<P> Fuse wire as sold to UK consumers </P> <P> The Wylex standard consumer unit was very popular in the United Kingdom until the wiring regulations started demanding residual - current devices (RCDs) for sockets that could feasibly supply equipment outside the equipotential zone . The design does not allow for fitting of RCDs or RCBOs . Some Wylex standard models were made with an RCD instead of the main switch, but (for consumer units supplying the entire installation) this is no longer compliant with the wiring regulations as alarm systems should not be RCD - protected . There are two styles of fuse base that can be screwed into these units: one designed for rewirable fusewire carriers and one designed for cartridge fuse carriers . Over the years MCBs have been made for both styles of base . In both cases, higher rated carriers had wider pins, so a carrier couldn't be changed for a higher rated one without also changing the base . Cartridge fuse carriers are also now available for DIN - rail enclosures . </P> <P> In North America, fuses were used in buildings wired before 1960 . These Edison base fuses would screw into a fuse socket similar to Edison - base incandescent lamps . Ratings were 5, 10, 15, 20, 25, and 30 amperes . To prevent installation of fuses with an excessive current rating, later fuse boxes included rejection features in the fuse - holder socket, commonly known as Rejection Base (Type S fuses) which have smaller diameters that vary depending on the rating of the fuse . This means that fuses can only be replaced by the preset (Type S) fuse rating . This is a North American, tri-national standard (UL 4248 - 11; CAN / CSA - C22. 2 NO . 4248.11 - 07 (R2012); and, NMX - J - 009 / 4248 / 11 - ANCE). Existing Edison fuse boards can easily be converted to only accept Rejection Base (Type S) fuses, by screwing - in a tamper - proof adapter . This adapter screws into the existing Edison fuse holder, and has a smaller diameter threaded hole to accept the designated Type S rated fuse . </P> <Ul> <Li> <P> Edison base (left) and Type S fuses (right) </P> </Li> <Li> <P> An older fuse box of the type used in North America </P> </Li> </Ul>

Where is a fuse fitted in a circuit