Electrical circuit breaker is a switching device which can be operated manually and automatically for controlling and protecting of electrical power system from damage caused by overcurrent, typically resulting from an overload or short circuit respectively. As the modern power system deals with huge currents, the special attention should be given during designing of circuit breaker for safe interruption of arc produced during the operation of circuit breaker. Circuit breaker can be reset (either manually or automatically) to resume normal operation.
Main three objectives of a circuit breaker in electrical power management system:
(a) In normal condition it works like a switch for connecting or disconnecting with the live circuit manually or remote control.
(b) In abnormal situation, isolate a faulty section of electrical power system from rest of the live system automatically.
(c) After clearing the fault circuit will be established again by manually or automatically.
The circuit breaker mainly consists of fixed contacts and moving contacts. In normal "ON" condition of circuit breaker, these two contacts are physically connected to each other due to applied mechanical pressure on the moving contacts.
Figure shows that the circuit consists of trip coil, battery, relay coil current transformer and circuit breaker. Under normal operating conditions, the contacts remain closed and the circuit breaker carries the load current. The protective relay is connected in the secondary of a current transformer. When fault occurs on a system heavy current flows through the current transformer which increase the secondary current. This current when flows through relay coil, it closes the trip coil circuit.
According different criteria there are different types of circuit breaker. According to their arc quenching media the circuit breaker can be divided as-
According to their arc quenching media the circuit breaker can be divided as-
According to their services the circuit breaker can be divided as-
According to the operating mechanism of circuit breaker they can be divided as-
According to the voltage level of installation types of circuit breaker are referred as-
When a short-circuit occurs, a heavy current flows through the contacts of the circuit breaker before they are opened by the protective system. 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 vapor acts as conductor and an arc is struck between the contacts. The p.d. 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.
During the arcing period, the current flowing between the contacts depends upon the arc resistance. The greater the arc resistance, the smaller the current that flows between the contacts. The arc resistance depends upon the following factors :
(1) Degree of ionization— the arc resistance increases with the decrease in the number of ionized particles between the contacts.
(2) Length of the arc— the arc resistance increases with the length of the arc i.e., separation of contacts.
(3) Cross-section of arc— the arc resistance increases with the decrease in area of X-section of the arc.
There are two methods of extinguishing the arc in circuit breakers viz.
a. High resistance method. In this method, arc resistance is made to increase with time so that current is reduced to a value insufficient to maintain the arc. Consequently, the current is interrupted or the arc is extinguished. The principal disadvantage of this method is that enormous energy is dissipated in the arc. Therefore, it is employed only in d.c. circuit breakers and low-capacity a.c. circuit breakers.
The resistance of the arc may be increased by:
(1) Lengthening the arc. The resistance of the arc is directly proportional to its length. The length of the arc can be increased by increasing the gap between contacts.
(2) Cooling the arc. Cooling helps in the deionization of the medium between the contacts. This increases the arc resistance. Efficient cooling may be obtained by a gas blast directed along the arc.
(3) Reducing X-section of the arc. If the area of X-section of the arc is reduced, the voltage necessary to maintain the arc is increased. In other words, the resistance of the arc path is increased. The cross-section of the arc can be reduced by letting the arc pass through a narrow opening or by having smaller area of contacts.
(4) Splitting the arc. The resistance of the arc can be increased by splitting the arc into a number of smaller arcs in series. Each one of these arcs experiences the effect of lengthening and cooling. The arc may be split by introducing some conducting plates between the contacts.
b. Low resistance or Current zero method. This method is employed for arc extinction in a.c. circuits only. In this method, arc resistance is kept low until current is zero where the arc extinguishes naturally and is prevented from re-striking in-spite of the rising voltage across the contacts. All modern high power a.c. circuit breakers employ this method for arc extinction. In an a.c. system, current drops to zero after every half-cycle. At every current zero, the arc extinguishes for a brief moment. Now the medium between the contacts contains ions and electrons so that it has small dielectric strength and can be easily broken down by the rising contact voltage known as re-striking voltage. If such a breakdown does occur, the arc will persist for another half- cycle. If immediately after current zero, the dielectric strength of the medium between contacts is built up more rapidly than the voltage across the contacts, the arc fails to restrike and the current will be interrupted. The rapid increase of dielectric strength of the medium near current zero can be achieved by: