Pdf Power Circuit Breaker Theory And Design

pdf power circuit breaker theory and design

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The great Chinese, Greek and Roman civilisations all added to the new use of materials, and sculpture and architecture went hand in hand w He went to the Metropolitan-Vickers Electrical Co. During the war he was responsible for the development of a series of electrical components for aircraft, including under-carriage systems and miniature circuit breakers.

Circuit Breaker

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.

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, or fuse , as an automatic means of removing power from a faulty system is often abbreviated as OCPD Over Current Protection Device. An early form of circuit breaker was described by Thomas Edison in an patent application, although his commercial power distribution system used fuses.

Interconnection of multiple generator sources into an electrical grid required the development of circuit breakers with increasing voltage ratings and increased ability to safely interrupt the increasing short-circuit currents produced by networks. Simple air-break manual switches produced hazardous arcs when interrupting high voltages; these gave way to oil-enclosed contacts, and various forms using the directed flow of pressurized air, or of pressurized oil, to cool and interrupt the arc.

By , the specially constructed circuit breakers used at the Boulder Dam project use eight series breaks and pressurized oil flow to interrupt faults of up to 2, MVA, in three cycles of the AC power frequency.

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 transformers , protective relays , and an internal control power source. Once a fault is detected, the circuit breaker contacts must open to interrupt the circuit; this is commonly done using mechanically stored energy contained within the breaker, such as a spring or compressed air to separate the contacts.

Circuit breakers may also use the higher current caused by the fault to separate the contacts, such as thermal expansion or a magnetic field. Small circuit breakers typically have a manual control lever to switch off the load or reset a tripped breaker, while larger units use solenoids to trip the mechanism, and electric motors to restore energy to the springs.

The circuit breaker contacts must carry the load current without excessive heating, and must also withstand the heat of the arc produced when interrupting opening the circuit. 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. When a high current or voltage is interrupted, an arc is generated. The length of the arc is generally proportional to the voltage while the intensity or heat is proportional to the current.

This arc must be contained, cooled and extinguished in a controlled way, so that the gap between the contacts can again withstand the voltage in the circuit. Different circuit breakers use vacuum , air, insulating gas , or oil as the medium the arc forms in. Different techniques are used to extinguish the arc including:.

Finally, once the fault condition has been cleared, the contacts must again be closed to restore power to the interrupted circuit.

Low-voltage miniature circuit breakers MCB use air alone to extinguish the arc. These circuit breakers contain so-called arc chutes, a stack of mutually insulated parallel metal plates that divide and cool the arc. By splitting the arc into smaller arcs the arc is cooled down while the arc voltage is increased and serves as an additional impedance that limits the current through the circuit breaker. The current-carrying parts near the contacts provide easy deflection of the arc into the arc chutes by a magnetic force of a current path, although magnetic blowout coils or permanent magnets could also deflect the arc into the arc chute used on circuit breakers for higher ratings.

The number of plates in the arc chute is dependent on the short-circuit rating and nominal voltage of the circuit breaker. In larger ratings, oil circuit breakers rely upon vaporization of some of the oil to blast a jet of oil through the arc. Gas usually sulfur hexafluoride circuit breakers sometimes stretch the arc using a magnetic field, and then rely upon the dielectric strength of the sulfur hexafluoride SF 6 to quench the stretched arc.

Vacuum circuit breakers have minimal arcing as there is nothing to ionize other than the contact material. Vacuum circuit breakers are frequently used in modern medium-voltage switchgear to 38, volts. Air circuit breakers may use compressed air to blow out the arc, or alternatively, the contacts are rapidly swung into a small sealed chamber, the escaping of the displaced air thus blowing out the arc.

Circuit breakers are usually able to terminate all current very quickly: typically the arc is extinguished between 30 ms and ms after the mechanism has been tripped, depending upon age and construction of the device. The maximum current value and let-through energy determine the quality of the circuit breakers.

Circuit breakers are rated both by the normal current that they are expected to carry, and the maximum short-circuit current that they can safely interrupt.

This latter figure is the ampere interrupting capacity AIC of the breaker. Under short-circuit conditions, the calculated or measured maximum prospective short-circuit current may be many times the normal, rated current of the circuit.

When electrical contacts open to interrupt a large current, there is a tendency for an arc to form between the opened contacts, which would allow the current to continue. This condition can create conductive ionized gases and molten or vaporized metal, which can cause the further continuation of the arc, or creation of additional short circuits, potentially resulting in the explosion of the circuit breaker and the equipment that it is installed in.

Therefore, circuit breakers must incorporate various features to divide and extinguish the arc. The maximum short-circuit current that a breaker can interrupt is determined by testing. Application of a breaker in a circuit with a prospective short-circuit current higher than the breaker's interrupting capacity rating may result in failure of the breaker to safely interrupt a fault. In a worst-case scenario, the breaker may successfully interrupt the fault, only to explode when reset.

Miniature circuit breakers used to protect control circuits or small appliances may not have sufficient interrupting capacity to use at a panel board; these circuit breakers are called "supplemental circuit protectors" to distinguish them from distribution-type circuit breakers. 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 ampere "frame size" might have its overcurrent detection set to operate at only amperes, to protect a feeder cable.

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.

For the protection of loads that cause frequent short duration approximately ms to 2 s current peaks in normal operation. For the protection of loads such as semiconductor devices or measuring circuits using current transformers.

Circuit breakers are also rated by the maximum fault current that they can interrupt; this allows use of more economical devices on systems unlikely to develop the high short-circuit current found on, for example, a large commercial building distribution system. In the United States, Underwriters Laboratories UL certifies equipment ratings, called Series Ratings or "integrated equipment ratings" for circuit breaker equipment used for buildings.

Power circuit breakers and medium- and high-voltage circuit breakers used for industrial or electric power systems are designed and tested to ANSI or IEEE standards in the C37 series. 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 less than 1, V AC types are common in domestic, commercial and industrial application, and include:.

The characteristics of low-voltage circuit breakers are given by international standards such as IEC These circuit breakers are often installed in draw-out enclosures that allow removal and interchange without dismantling the switchgear. Large low-voltage molded case and power circuit breakers may have electric motor operators so they can open and close under remote control.

These may form part of an automatic transfer switch system for standby power. Low-voltage circuit breakers are also made for direct-current DC applications, such as DC for subway lines. Direct current requires special breakers because the arc is continuous—unlike an AC arc, which tends to go out on each half cycle, direct current circuit breaker has blow-out coils that generate a magnetic field that rapidly stretches the arc.

Small circuit breakers are either installed directly in equipment, or are arranged in a breaker panel. The DIN rail -mounted thermal-magnetic miniature circuit breaker is the most common style in modern domestic consumer units and commercial electrical distribution boards throughout Europe.

The design includes the following components:. Solid-state circuit breakers , also known as digital circuit breakers are a technological innovation which promises advance circuit breaker technology out of the mechanical level, into the electrical. This promises several advantages, such as cutting the circuit in fractions of microseconds, better monitoring of circuit loads and longer lifetimes.

Magnetic circuit breakers use a solenoid electromagnet whose pulling force increases with the current. Certain designs utilize electromagnetic forces in addition to those of the solenoid. The circuit breaker contacts are held closed by a latch.

As the current in the solenoid increases beyond the rating of the circuit breaker, the solenoid's pull releases the latch, which lets the contacts open by spring action. They are the most commonly used circuit breakers in the USA. Thermal magnetic circuit breakers , which are the type found in most distribution boards in Europe and countries with a similar wiring arrangements, incorporate both techniques with the electromagnet responding instantaneously to large surges in current short circuits and the bimetallic strip responding to less extreme but longer-term over-current conditions.

The thermal portion of the circuit breaker provides a time response feature, that trips the circuit breaker sooner for larger over currents but allows smaller overloads to persist for a longer time.

This allows short current spikes such as are produced when a motor or other non-resistive load is switched on.

With very large over-currents during a short circuit, the magnetic element trips the circuit breaker with no intentional additional delay. A magnetic-hydraulic circuit breaker uses a solenoid coil to provide operating force to open the contacts.

Magnetic-hydraulic breakers incorporate a hydraulic time delay feature using a viscous fluid. A spring restrains the core until the current exceeds the breaker rating. During an overload, the speed of the solenoid motion is restricted by the fluid. The delay permits brief current surges beyond normal running current for motor starting, energizing equipment, etc.

Short-circuit currents provide sufficient solenoid force to release the latch regardless of core position thus bypassing the delay feature. Ambient temperature affects the time delay but does not affect the current rating of a magnetic breaker. Large power circuit breakers, applied in circuits of more than volts, may incorporate hydraulic elements in the contact operating mechanism. Hydraulic energy may be supplied by a pump, or stored in accumulators.

These form a distinct type from oil-filled circuit breakers where oil is the arc extinguishing medium. To provide simultaneous breaking on multiple circuits from a fault on any one, circuit breakers may be made as a ganged assembly.

This is a very common requirement for 3 phase systems, where breaking may be either 3 or 4 pole solid or switched neutral. Some makers make ganging kits to allow groups of single phase breakers to be interlinked as required. In the US, where split phase supplies are common, in branch circuits with more than one live conductor, each live conductor must be protected by a breaker pole. To ensure that all live conductors are interrupted when any pole trips, a "common trip" breaker must be used.

These may either contain two or three tripping mechanisms within one case, or for small breakers, may externally tie the poles together via their operating handles.

Circuit breaker

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Feb 12, - Power Circuit Breaker Theory and Design PDF By:C. H. Flurscheim,Charles H. Flurscheim Published on by IET The book has 13 chapters.


Power circuit breaker theory and design

In the electrical and electronics world, there are many cases where mishap takes place. It will lead to severe damage to buildings, offices, houses, schools, industries, etc. Trusting voltage and current are not correct, though safety measures are taken. Once circuit breakers are installed it will control the sudden rise of voltage and current.

A circuit breaker is a switching device that interrupts the abnormal or fault current. It is a mechanical device that disturbs the flow of high magnitude fault current and in additions performs the function of a switch. The circuit breaker is mainly designed for closing or opening of an electrical circuit, thus protects the electrical system from damage. Circuit breaker essentially consists of fixed and moving contacts.

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