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Sunday, 19 April 2015

Initiation of Arc in Electric Circuit Breakers and it’s Interruption theory

Initiation of Arc in an Electric Circuit Breaker:

The electric arc is a type of electric discharge between the contacts of the circuit breaker. Arc plays an important role in the behavior of an electric circuit breaker. A circuit breaker should be capable of extinguishing the arc without getting damaged.

As the contacts of a circuit breaker begin to separate, the voltage is appreciable and the distance of separation is very small. Therefore, a large voltage gradient occurs at the contact surface. When the voltage gradient attains a sufficiently high value (106 V/cm) electrons are dragged out of the surface causing ionization of the particles between the contacts. The emission of electrons because of the high value of voltage gradient is known as field emission.

Although this high voltage gradient exist only for a fraction of micro-seconds, but a large number of electrons are liberated from the cathode because of this. These electrons move towards the positive contact i.e. anode at a very rapid pace. On their way to anode, these electrons collide with the atoms and molecules of the gases and vapour existing between the contacts. Hence, each liberated electron tends to create other electrons. If the current is high, which is certainly in case of an electric fault, the discharge attains the form of an arc.  

The temperature of arc is high enough and causes thermal ionization. The liberation of electrons because of high temperature is called thermal emission. Thus, in an electric circuit breaker, an arc is initiated because of field emission but is maintained due to thermal ionization.

Arc Extinction in Circuit Breakers:

As per the above discussion, in every electric circuit breaker the arc is there to be formed. So there should be ways and methods to extinct or quench the arc. There are two methods of arc extinction. They are:
1.       High resistance method, and
2.       Low resistance or Current zero Interruption method.

High Resistance Method of Arc Extinction:

In this method, the arc resistance is increased with time to such a high value that the current is reduced to a value insufficient to maintain the arc. The rate at which the resistance is increased or the current is decreased should be such that no harmful voltages are induced in the system.

The resistance of an arc can be increased by any or all of the following de-ionization methods:
1.       Lengthening the arc by increasing the gap between the contacts.
2.    Constraining the arc; the arc is constrained to a very narrow channel. The arc resistance increases with the decrease in the cross section of the arc.
3.    Cooling the arc; the voltage required to maintain ionization increases with the decrease in temperature.
4.     Splitting the arc; the arc resistance can be increased by splitting the arc into a number of smaller arcs in series.

Now the question arises where, I mean, in which circuit breaker, this method of arc extinction is used? Since the energy dissipated in the arc is high and therefore, the high resistance method of arc extinction is employed only in low and medium power AC circuit breakers and in DC circuit breakers.

Low Resistance or Current Zero Method of Arc Extinction:

In AC circuits there is a natural zero of current present in the system. This property of AC circuits is used for the interruption purpose and the current is not allowed to rise again after a zero occurs. It is undesirable to cut-off the current at any other point, other than natural zero, because this may induce a high voltage in the system. 

The current zero method of circuit interruption is adopted only in AC circuits and is employed in all modern AC circuit breakers.
The two main theories explaining current zero interruption are:
1.       Energy Balance or Cassie theory: According to this theory, the interruption or re-establishment of the arc is an energy balance process. If the rate at which the heat is generated between the breaker contacts is lower than the rate at which heat is dissipated the arc will be extinguished or else it will re-strike. The amount of heat generated is variable and depends on the separation of contacts. Initially when the contacts are about to open the heat generated is zero as the re-striking voltage is zero. The heat generated is again zero when the contacts are fully open as the space between the breaker contacts has become de-ionized and hence the resistance is very high. The heat generation rises to a maximum in between the two states of the breaker contacts. This theory is also known as Cassie theory.

2.       Slepian theory: Current zero is the stage where the degree of ionization is minimum. The ionization at current zero depends upon the voltage appearing across the breaker contacts. This voltage is known as re-striking voltage. If at this current zero, the electrons and ions between the contact space can be removed either by recombining them with neutral molecules or by sweeping them away by forcing the insulation at a faster rate than the rate of ionization, the arc will be interrupted. The recombination can be accelerated by cooling and by increasing the pressure in the arc space. This theory was given by Dr. J. Slepian and therefore called the Slepian theory.