الأربعاء، 2 فبراير 2011

Circuit Breaker Operating Principle

 Circuit Breaker Operating Principle

The primary function of the circuit breakers mechanism is to provide the means for opening and closing the contacts. Initially, this seems to be a rather simple and straightforward requirement. However, when one considers the fact that most circuit breakers, once placed into service, will remain in the closed position for long periods of time, and yet on the few occasions when they are called upon to open or close, they must do so reliably, without any delay or sluggishness, then one realizes that the demands on the mechanisms are not as simple as was first thought.
A circuit breaker essentially consists of fixed and moving contacts. These contacts are called electrodes. The need for carrying the continuous current and for withstanding a period of arcing makes it necessary to use two sets of contacts in parallel, one is the primary contact and the second is the arcing contact. The primary contact is always made of a high conductive material such as copper and the arcing contact is made of arc resistance material such as tungsten or molybdenum, which has a much lower conductivity than those used for primary contacts. When the circuit breaker opens to interrupt the current, the primary contacts open before the arcing contacts.

Under the normal operating conditions, these contacts remain closed and are not open automatically until and unless the system becomes faulty. Of course, the contacts can be opened manually or by remote control when ever desired. When a fault occurs on any part of the system, the trip coils of the circuit breaker get energized and the moving contacts are pulled apart by some mechanism, thus opening the circuit.

When the contacts of a circuit break are separated under fault conditions, an arc is struck between them. The current is thus able to continue until the discharge ceases. The production of the arc not only delays the current interruption process but it also generates enormous heat which may cause damage to the system or to the circuit breaker itself. Therefore, the main problem in the circuit breaker is to extinguish the arc within the shortest possible time so that heat generated by it may not reach a dangerous value.

In single phase (1-phase) circuits (i.e., lighting circuits etc.), a switch is located in only one of the two conductors to the load. However in the power circuits, a circuit interrupting device (i.e., circuit breaker) is put in each phase or conductor. These are, sometimes, called 3-pole circuit breakers.

Arc Phenomenon

Arc in an ac circuit breaker occurs in two ways:

1- 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.

2- The other way of occurrence of an arc happens only in ac circuit breakers. In such case, the arc is extinguished every time the current passes through zero and can restrike 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 restriking of the arc, which depends upon the following important factors:
- The nature and pressure of the medium of arc
- The external ionizing and de-ionizing agents present
- The voltage across the electrodes and its variation with time
- The material and configuration of the electrodes
- The nature and configuration of the arcing chamber

Categories of Arcs :

Arcs in the circuit breakers are categorized as: 
a) High-pressure arcs: with ambient pressures of 1 atm and above
b) Vacuum arcs: with ambient pressures below 10-4 torr

Arc is an essential part of any circuit breaker operation where contact is physically parted. Before they are opened by the protective system, a heavy current flows through the contacts of circuit breaker, when a short circuit occurs.
When two current carrying contacts open, an arc bridges the contact gap and prevents an abrupt interruption of the current. At the instant when the contacts begin to separate, the contact area decreases rapidly and large fault current causes increased current density and hence rise in temperature. The heat produced in the medium
between contacts (usually the medium is oil or air) is sufficient to ionize the air or vaporize and ionize the oil. The ionized air or vapors act as conductor and an arc is struck between the contacts. The potential difference between the contacts is quite small and is just sufficient to maintain the arc. The arc provides a low resistance path and consequently the current in the circuit remains uninterrupted so long as the arc persists.

The arc is useful in a way as it provides a low resistance path for the current after contact separation. It prevents current chopping and associated abnormal switching over-voltages in the system. 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 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. Therefore, it permits the disconnection to take place at zero current without indicating 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.

Significance of Arc Resistance :

During the arcing period, the current flowing between the contacts depends upon the arc resistance. The greater the arc resistance, the smaller is the current that flows between the contacts. The arc resistance depends upon the following factors:
- Degree of Ionization
- Length of the Arc
- Cross-section of the Arc
a) Degree of Ionization
The arc resistance increases with the decrease in the number of ionized particles between the contacts.
b) Length of the Arc
The arc resistance increases with the length of the arc i.e., separation of contacts.
c) Cross-section of the Arc
The arc resistance increases with the decrease in the area of cross-section of the arc.

Principle of Arc Extinction :
Prior to discussing the methods of arc extinction, it is essential to scrutinize the factors accountable for the maintenance of arc between the contacts. These are:
- Potential difference between the contacts
- Ionized particles between the contacts
i) Potential Difference between the Contacts
When the contacts have small separation, the potential difference between them is sufficient to maintain the arc. One way to extinguish the arc is to separate the contacts to such a distance that potential difference becomes inadequate to maintain the arc. However this method is impracticable in high voltage systems where a separation of many meters may be required.
ii) Ionized Particles between the Contacts
The ionized particles between the contacts tend to maintain the arc. If the arc path is de-ionized, the arc extinction will be facilitated. This may be achieved by cooling the arc or bodily removing the ionized particles from the space between the contacts.

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