OVERCURRENT PROTECTION DEVICE

摘要

A microprocessor based overcurrent trip unit in- cludes an analog circuit for monitoring phase currents in the electrical interrupting device. The analog circuit is adapted to interface with the microprocessor over a range of selectable setpoints for instantaneous tripping and at the same time accurately monitor the phase currents even during a condition when the current sensing transformers are saturated. The overcurrent trip unit also allows the function of the long time delay characteristics of protec- tion curve to be varied in order to provide better coordi- nation with the characteristics of other overcurrent trip- ping devices in an electrical distribution system, such as a current limiting fuse, thereby enhancing the overall reliability of the system. For example, the function may be varied to provide either a fixed or FLAT, It, I2t and I4t function. The overcurrent tripping device also allows for relatively wide adjustment ranges for the long tine delay and short time delay protection portions of an overcurrent protection curve. In order to prevent any overlapping of the protection ranges, the long time delay portion is prevented from tripping the interrupting device prior to the short time delay portion.