Littelfuse Plug fuses provide overcurrent protection to general purpose circuits and small motor loads. Edison-base Plug fuses (TOO and TLO series) feature metal threads similar to incandescent lamp bases. Type S Plug fuses (SOO and SLO series) utilize a nonmetallic threaded base and must be used with Type S adapters (SAO series). The National Electrical Code permits Edison base Plug fuses to be used for replacement purposes only. Type S fuses and adapters prevent shunting open fuses and over-fusing.