The National Electric Code now permits Edison base fuses to be used only for replacements in existing installations and then, only when there is no evidence of overfusing or tampering. All new Edison-based fuse holders must have Type S fuse adapters installed before they are placed in service. Designed to prevent installation of wrong size fuses, Type S Adapters (SAO series) screw into Edison base fuse holders so they may accept Type S fuses. Once installed the adapters cannot be removed. To prevent shunting of open fuses or over-fusing and to reduce nuisance fuse opening, it is recommended that SAO adapters with properly rated SLO or SOO series fuses be installed in all Edison base fuse holders.