Fuse Box Upgrade in the UK According to BS 7671 (Amendment 3:2024)

Introduction

A fuse box upgrade, also known as a consumer unit replacement, is one of the most critical tasks in modernising an electrical installation in a UK property. According to BS 7671:2018 with Amendment 3:2024, upgrading a fuse box ensures not only compliance with regulations but also greatly improves safety against electric shock, fire risk, and power disruptions.

Why Upgrade a Fuse Box?

In many homes across the UK, outdated fuse boxes with rewirable fuses (BS 3036) or MCBs without any RCD protection are still in service. These older units fall significantly short of current safety expectations. The 2024 amendment mandates that any circuits supplying socket outlets, lighting in domestic dwellings, bathroom circuits, kitchen installations, outbuildings, and all circuits involving fixed metallic appliances must be protected by an RCD with a current rating not exceeding 30 mA.

When Are RCDs and RCBOs Required?

More importantly, the use of RCBOs (devices that combine MCB and RCD functionality) is strongly recommended — and in some cases essential — where disconnection of other circuits would cause unacceptable inconvenience or safety risks. For example, circuits serving fridges, medical equipment, security systems, smoke alarms, or installations in properties with vulnerable occupants (such as the elderly or those with disabilities) should be individually protected with RCBOs to ensure circuit isolation without compromising the rest of the installation. Find more info in our RCD/RCBO article.

Surge Protection and AFDDs

Amendment 3:2024 also strengthens requirements around surge protection. SPD (Surge Protection Device) installation is no longer optional in most cases. Where properties contain electronic devices (e.g. computers, boilers, alarms) or are supplied by overhead lines or are in rural areas, a Type 2 SPD must be installed unless a documented risk assessment explicitly proves it unnecessary.

Arc Fault Detection Devices (AFDDs) are now considered best practice for circuits supplying bedrooms, timber-structured buildings, or locations with increased fire risk. While not yet mandatory across all installations, they offer a high level of fire prevention by detecting arcing faults, which are not caught by standard RCDs or MCBs.

Construction and Regulation Compliance

Physically, all newly installed consumer units must have a non-combustible enclosure — typically steel — compliant with BS EN 61439-3. They must be fully accessible for inspection and testing, with proper labelling of all circuits, functional test access, and a main double-pole isolator capable of switching both live and neutral conductors.

Testing and Certification

Testing after installation is mandatory under Section 6 of BS 7671. This includes polarity checks, continuity of protective conductors, insulation resistance, Zs loop measurements, prospective fault current (PFC), RCD trip times, SPD/AFDD function checks (if applicable), and a full visual inspection. An Electrical Installation Certificate (EIC) must be issued, and Part P notification submitted to Building Control via a registered competent person scheme (NICEIC, NAPIT).

Example Circuit Chart for Full RCBO Layout