SD Series Guide to applications and selection The SD Series of signal protection devices includes models for a full range of applications operating at voltages up to 250V ac. The optional fuse/disconnect package provides both fused protection against fault currents and a convenient method of isolating field circuitry from protected circuitry without requiring additional disconnect terminals. The standard fuse (replaceable) is rated 250mA. 50mA fuses are available by special request. Solid links can be used in applications where only the disconnect feature is required.