Understanding Consumer Unit Components & Protective Devices

Scope of this guide

This article provides objective technical definitions for general reference. Device selection, circuit design, and certification are governed by BS 7671 and must be carried out by a competent, registered electrical contractor.

Key takeaways

Each device in a consumer unit has a distinct, defined function — protection roles cannot be assumed to overlap.
RCDs protect against electric shock; MCBs protect cables — they operate on completely different principles.
RCBOs combine both functions per circuit and are the current best-practice device for discrimination.
Amendment 4 (2026) requires a documented SPD risk assessment for every new installation or upgrade.
AFDDs address arc-fault fire risk — a hazard that standard breakers and RCDs are not designed to detect.

Introduction

A modern consumer unit (often referred to as a fuse box) is the central safety hub of a domestic electrical installation. Under BS 7671 Amendment 4 (2026), these units incorporate sophisticated technology designed to protect both the building’s infrastructure and its occupants.

Each circuit in a property originates at the consumer unit. The assembly of devices within it must be correctly selected, coordinated, and installed to satisfy the Regulations. Domestic units installed since 2016 are generally required to use a non-combustible metal-clad enclosure, a requirement driven by fire safety statistics linked to older plastic boards.

The sections below define each major component type in the order in which current flows and protection is applied, from the incoming supply through to individual circuit protection.

Component Reference

Primary Isolation

Main Switch

The main switch provides the primary means of double-pole isolation for the entire installation. It permits the manual disconnection of all live and neutral conductors simultaneously, allowing safe access to the consumer unit and its circuits for maintenance or in an emergency.

BS 7671 requires that a suitable means of isolation exists at the origin of every installation. On most domestic supplies, the main switch is the defined means of achieving this.

Standard ratings: 63 A, 80 A, or 100 A double-pole, to match supply capacity.

Shock Protection

RCD — Residual Current Device

An RCD is a life-safety device designed to protect against electric shock. It continuously monitors the residual current imbalance between live and neutral conductors: under fault conditions, leakage current to earth creates an imbalance, and the RCD disconnects the supply within milliseconds — typically in under 40 ms at rated sensitivity.

Standard domestic RCDs operate at a trip threshold of 30 mA, a level established by clinical research as the threshold below which cardiac fibrillation is unlikely in healthy adults. RCDs do not provide overcurrent protection; they must be used in conjunction with correctly rated overcurrent devices.

Note: Type AC RCDs detect sinusoidal AC fault currents only. Installations with electronics may require Type A (for pulsed DC) or Type B (for smooth DC, e.g. EV chargers and inverters).

Cable Protection

MCB — Miniature Circuit Breaker

An MCB is a dedicated overcurrent protection device. Its purpose is to protect the wiring of a circuit against thermal damage arising from sustained overcurrent or immediate disconnection under short-circuit conditions. It is not a life-safety device in the shock-protection sense; it protects the installation’s infrastructure.

MCBs are characterised by their current rating (e.g. 6 A, 16 A, 32 A) and trip curve (Type B for domestic lighting and general circuits; Type C for higher inrush loads; Type D for specialist equipment). Best practice dictates that the MCB rating is chosen to match the cable cross-section of the circuit it protects, not the maximum demand of the connected load.

Limitation: An MCB alone provides no protection against electric shock via earth leakage. An RCD or RCBO must be present on circuits requiring shock protection.

Integrated Protection — Best Practice

RCBO — Residual Current Breaker with Overcurrent

An RCBO integrates the functions of both an RCD and an MCB into a single device dedicated to one circuit. It provides simultaneous protection against electric shock (earth leakage ≥ 30 mA) and overcurrent / short circuit on that circuit alone.

This represents current industry best practice for new consumer unit installations. Because each circuit has its own independent residual-current element, a fault on one circuit — such as a failing appliance causing earth leakage — does not cause a total loss of power to the property. The affected circuit trips; all others remain energised. This significant improvement in resilience is why the RCBO arrangement supersedes split-load boards with a shared RCD protecting multiple circuits.

Regulatory context: Regulations state that discrimination between protective devices must be considered at design stage. RCBO-per-circuit arrangements inherently satisfy this requirement for residual current discrimination.

Transient Overvoltage Protection

SPD — Surge Protection Device

An SPD limits transient overvoltages from external sources — primarily lightning-related events on the distribution network and switching transients from grid operations — by diverting surge energy to earth before it can propagate to connected equipment.

It is a mandatory consideration under BS 7671:2018+A4:2026: the designer must carry out a documented risk assessment and either fit an SPD or formally justify its omission. Modern homes contain increasing volumes of vulnerable electronics: televisions, smart heating systems, boiler control boards, EV chargers, and battery storage inverters. These devices are disproportionately susceptible to voltage transients that would not affect older resistive loads. SPDs are classed as Type 1, 2, or 3 depending on their position in the installation and energy handling capacity.

Coordination: SPDs must be selected and coordinated with upstream overcurrent protection. An SPD module incorporates a condition indicator; it requires periodic inspection and replacement at end-of-life.

Fire Prevention — Amendment 4

AFDD — Arc Fault Detection Device

An AFDD is an advanced safety component that monitors the current waveform on a circuit for electrical signatures consistent with series or parallel arcing faults. Arcing at loose or damaged connections, deteriorated insulation, or crushed cable can ignite surrounding material at temperatures far below what an MCB or RCD is designed to detect: the fault current may remain below the overcurrent trip threshold while sustained heat builds at the arc site.

It is required on specified circuit types under BS 7671 where the risk of ignition and its consequences are greatest — for example, final circuits in certain categories of sleeping accommodation. Beyond prescribed requirements, it represents a proportionate mitigation for installations in older properties, timber-framed buildings, or situations where wiring condition is uncertain. An AFDD is not a substitute for a sound installation; it is a supplementary safeguard for faults that conventional devices cannot reliably detect.

Scope: An AFDD provides fire risk mitigation, not shock protection. It must be used in conjunction with appropriate RCD protection on circuits requiring both.

Device Summary

Device	Primary Function	Protects	A4 / 2026 Relevance
Main Switch	Double-pole isolation	Entire installation	Required at origin
RCD	Earth leakage detection	Persons (shock)	Type A/B required for electronics
MCB	Overcurrent / short circuit	Cable / infrastructure	Rating matched to cable size
RCBO	RCD + MCB per circuit	Persons & cable, individually	Best practice; ensures discrimination
SPD	Transient overvoltage clamping	Equipment / electronics	Mandatory risk assessment; often fitted
AFDD	Arc fault waveform analysis	Building (fire risk)	Required in specified premises types