These three positions tell the operator what condition the contacts are in when the handle is viewed. The operating mechanism may be actuated to move the contacts into a second, open position. The trip mechanism is automatically Releasable to effect tripping operations and manually resettable following tripping operations. The trip mechanism may include a thermal trip capability, which responds to persistent low level over currents. The thermal-magnetic device includes a "Bimetal" which heats up and bends in response to persistent over current conditions to unlatch a spring powered operating mechanism which opens the contacts which opens the separable contacts of the circuit breaker to interrupt current flow in the protected power system. For short circuit protection, an armature, which is attracted by the sizable magnetic forces generated in a magnetic core by a short circuit; unlatches the operating mechanism.
In some modern types of MCBs arc fault or ground fault mechanism are operative. The tripping is effected in the event of a line to ground fault or neutral to ground fault. In such case a torridal C.T. is used for differential current sensing.
Nowadays a Molded Case Circuit Breaker (MCCB) is available which is the same as MCB and hence the further operation of MCB is explained in MCCB.
MOLDED CASE CIRCUIT BREAKER(MCCB):
Although there are many types of molded case circuit breakers manufactured, all made up of five main components. These are:
•Molded Case or Frame.
•Operating Mechanism .
•Arc Extinguisher.
•Contacts .
•Trip Units.
The function of the frame is to provide an insulated housing to mount all of the circuit breaker components. The frame is often a glass-polyster material or thermo set composite resin that combines ruggedness and high Dielectric strength in a compact design. The frame is also known as a molded case.
A frame designation is assigned for each different type and size of molded case:
This designation is used to describe the breaker's characteristics such as maximum voltage and current ratings however, each manufacturer has their own identification system to account for the differences between breaker characteristics.
Operating Mechanism:
Increasingly, molded case circuit with conventional thermal magnetic trip units are being replaced by breakers with electronic trips units. These units provide increased accuracy and Repeatibility. 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 composed to three components:
•A current transformer (sensor) is used on each phase to monitor the current. It also reduces the current to the proper level of an input to a printed circuit board.
•Electronic circuitry (printed circuit board) that intreprets 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 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 because the entire tripping system has very low pwoer requirements.
Basics of MCCB:
The traditional molded-case circuit breaker uses electromechanical (thermal magnetic) trip units that may be fixed interchangeable. An MCCB provides protection by combining a temperature sensitive device with a current sensitive electromagnetic device. Both these devices act mechanically on the trip mechanism.
Depending upon the application and required protection, an MCCB will use one or a combination of different trip elements that protect against the thermal overload, short circuit and ground fault.
Thermal overload, in an overload condition, there's a temperature buildup between the insulation and conductor. If left unchecked, the insulation's life will drastically reduce, ultimately resulting in a short circuit. This heat is a function of I² RT.
Short-circuit condition:
Usually, a short circuit occurs when abnormally high currents flow as a result of the failure of an insulation system. This high current flow, termed short-circuit current, is limited only by the capabilities of the distribution system. To stop this current flow quickly so that major damage can be prevented, the short circuit element of an MCCB is used.
Ground fault condition:-
A ground fault actually is a type of short circuit, only it's phase-to-ground, which probably is the most Common type of fault on low-voltage systems (600 V or less). Today's modern electronif C.B. has the ground fault protection as an integral part of the trip unit.
Overload trip action:-
Overload or thermal trip action uses a piece of bimetal heated by the load current. This bimetal is actually two strips of metal bonded together, with each having a different thermal rate of heat expansion. They are factory-calibrated and not field-adjustable.
Short circuit trip action:-
Short-circuit trip action uses an electromagnet having a winding that's in series with the load current. When a short circuit occurs, the current flowing through the circuit conductor causes the magnetic field strength of the electromagnet to increase rapidly and attract the armature. When this happens, the armature rotates the trip bar, causing the C.B. to trip.
The only time delay factor involves the time it takes for the contacts to physically open and extinguish the arc, this usually is less than one cycle.
Magnetic elements are either fixed or adjustable,depending upon the type of C.B. and frame size. For example, most thermal magnetic breakers above the 150 A frame size have adjustable magnetic trips.
A thermal magnetic trip unit is best suited to most general-purpose applications as it's temperature sensitive and automatically will follow safe cable and equipment loadings. These loadings will vary with ambient temperatures. Thermal magnetic units do not trip if the overload is not dangerous, but will trip instantly with heavy Short-circuit circuit currents.
Electronic trip units:-
Electronic trip units typically consist of a current transformer (CT) for each phase, a printed circuit board, and a shunt trip. The CTs monitor current and reduce it to the required ratio for direct input into the printed circuit board, the brains of the electronic trip unit. The circuit board then interprets current flow information, makes trip decisions on predetermined parameters, and tells the shunt trip unit to the trip the breaker.
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