Fuse (electrical) - Fuse Boxes

Fuse Boxes

• Rewirable fuses

• MEM rewirable fuse box

• MEM rewirable fuse holders (30 A and 15 A)

• Wylex fuse box

• fuse wire as sold to UK consumers

In the UK, older electrical consumer units (also called fuse boxes) are fitted either with semi-enclosed (rewirable) fuses (BS 3036) or cartridge fuses (BS 1361). (Fuse wire is commonly supplied to consumers as short lengths of 5 A-, 15 A- and 30 A-rated wire wound on a piece of cardboard.) Modern consumer units usually contain miniature circuit breakers (MCBs) instead of fuses, though cartridge fuses are sometimes still used, as MCBs are prone to nuisance tripping.

Renewable fuses (rewirable or cartridge) allow user replacement, but this can be hazardous as it is easy to put a higher-rated or double fuse element (link or wire) into the holder (overfusing), or simply fitting it with copper wire or even a totally different type of conducting object (hairpins, paper clips, nails, etc.) to the existing carrier. One form of fuse box abuse was to put a penny in the socket, which defeated overcurrent protection and resulted in a dangerous condition. Such tampering will not be visible without full inspection of the fuse. Fuse wire was never used in North America for this reason, although renewable fuses continue to be made for distribution boards.

The fuse boxes pictured in this section are (right) a MEM consumer unit with four rewirable fuse holders (two 30 A and two 15 A) installed c. 1957 (cover removed); a Wylex standard unit with eight rewirable fuse holders.

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.

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 and 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.

Some companies manufacture resettable miniature thermal circuit breakers, which screw into a fuse socket. Some installations use these Edison-base circuit breakers. However, any such breaker sold today does have one flaw. It may be installed in a circuit-breaker box with a door. If so, if the door is closed, the door may hold down the breaker's reset button. While in this state, the breaker is effectively useless: it does not provide any overcurrent protection.

In the 1950s, fuses in new residential or industrial construction for branch circuit protection were superseded by low voltage circuit breakers.