MF-SD/250 Telecom PTC Resettable Fuses Bourns MF-SD/250 Telecom PTC Resettable Fuses feature a surface-mount package containing twin 250V polymer fuses to save space. Bourns MF-SD/250 twin pack provides tip and ring overcurrent protection with high voltage surge capabilities. These fuses are designed for secondary overcurrent protection in customer premise equipment (CPE), central office (CO), and subscriber line interface card (SLIC) applications.