MCCB(molded case circuit breaker) is typically used because it has superior characteristics that fuses do not possess, such as safety, controllability and ability to collaborate with other devices. It has also been used for the purpose of protecting load equipments or lines installed in the low-voltage distribution panel of high-voltage accommodation or industrial low-voltage distribution panels. MCCB is classified as the current limiting type which interrupts the fault current by decreasing the let-through-current and as the none current limiting type which interrupts current regardless of current limiting characteristics. When short circuit current occurs by fault, the moving contactor moves to open-position before the trip unit detects the fault current whereas the none current limiting type detects the fault current. Therefore, the current limiting type has the advantage of shortening the breaking time, but also in protection coordination. MCCB is composed of mechanism unit, arc quenching unit and the trip unit. These are assembled within the molded case. The mechanism unit has a role which separates and contacts the tips of moving and the fixed. It''s termed "ON" when both contacts meets each other and "OFF" when both contacts have a distance between them. During mechanical operation with the current, the effect of the arc on the contacts can be quite severe. The prolonged arcing time may be a cause of instability of contact structures or adhesion between the contacts. The structures of moving contactor of present mechanism may be damaged when both contacts meet by means of increasing contact force and tips can be broken by impact when they collide. Also, the contact force can be increased during the "ON" time which may lead to imperfect "ON" state. Consequently this can take effect on equipments but also cause faults. The arc quenching unit is composed of laminated metal named grid which has array, direction and angle. Its function is splitting and cooling of the arc by inducing it into grids at the time of event. The present arc chamber unit is disadvantageous to extinguish arc. This is because the arc column cannot be prolonged enough to reach to the upper grid. Also the low gas pressure and arc voltage may cause arc ignition again. Like this, arc quenching phenomenon is so complicate, being coupled with electrical characteristics. To improve the current limiting characteristics, it may require coupling analysis of dynamics, heat and fluid, electric and arc but it is impossible to come up with a solution. Hence various methods have been adopting to solve the problem by trial and error.
This thesis applied dynamic and electromagnetic analysis, also the shaft assembly and arc quenching unit were optimized and then verified by experimental methods to improve current limiting characteristics. The conclusions are as follows:.
1. Newly designed shaft assembly reduces interruption time by about 6 ms. Also it minimizes the consumption of the contact tip, prevent welding and imperfect making. Like this toggle structure of the shaft assembly is very advantageous to provide a constant current and interrupt short circuit.
2. Newly adopted contact tips reduced the temperature rising by about 15 ℃ compare to the AgCdO tip. It can be useful to achieve high performance of short circuit breaking.
3. Newly designed arc quenching unit maintained high arc voltage and obtained excellent results for three-phase short circuit breaking test. And it shows that rear vent and grid feature can induce the arc rapidly outside. So the test result of discrimination with optimized feature shows superior and effective discrimination of device which will minimize the damage at fault.
As mentioned above, this thesis suggests new mechanism and arc quenching chamber to enhance current limiting characteristics. Like these structures could contribute to make effective discrimination systems and could be used to achieve high voltage performance.