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Molded Case Circuit Breakers are designed to provide circuit protection for low voltage distribution systems. They will protect connected devices against both overloads and short circuits. They are most-commonly-used in panelboards and switchboards where they are fixed mounted, though some of the larger MCCBs available may be available in a drawout mount design.
The Operating Mechanism handles the opening and closing of the contacts. The speed that the contacts open or close is independent of how fast the handle is moved. This is known as "quick-make, quick-break". The breaker cannot be prevented from tripping by holding the handle in the on position. This is known as "trip-free". The position of the handle indicates the status of the contacts - whether they are closed, open, or tripped. The handle will be in a midway position when the contacts are tripped, for example. In the event of a trip, the handle must first be moved to the off position from its center-tripped position, and then to the on position. When breakers are mounted in a group such as in a panelboard, the distinct handle position will clearly indicate the faulted circuit. Some breaker designs may also incorporate a push-to-trip mechanism which allows for a manual means to trip the breaker and test the mechanism.
To provide short-circuit protection, electromechanical trip circuit breakers have adjustable magnetic elements. To provide overload protection, electromechanical trip circuit breakers contain thermal trip elements. Breakers that use a combination of magnetic elements and thermal elements are often called thermal magnetic breakers. Increasingly, molded case circuit breakers with conventional thermal magnetic trip units are being replaced by breakers with electronic trip units. These units provide increased accuracy and repeatability. Additionally, some units have built-in ground fault protection, removing the need for separate ground fault relays and shunt trips. Some units can also provide system monitoring, data gathering and communication to energy management systems.
In general, electronic trip systems are made up of three components:
* A current transformer (sensor) is used on each phase to monitor the current. It also reduces the current to the proper level for input to a printed circuit board.
* Electronic circuitry (printed circuit board) that interprets the input and makes a decision based on predetermined values. A decision to trip results in sending an output to the next component.
* A low power flux-transfer internal shunt trip that trips the breaker. This is typically a mechanical, spring loaded device held in place by a permanent magnet.
When a tripping signal is received from the electronic circuitry, the effects of the permanent magnet are momentarily counteracted by the tripping pulse, allowing the mechanical tripping action to take place. There is no need for an external source of tripping power, since the entire tripping system has very low power requirements.
Types of MCCB Circuit Breaker by Application:
Molded case circuit breakers can have very high current ratings, which allows them to be used in heavy duty applications. The following are some typical uses of an MCCB:
Main electric feeder protection – The electric feeder circuits that supply power to large distribution boards normally have very high currents, of hundreds of amperes. In addition, if more circuits are added to the system in the future, it may be necessary to adjust the circuit breaker trip settings. Therefore, a molded-case circuit breaker is required.
Capacitor bank protection – Capacitor banks are a very important component of commercial and industrial electrical systems, since they allow power factor correction – reducing line currents and preventing fees from the electric utility company. Large capacitor banks may draw high currents and will require MCCB protection.
Generator protection – Large electrical generators may provide an output of hundreds of amperes. In addition, gen-sets are normally very expensive. The high current ratings of molded case circuit breakers allow them to provide reliable protection in this application.
Welding applications – Some welding machines may draw very high currents that exceed the capabilities of miniature circuit breakers, requiring the use of an MCCB.
Low current applications that require adjustable trip settings – MCCBs are not only for high current applications. There are models rated below 100 amperes for when low current equipment requires the adjustable trip settings provided by MCCBs.
Motor protection – The reliable protection capabilities of MCCBs make them an adequate choice for motor protection. A molded case circuit breaker can be adjusted to provide overload protection without tripping during the inrush current of an electric motor.
In summary, an MCCB offers adequate protection whenever an application requires a high current rating, adjustable trip settings, or a combination of both factors.