Operating in the ‘Danger Zone’: Contactor Dropout vs. 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 ’dropout’. For low-level faults which 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 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 or complex and expensive supplemental power supply circuits are added to ensure the holding coil remains energized until the fuse clears the fault. The trade-offs of short-circuit clearing time versus rated current to achieve a take-over 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.