Project Title: Improvement in Thermal Performance of A Molded Case Circuit Breaker

Abstract

In electrical systems, the MCCB is used to protect the electrical circuit and its components from excessive current which is required to be isolated at the right time thereby preventing an overload or a short circuit. During the overload conditions of an MCCB, tripping characteristics should perform as per the applicable standard IEC 60947 Part 2. The standard requirement is that at 1.05 times the rated current, the MCCB should not trip for 2 hours, then immediately switching to 1.3 times the rated current, the MCCB should trip within 2 hours. In case of the considered MCCB (Model DN1/DU250), the target performance (100 % performance as per IEC standard) was the set objective for the present study. The tripping characteristics of a MCCB mainly depends on two components - the heater and the bimetallic strip. As a part of study, the heater and the bimetallic strip were analysed and it was observed that the current distribution in the heater component and the deflection rate of the bimetallic strip were the main aspects affecting the thermal performance of DN1 MCCB. It was observed that key factor in the heater performance was current density which was also indirectly affecting the performance of bimetallic strip. Thus redesign of heater was done keeping in mind the proper distribution of current so that necessary amount of heat was delivered to the bimetallic strip and thereby improving the thermal performance of the MCCB. Based on the analysis, innovative concepts for the heater redesign were introduced and the redesigned MCCB were tested as per the standard testing methods prescribed for Low Voltage Switch Gear. Based on the testing of these MCCB's, it was found that the new heater design was working efficiently in 250A MCCB, but when the same heater design was tested for lower variant MCCB's (i.e 200A, 160A &125A) due to less amount of current, heat generation in the heater reduced which led to the breakers not passing as per standard tests. Hence for improving the heat generation in heater for lower variant MCCB's some geometrical changes were made such as reduction in thickness and change of material from Copper to Brass. Thus again, the redesigned heater for lower variant MCCB's were tested as per standards and passed in every test. Thereafter numerical investigation was also carried out in ANSYS Mechanical to monitor the deflection of bi-metal with change in heater design. This provided a brief idea of how the deflection of bi-metal changed with change in design and change in material of heater. Based on the experimental and numerical investigations, it can be concluded that the heater redesign was working efficiently and can be implemented. Keywords: MCCB, Heater, Bimetallic strip, Current Density, ANSYS