An Improved Overcurrent Tripping Device for Low-Voltage Circuit Breakers

The necessity for greater service continuity on low voltage systems has focused attention on the limitations of present delayed overcurrent tripping devices and created a demand for their improvement. This paper reviews the limitations of present devices and states the requirements for delayed overcurrent trips. A new device meeting all of the requirements is described and its characteristics shown. The new device is universal in its application in that the same elements provide for circuit breaker selectivity up to the highest values of fault current and yet give correct timing to permit motor starting. Tests are described which show the adequacy of the new device for its intended service as well as indicating an unusual degree of reliability.