Circuit Breaker

Circuit Breaker

Introduction :

                            A circuit breaker is an automatically operated electrical switch designed to protect an electrical circuit  from damage caused by excess current from an overload or short circuit. Its basic function is to interrupt current flow after a fault is detected. Unlike a fuse, which operates once and then must be replaced, a circuit breaker can be reset (either manually or automatically) to resume normal operation.

Origins :

                           Circuit breakers are made in varying sizes, from small devices that protect low-current circuits or individual household appliance, up to large switchgear designed to protect high voltage circuits feeding an entire city. The generic function of a circuit breaker,RCD or a fuse, as an automatic means of removing power from a faulty system is often abbreviated as OCPD (Over Current Protection Device). Its purpose was to protect lighting circuit wiring from accidental short circuits and overloads.Interconnection of multiple generator sources into an electrical grid required development of circuit breakers with increasing voltage ratings and increased ability to safely interrupt the increasing short-circuit currents produced by networks. 

Operations :

                                     All circuit breaker systems have common features in their operation, but details vary substantially depending on the voltage class, current rating and type of the circuit breaker.

                             The circuit breaker must first detect a fault condition. In small mains and low voltage circuit breakers, this is usually done within the device itself. Typically, the heating or magnetic effects of electric current are employed. Circuit breakers for large currents or high voltages are usually arranged with protective relay pilot devices to sense a fault condition and to operate the opening mechanism. These typically require a separate power source, such as a battery, although some high-voltage circuit breakers are self-contained with current transformer, protective relays, and an internal control power source.

                   Contacts are made of copper or copper alloys, silver alloys and other highly conductive materials. Service life of the contacts is limited by the erosion of contact material due to arcing while interrupting the current. Miniature and molded-case circuit breakers are usually discarded when the contacts have worn, but power circuit breakers and high-voltage circuit breakers have replaceable contacts.

Standard current ratings :

Circuit breakers are manufactured in standard sizes, using a system of preferred numbers to cover a range of ratings. Miniature circuit breakers have a fixed trip setting; changing the operating current value requires changing the whole circuit breaker. Larger circuit breakers can have adjustable trip settings, allowing standardized elements to be applied but with a setting intended to improve protection. For example, a circuit breaker with a 400 ampere "frame size" might have its overcurrent detection set to operate at only 300 amperes, to protect a feeder cable.

 The commonly available preferred values for the rated current are1A,2A,4A, 6 A, 10 A, 13 A, 16 A, 20 A, 25 A, 32 A, 40 A, 50 A, 63 A, 80 A, 100 A, and 125 A. The circuit breaker is labeled with the rated current in amperes, but excluding the unit symbol, A. Instead, the ampere figure is preceded by a letter, B, C, or D, which indicates the instantaneous tripping current — that is, the minimum value of current that causes the circuit breaker to trip without intentional time delay (i.e., in less than 100 ms), expressed in terms of I_n.

Type	Instantaneous tripping current
B	Above 3 In
C	Above 5 In up to and including 10 In
D	Above 10 In up to and including 20 In
K	Above 8 In up to and including 12 In For the protection of loads that cause frequent short duration (approximately 400 ms to 2 s) current peaks in normal operation.
Z	Above 2 In up to and including 3 In for periods in the order of tens of seconds. For the protection of loads such as semiconductor devices or measuring circuits using current transformers.

 Types of circuit breaker :        

                         Many classifications of circuit breakers can be made, based on their features such as voltage class, construction type, interrupting type, and structural features.

• Low-voltage circuit breakerss
• Magnetic circuit breakers
• Thermal magnetic circuit breakers
• Magnetic-hydraulic circuit breakers
• Common trip breakers
• Medium-voltage circuit breakers
• High-voltage circuit breakers
• Sulfur hexafluoride (SF₆) high-voltage circuit breakers
• Disconnecting circuit breaker (DCB)
• Carbon dioxide (CO₂) high-voltage circuit breakers