1. Structure and principle of miniature circuit breaker
As an overcurrent protection device, our circuit breakers use current control switches to switch indoor wiring systems. Important components within electrical devices are effectively protected. They are primarily designed to protect against short circuiting and serious overloading. Circuit breakers are important and are used in load protection on industrial machinery.

It will allow the current to flow when using the handle is pulled into position. When the circuit is overloaded or a short circuit occurs, it will automatically release from the power position. Once the problem has been identified and resolved, the handle can be pulled into the proper position and the power can be restored.

Our circuit breakers feature points for the addition of springs and arc extinguishing devices which is different from normal power switches. In the spring break or closed in process of operation, the pre-storage spring force will sometimes reach the critical point and bounce off and on contact. The operation speed is not affected by the speed of operation of the hand. The arc suppression device eliminates the operation of the internal contacts generated by the spark arc chamber. Any contact to open the load current will generate an arc (or spark). The arc is a conductive air column that generates a high temperature and amperage according to the right=hand rule. The device acts to form a strong magnetic field in the arc chamber between the Fleming's right hand rule. The air conductor is fast and elongates and bends the arc to extinguish the spark more quickly.

If the load is on an electric motor, the normal start can be increased to a higher current, so it should be used 1.5 to 2 times the rated current of the circuit breaker.

2. Reliability detecting device for miniature circuit breaker
The miniature circuit breaker is a kind of protective equipment, which is used to maintain the safety and operation of electrical circuits and equipment. When the electrical circuit or the electric equipment load experiences a short circuit fault, the tripping device immediately goes into action, reliably cutting off the circuit. When the electrical circuit or the electric equipment is in normal condition, operation is unaffected.

Introduction
The miniature circuit breaker is a protection type electrical appliance, which is used for maintaining the safety of electric circuits and equipment. When the electrical circuit or the electric equipment load experiences a short circuit fault, the tripping device will automatically and reliably cut off the circuit. When the electrical circuit or the electric equipment is in normal condition, the main contact can be reliably connected to the circuit. It is necessary to carry out a reliability test and the instantaneous reliability test on the miniature circuit breaker. In order to study the reliability of miniature circuit breakers, in addition to study the reliability index and test plan, the most important thing is to develop a set of reliability test equipment used to carry out the reliability standard. The only way to the research the reliability of miniature circuit breakers is for small circuit breaker manufacturers to provide an effective means to enhance the competitiveness of products and improve product reliability.

Using a self-designed apparatus, the miniature circuit breakers will be tested according to specific requirements for instantaneous reliability testing. The final test data will help us identify the most effective designs of our small circuit breakers.

Hardware design of reliability test equipment
The hardware of small circuit breaker reliability testing equipment is mainly composed of the test control cabinet, the test cabinet, the large current cabinet and the load circuit board. The control cabinet is composed of a computer, printer and Advantech PCL-720 control card and PCL-818 card series.

The test cabinet is mainly composed of eight electric motors. The industrial control machine is used to control the motor driven mechanical handle, collecting the voltage at both ends of the test product contacts to determine the point at which failure occurs as shown in figure1.

The computer of the PCL-720 control digital output card tests the motor control circuit. It switches the motor forward and backward to perform driving tests on equipmet. The Holzer element is provided with mechanical operation of motor control, when the motor rotates in place after the Holzer component can output signals in place, through the PCL-720 control digital input card to that place and motor control through PCL-720 digital output card and the motor forward or reverse circuit is disconnected, the motor stops rotating. In the process of motor forward or backward in through the data acquisition card contacts at both ends of the voltage closed and open voltage detection voltage is not meet the requirements. The data is then recorded.

Fig.2 Circuit model for data acquisition. When the contact is closed to be measured when the contact voltage is 0V, voltage collected by acquisition card value should be approximately 0V. When the measured contact is opened, the contact voltage is 24V, the maximum range of PCL-818HG data acquisition card for analog acquisition by 10V acquisition card before measuring, dividing and measuring by R1 and R2.

The high current cabinet is mainly composed of the main circuit contactor, the voltage regulator and the transformer. For C type miniature circuit breakers, the transient reliability test current is 5 and 10 times the rated current. When 5 times the rated current, the small breaker shall not operate. When the 10 times the rated current, the small breaker will activate and reliably cut off the circuit. In the process of transient reliability test, the current value can reach 5 or 10 times by adjusting the voltage regulator. Then, close the test loop to detect, through the PCL-818HG acquisition card to collect the voltage at both ends of the contact, with the collected voltage and the allowable value of the comparison, to determine whether the failure of the test.

The testing conditions of resistance load provided by load circuit board is 24V, 0.1A. Figure 2 is the schematic diagram of the load circuit board and its sampling circuit. When the external power supply is 24V, the load resistance is 0.24k, the load current of the circuit breaker contact is 0.1A.

Software design of reliability test device
Software is an important part of the hardware realization of operational reliability testing and transient reliability testing on small circuit breakers. The test device controls interface by VC++/MFC and assembly language. It is necessary to use this programming method in Windows 98 environment due to its incomplete protection.

The Figure 3 is interface of the reliability test device of miniature circuit breaker. It can test the miniature circuit breaker by clicking on the corresponding menu item.

In this interface, it will pop up dialog box or a drop-down menu by clicking on the corresponding menu. The main functions of the application interface are test initialization, test parameter setting, test auxiliary operation, operation reliability test, transient reliability test, view data and help.

Operation flow chart application interface as shown in Figure 4, enter the application interface, the first test parameters, test initialization, if the initialization is not detected sample, the system can give reliability test tips. If the test device is tested to be tested, then click on the start of the operating reliability or the instant reliability start button.

3. The difference between MCB and MCCB
1) Circuit breaker can up the coarse wire more than 35 square. Small circuit breakers are only suitable for the following 10 square meters wire.
2) The end of circuit breaker is used for connecting to the nut, easy compression and good contact. Its small clamping terminal can only be tightened with a screwdriver, but its torque is not enough, not easy to press and contact is not good.
3) The circuit breaker over-current and short-circuit protection devices use two different sets respectively, can adjust over-current protection action value manually. The flow and short circuit protection devices of small circuit breaker share a device, the current cannot be adjusted.
4) MCCB is white, using arc shield with strong discharge ability and can withstand greater short-circuit currents. The short is not easily caused. Long life.
5) A small circuit breaker MCCB with high sensitivity ratio.