Construction of MCCB, working principle of MCCB, Operating Mechanism of MCCB

Melded Case Circuit Breaker (MCCB) :

• Molded case circuit breakers are tested and rated according to the UL489 Standard. Their current carrying parts, mechanisms and trip devices are completely contained within a molded case of insulating material. MCCBs are available in various frame sizes with various interrupting ratings for each frame size.
• Molded case circuit breakers are designed to provide circuit protection for low voltage distribution systems. They protect connected devices against overloads and/or short circuits. They are used primarily in panel boards and switchboards where they are Fixed Mounted. Some of the larger MCCBs are available in Drawout Mount design.
• Molded case circuit breakers are available with special features making them suitable for the protection of motor circuits when used in conjunction with separate overload protection device. In the applications, they are often referred to as motor circuit protectors (MCPs).
• Although there are many types of molded case circuit breakers manufactured, all are made up of five main components. These are :
1. Molded Case or Frame
2. Operating Mechanism.
3. Arc Extinguishers.
4. Contacts
5. 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 of a glass-polyester material or thermoset 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 :

• The operating mechanism is the means to ope close the contacts. The speed with which the con 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 know as Trip-Free. The handle position indicates the statue don of the contacts-closed, open or tripped. When the contacts are in the tripped position, the handle is in a midway position.
• To restore service after the breaker trips, the handle must be moved first to the OFF position from its center tripped position. Then the handle must be moved to the ON position. When breakers are mounted in a group, as in a panel board, the distinct handle position clearly indicates the faulted circuit. Some breaker designs also incorporate a push-to-trip mechanism. This allows a manual means to trip the breaker and test the mechanism.

Arc Extinguisher :

• Whenever a circuit breaker interrupts current flow, an arc is created. The function of the are extinguisher is to confine and divide that arc, thereby extinguishing it. Each arc extinguisher is made up of a stack of steel plates held together by two insulator plates.
• When an interruption occurs and the contacts separate the current flow through the ionized region contacts induces a magnetic field around the arc and the arc extinguisher. (Module 5, “Fundamentals of Circuit Breakers," covers this topic in detail.
• The lines of magnetic flux created around the arc and its force drives the arc into the steel plates. The gas goes through Deionization and the arc divides, allowing it to cool.

• Standard molded case circuit breakers use a linear current flow through the contacts. Under short circuit conditions, a small blow-apart force is created, which helps open the contacts. The majority of the opening action comes from mechanical energy stored in the trip mechanism itself. This is because the current in both contacts are going in the same direction and attract each other.
• Newer design breakers use a reverse loop of current flowing in essentially opposite paths. This creates a repulsion action and results in a greater blow-apart force. This force assists with rapid arc extinguishing by causing the contact to open faster. The force is directly Proportional to the size of the Fault Current. The greater the fault, the greater the force, and the faster the contacts open.

Trip Unit

• The trip unit is the brains of the circuit breaker. The function of the trip unit is to trip the operating mechanism in the event of a short circuit or a prolonged overload of current. Traditional molded case circuit breakers use electromechanical (thermal magnetic) trip units (Module 5 covers this in more detail). Protection is provided by combining a on temperature sensitive device with a current sensitive electromagnetic device, both of which act mechanically on the trip mechanism. Electronic trip units are now available and they can provide much more sophisticated protection and monitoring.
• Most molded case circuit breakers utilize one or more different trip elements to provide circuit protection for different applications. These trip elements protect against thermal overloads, short circuits and arcing ground faults.
• Conventional MCCBs are available with either a fixed or interchangeable electromechanical trip unit. If a new trip rating is required for a Fixed Trip breaker, the entire breaker must be replaced. With an Interchangeable Trip Unit, as its name implies, only trip unit needs to be changed, up to the maximum current rating of the breaker frame. Interchangeable units are also called Rating Plugs. Some breakers offer interchangeability between electromechanical and electronic trip units within the same frame.
• 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 using the 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 repeat ability. 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 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 because the entire tripping system has very low power requirements.