14-11-2012, 02:36 PM
Circuit breaker
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Origins
Inspired by the works of American scientist Joseph Henry
and English scientist Michael Faraday, the circuit breaker
was invented in 1836 by an American, Charles Grafton Page.
[1]
An early form of circuit breaker was described by Thomas
Alva Edison in an 1879 patent application, although his
commercial power distribution system used fuses.[2] Its
purpose was to protect lighting circuit wiring from accidental
short-circuits and overloads. A modern miniature circuit
Circuit breaker - Wikipedia, the free encyclopedia Page 1 of 10
Operation
All circuit breakers have common features in their operation, although details vary substantially
depending on the voltage class, current rating and type of the circuit breaker.
The circuit breaker must detect a fault condition; in low-voltage circuit breakers this is usually done
within the breaker enclosure. Circuit breakers for large currents or high voltages are usually arranged
with pilot devices to sense a fault current and to operate the trip opening mechanism. The trip
solenoid that releases the latch is usually energized by a separate battery, although some highvoltage
circuit breakers are self-contained with current transformers, protection relays, and an
internal control power source.
Once a fault is detected, contacts within the circuit breaker must open to interrupt the circuit; some
mechanically-stored energy (using something such as springs or compressed air) contained within
the breaker is used to separate the contacts, although some of the energy required may be obtained
from the fault current itself. Small circuit breakers may be manually operated; larger units have
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 current is interrupted, an arc is generated. 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
in which the arc forms. Different techniques are used to extinguish the arc including:
Short-circuit current
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.
Under short-circuit conditions, a current many times greater than normal can exist (see maximum
prospective short circuit current). 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
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.
In air-insulated and miniature breakers an arc chute structure consisting (often) of metal plates or
ceramic ridges cools the arc, and magnetic blowout coils deflect the arc into the arc chute. Larger
circuit breakers such as those used in electrical power distribution may use vacuum, an inert gas such
as sulphur hexafluoride or have contacts immersed in oil to suppress 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 panelboard; these circuit breakers are called "supplemental circuit
protectors" to distinguish them from distribution-type circuit breakers.
Magnetic circuit breakers
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 then allows the contacts
to open by spring action. Some types of magnetic breakers incorporate a hydraulic time delay feature
using a viscous fluid. The core is restrained by a spring until the current exceeds the breaker rating.