Benchmark Tests of Single-Break and Double-Break Design Principles

Abstract

In this paper test results of low voltage Molded Case Circuit Breakers (MCCB) are discussed with respect to two basic design principles: single-break design and double-break design. Experiment were carried out using a discharge bank test system at three different prospective fault current levels 10 kA, 24 kA, and 37 kA. Test results show that the two basic design principles have different influences on characteristic switching parameters such as arc voltage, arc current, I <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> t, VIdt, and pressure. At low current levels, the single-break design has advantages over the double-break design because the double-break design re-closes during short circuit events. The contact re-closing causes higher I <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> t and VIdt in comparison with those of the single-break design. While strong interaction between arc and chamber material in a small arc chamber volume as well as two arcs generated by the double-break design leads to effective current limiting with fast rise of arc voltage to a high arc voltage level, but it also causes strong material evaporation and high pressure. Test results also indicate that MCCB performance can vary significantly even though using the same design principle due to other design variations. In order to fully evaluate performances of these two design principles, power tests such as high fault current interruption, overload, and temperature rise tests required by UL or IEC standards need to be carried out.