Operating in the “Danger Zone”: Contactor Dropout Versus Fuse Clearing Time

The most common and economical arrangement for medium-voltage motor controllers is to self-supply from the primary motor circuit. In this arrangement, a short circuit collapses the line voltage, causing the contactor to “drop out.” For low-level faults that nevertheless exceed the interrupting rating of the contactor, it is common for the dropout time to be faster than the clearing time for the fuse. Although this fault current is extremely unlikely to occur, no provisions exist in current standards to permit the application of contactors in this manner. To prevent the contactor from attempting to clear a fault current above its rating, less than optimal fuses must be selected, and complex and expensive supplemental power supply circuits are added to ensure that the holding coil remains energized until the fuse clears the fault. The tradeoffs of the short circuit clearing time versus the rated current to achieve a takeover point below the interrupting rating of a contactor are illustrated. In response to a customer request, an alternative test procedure to demonstrate capability where contactor dropout precedes fuse clearing was performed; actual results of such tests are included.