typical of the heavy circuit regime of circuit breaker arcs.
  • A few years later, in 1943, Mayr proposed a somewhat improved model, in which the arc was assumed to be of a fixed diameter but of varying temperature and conductivity, and the power loss occurred from the surface of the arc only. This model was described by the differential equation:
`frac{Rd}{dt}left(frac1Rright)=frac1theta{left(frac {v_i}{w_0}right)^2-1}`
where i is the arc current at any instant and wo is the energy loss from periphery of arc at steady state.
  • It has been found that Cassie's model best describes the period before current zero whereas Mayr's model represents better the post-arc regime. In 1948, Browne developed a model of the arc that combined both these models. Many other models have also been proposed since then, but none of them relates the terms of the equation numerically with the physical properties of the interrupting medium. An exception, however, is the theory evolved by Butler and Whittaker in 1972.
  • The fact that energy is stored in the arc column means that the conductance will fall to zero only sometime after the current zero and some 'post-zero' current will flow. If immediately after current zero, the rate of rise of recovery voltage is greater than a critical value, the decaying arc channel is re established by ohmic heating. In this period, which is controlled by the energy balance, thermal failure can therefore occur. After a successful thermal interruption, the re-striking voltage can reach such a high peak value that the gap fails through dielectric breakdown.
  • This is called dielectric failure in the peak regime of re-striking voltage. A typical feature of this failure mode is that the voltage decay occurs so fast that it cannot be resolved on an oscillogram while the voltage in the thermal failure mode decreases and approaches the arc voltage during a time interval lasting for several micro-seconds. The nature of voltage variation with time under the two failure modes is illustrated in Fig. 1.
Figure 1: The nature of voltage variation with time under two failure modes

Arcing Process

  • Arcs in ac circuit breakers occur in two ways. When the current-carrying contacts are being separated, arcing is possible even when the circuit e.m.f. is considerably below the minimum cold electrode breakdown voltage, because of the ions neutralizing the electronic space charge and thus allowing large currents to flow at relatively low voltage gradients.
  • This way of occurrence of an arc is common to both dc and ac circuit breakers. Another way of occurrence of arc happens only in ac circuit breakers. In this case, the arc is extinguished every time the current passes through zero and can re-strike only if the transient recovery voltage across the electrodes,already separated and continuing to separate, reaches a sufficiently high value causing breakdown.
  • The function of an ac circuit breaker is to prevent re-striking of the arc, which depends upon the following important factors:
  1. The nature and pressure of the medium of arc
  2. The external ionising and de-ionising agents present
  3. The voltage across the electrodes and its variation with time
  4. The material and configuration of the electrodes
  5. The nature and configuration of the arcing chamber
  • To the physicist the arc appears in many forms and is a subject of great scientific interest.To the switchgear designer the arc all too often appears to have only one objective viz.self- preservation.The power arcs try to evade de-ionization systems by striking in new and unexpected directions and by re-igniting after they appear to have been successfully interrupted if the design has any weakness.
  • Thus, while the physicist would be interested in studying the arc plasma properties, including particle densities, temperatures and distributions, and electrical conductivity, the switchgear designer would be more concerned with the insulating and dielectric recovery properties of the arcing medium so that he can use them to interrupt as high a current and voltage as possible.

Conclusion

The occurrence of arcs in circuit breakers demonstrates the complex interactions required to safely and effectively interrupt electrical currents. Knowing how arcs are created, how they act, and how they stop is important for developing and running dependable electrical systems. By integrating advanced arc control techniques and technologies, engineers can boost the effectiveness, security, and durability of circuit breakers in various industries and applications.

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