There has been a continual evolution of the MCCB due to a number of factors, such as market demands, performance improvements, the development of new materials, and advances in technology such as solid-state and microprocessor trip and control systems. These developments have greatly increased the system flexibility of MCCBs and it is now possible to tailor an MCCB to satisfy a wide range of applications.
It is interesting to note that, although the MCCB’s primary function is to protect downstream circuit elements in the event of an overcurrent, most MCCBs spend their entire lives in a closed position with perhaps a few operations to switch normal load currents and in isolating electrical circuits.
An MCCB has many internal and external components. Each component is important, but the MCCB must be able to interrupt the circuit.
MCCB (Molded case circuit breaker) components
MCCB (Molded case circuit breaker) can be operated easily and have excellent switching performance and breaking performance. The major components of an MCCB are the following:
The external parts of an MCCB are the case and terminals. The molded case provides both the insulation and the support structure for mounting all other components. The material is typically a reinforced polymer such as glass polyester which possesses both mechanical and dielectric strength. Typical construction consists of a case and a cover. The entire internal assembly is first mounted into the case and the cover is put on during the final assembly.
The breaker case involves a multitude of design considerations. For example, it is important to consider not only the mechanical strength required to hold both the stationary and moving parts, but also the strength to withstand the magnetic force exerted through the current-carrying members, the thermal loading during short-circuit interruption, and the gas pressure generation during high current arcing.
For low voltages, the dielectric strength of the case is usually more than adequate, although surface degradation may occur along the inside of the case which faces the arcing chamber. The dielectric strength can be increased by appropriate selection of case material or by application of an arc-resisting layer.
The trip system provides the function of activating the operating mechanism in the event of a prolonged overload or of a high fault current. The overload trip action is usually provided by heating a bimetal element. This can be done by direct heating, indirect heating, or induction heating.
As the overload current flows through the heater adjacent to the bimetal, the bimetal bends as its temperature increases. A trip action is accomplished when the bimetal deflects sufficiently to unlatch the operating mechanism.
The mechanism provides the means to execute “open” and “close” functions of the typical MCCB. The operating handle links the manual push or pull to the mechanism. The closing operation also charges a spring, which provides the force for the opening operation either through the handle or through the trip unit. A quick-make, quick-break operation is provided by a toggle-type mechanism. The contacts snap closed and open independently of the handle and of the trip unit speed.
The arc chamber is where the crucial function of circuit interruption occurs. In MCCBs, there are three major arc chamber components: (1) contacts; (2) arc runners; and (3) arc chute. Each component serves a specific function in the interruption process. The following discussion addresses each of these components and their functions.
The contacts are involved in both the conduction and interruption functions. In the closed position, the mating interface must have low resistance for carrying the continuous current without overheating. Upon opening under load or overcurrent conditions, an arc will form. The arc is a high-conductivity plasma column with a temperature greater than 10 x 10³ K. Here the contacts serve as the electrodes (anode and cathode) in direct contact with the arc column. In such a high-temperature environment, a certain portion of the contact material will vaporize, leading to contact erosion. Furthermore, being exposed to air, oxides can form, leading to increased contact resistance. The ideal material would be one that has low contact resistance, high resistance to arc erosion, soft oxides such that the oxide layer is easily broken by the closing operation, and structural strength to withstand the impact of the mechanical operation. Powder metallurgy has been successful in providing many useful contact materials. Those commonly found in MCCBs are Ag-W, Ag-Ni, Ag-C, Ag-metal oxide (e.g., CdO, Sn02), Ag-Mo, and Ag-WC. The refractory part provides the high-temperature withstand and structural strength, while the Ag provides the high conductivity and low contact resistance.
For high-continuous-current-duty breakers such as those rated at 1000 A or higher, parallel contacts are used to separate the conduction function from the interruption function.
The arc runners are extensions of the contact structures having the same electrical potential as the contacts. The functions of the arc runners are to stretch the arc as it moves off the contacts onto the diverging portion of the runner, to quickly channel the arc into the arc chute away from the contact region, and to quickly establish the arc in a noncontact region to prolong the contact life. In other words, the arc runners serve as new electrodes for the arc.
The common material used to make the arc runners are copper and iron. The surfaces may be plated to reduce corrosion or to aid arc movement.
Modern MCCBs use variations of the basic deion plate to form the arc chute. Usually, the arc chute is a stack of closely spaced steel plates supported by insulating material. Occasionally, other materials, such as copper or ceramic, are used instead of steel.
The terminals are used to connect MCCB and external conductors. Improper connection may cause abnormal heat generation. Therefore, it is necessary that the terminals can be connected easily and surely. Generally, crimp-style terminals and conductors which have high reliability are connected.
The trip button is a pushbutton for mechanically tripping the circuit breaker from the outside. A circuit breaker with a trip button can be easily tripped by pressing the trip button without electrical tripping by a voltage trip device or under-voltage tripping device or overcurrent tripping by application of current higher than the rated current to the circuit breaker. Therefore, it is easy to make sure that the circuit breaker has been reset and the external operation handle has been operated to reset, and, on circuit breakers with accessories, such as alarm switches, the control circuits can be checked easily.