What are rewireable and cartridge fuses?

Glossary · Consumer units and protection

How a fuse works

A fuse is a safety device with a small piece of wire inside. NICEIC says that when the current exceeds a set level, the wire melts, cutting the electricity to the circuit and preventing damage.^{[1]NICEIC, Consumer units and fuse boxes: installation and replacement} The IET says fuses protect against overcurrent and short-circuit faults, and that the circuit breaker is the modern equivalent.^{[2]IET, Consumer units: safe fuse-box upgrades for modern homes}

Each fuse is rated in amps, and there is one per final circuit, such as the cooker, the sockets or the lights.^{[2]IET, Consumer units: safe fuse-box upgrades for modern homes} A fuse that blows has done its job. The cause of the overload or fault still has to be found. See overload and short circuit.

Rewireable and cartridge fuses

NICEIC says a blown fuse must either be replaced with one of the same rating or, for rewireable fuses, have its wire replaced to restore the fuse box.^{[1]NICEIC, Consumer units and fuse boxes: installation and replacement} The rating matters because it sets the point at which the circuit is cut. Work on a fuse box is for a registered electrician, so this page does not describe how to change a fuse or its wire.

The IET notes that in the 1960s it was acceptable to install a consumer unit with a wooden back box and rewireable fuses.^{[2]IET, Consumer units: safe fuse-box upgrades for modern homes} NICEIC says fuse boxes began to be phased out in the mid-20th century in favour of consumer units with circuit breakers, which cut the circuit faster and can be safely reset.^{[1]NICEIC, Consumer units and fuse boxes: installation and replacement} See circuit breaker.

Do old fuses have to be replaced?

Not always. Electrical Safety First says a consumer unit need not be replaced only because it has rewireable fuses, cartridge fuses or older type circuit breakers, as these can provide satisfactory overcurrent protection. It also says a unit need not be replaced because it does not incorporate RCD protection, as there may be other ways to provide that protection.^{[3]Electrical Safety First, Best Practice Guide 1: Replacing a consumer unit in residential and similar premises}

NICEIC takes a firmer view. It says that if your home still has a fuse box, it recommends replacing it with a modern consumer unit, because fuse boxes do not offer the same level of protection.^{[1]NICEIC, Consumer units and fuse boxes: installation and replacement} The two views fit together: a fuse can protect a circuit from overcurrent, but it does not give the RCD protection that modern units provide. An EICR tells you whether a particular unit is still safe.^{[4]Electrical Safety First, Advice for home buyers}

Replacing a consumer unit is notifiable work under Part P of the Building Regulations in England and Wales. It must be done by a registered electrician or notified to building control.^{[1]NICEIC, Consumer units and fuse boxes: installation and replacement}

Example

Margaret has lived in her Armley terrace for forty years. Her fuse box has rewireable fuses and no RCD. She has an EICR before selling. The inspector looks for physical damage, heat damage and discolouration on the box, as Electrical Safety First advises for any older unit, and reports what is found.^{[4]Electrical Safety First, Advice for home buyers}

The buyer asks for a modern consumer unit. A registered electrician replaces it, tests the installation and issues the certificate.

What people get wrong

The usual mistake is to think a blown fuse is a fault in the fuse. It is the fuse doing its job. Something on the circuit drew too much current. Fitting a higher rating or wire to stop it blowing removes the protection and leaves the cable at risk.

Many people also assume an old fuse box is automatically unsafe. Electrical Safety First says older units may not have an RCD or circuit breakers, but that does not necessarily mean they are unsafe or need upgrading.^{[4]Electrical Safety First, Advice for home buyers}

The bottom line

A fuse protects a circuit from overload and short circuit by melting. Rewireable and cartridge fuses do the same job in different formats. They do not protect people from shock, and NICEIC recommends a modern consumer unit for that reason. Whether an old fuse box can stay is a question for an EICR and a registered electrician.