Consumer Units, MCBs, and RCBOs: Selecting and Installing a Split-Load Board to BS 7671:2018

The consumer unit — commonly called a fuse board or distribution board — is the heart of any domestic or commercial electrical installation. It houses the main switch, overcurrent protection devices (MCBs or fuses), and residual current devices (RCDs or RCBOs) that protect individual circuits. Since BS 7671:2018 (the 18th Edition IET Wiring Regulations) and Amendment 2 (2022), the requirements for consumer unit design, RCD protection, and surge protection have significantly changed. This guide covers consumer unit types, circuit protection selection, the 18th Edition requirements, and practical installation guidance for qualified electricians.

Consumer Unit Types

Main Switch Board

The simplest type: a single main switch (100A double-pole) with individual MCBs. No RCD protection built in — only suitable for older rewires where individual RCBOs are used on every circuit. Rare in new installations.

Dual RCD (Split-Load) Board

The most common domestic type before 2019. A 100A main switch feeds two RCD-protected busways, each covering half the circuits. The split-load design means a nuisance RCD trip only isolates half the installation, preserving lighting or sockets while the fault is traced.

High-Integrity / All-RCBO Board

Each circuit has its own RCBO (combined MCB + RCD). The main switch feeds a single busway; each RCBO provides individual overcurrent and RCD protection. Under Amendment 2 (2022), this configuration (or equivalent) is required for new installations because it prevents a single RCD fault from taking out multiple circuits.

AFDD-Enabled Boards

Amendment 2 recommends (and in some circumstances requires) Arc Fault Detection Devices on circuits supplying bedrooms and sleeping accommodation. AFDD-equipped consumer units have AFDD combination devices (AFDD + RCBO) or separate AFDD modules in series with MCBs. See the Surge Protection Devices and AFDDs guide (article #96) for more detail.

BS 7671:2018 Amendment 2 (2022) — Key Requirements

The 2022 amendment to the 18th Edition introduced significant changes for consumer units:

RCD Protection (Regulation 411.3.4)

All socket outlet circuits rated up to 32A must have 30mA RCD protection, regardless of installation method. All circuits in bathrooms and kitchens require RCD protection. Circuits supplying luminaires in dwellings are recommended to have RCD protection.

No Single RCD to Protect All Circuits

Regulation 531.3.6: Where more than one RCD is used, the circuits must be split so that no single device protects all circuits simultaneously. A single 100A main switch board with one RCD covering all circuits does not comply. The solution is either a split-load dual-RCD board (at minimum) or individual RCBOs per circuit.

Individual RCD Protection Preferred

The preferred method under the 18th Edition (and aligned with the consumer protection intent) is one RCBO per circuit. This:

• Prevents nuisance tripping affecting multiple circuits
• Simplifies fault-finding (the tripped device identifies the faulty circuit)
• Provides both overcurrent and RCD protection in a single device

Surge Protection Devices (SPDs)

Regulation 443: SPDs are now required in most dwellings where the consequence of a transient overvoltage would be intolerable (i.e., virtually all domestic premises with electronic equipment). Type 2 SPDs (plug-in or DIN-rail mounted) should be installed at the consumer unit. See article #96 for SPD types and selection.

Metal Clad Consumer Units

Since 2016 (Part P amendment), consumer units in domestic premises must have a non-combustible enclosure (typically metal-clad, 1mm steel). Plastic consumer units are permitted only where the complete enclosure is non-combustible or where it is installed in a location inaccessible to ordinary persons (e.g., commercial premises, locked enclosures).

IP Rating

Consumer units must meet minimum IP2XC (BS EN 61439-3) to prevent accidental contact with live parts. Most compliant consumer units are IP40 or better.

Overcurrent Protection: MCBs

Miniature Circuit Breakers (MCBs) protect circuits against overload and short-circuit current. They are characterised by:

MCB Current Rating

Matched to the cable current-carrying capacity (CCC) and the expected load. Common ratings:

• 6A: Lighting circuits (1.0mm² T&E)
• 16A: Immersion heater, freezer circuit, 2.5mm² T&E lighting
• 20A: 20A ring circuit, electric shower (small)
• 32A: Ring final circuit (socket outlets), 6mm² T&E, 7.2kW shower
• 40A: Cooker/oven circuit (6mm² or 10mm²)
• 45A or 50A: Electric shower (9.5–10.8kW)
• 63A or 80A: High-power heat pump, EV charger feeder

MCB Type (Tripping Characteristic)

The tripping characteristic determines the short-circuit current at which the MCB trips instantaneously:

• Type B (3–5× In): Residential lighting and socket circuits with few inductive loads
• Type C (5–10× In): Motors, fluorescent lighting, commercial equipment with high inrush
• Type D (10–20× In): Transformers, medical equipment, heavy inductive loads

Type B is standard for domestic circuits. Type C is used for circuits with motors (heat pumps, pumps, extraction fans with high inrush current). Mismatching type (e.g., using Type D on a domestic socket circuit) may prevent the MCB from tripping fast enough on fault, creating a safety hazard.

Breaking Capacity

MCBs are rated by their ability to interrupt fault current without damage. For domestic premises:

• 3kA (M3): Minimum — only for final circuits remote from the supply where fault current is low
• 6kA (M6): Standard for most domestic installations
• 10kA (M10): Required closer to the supply origin, or where fault current calculations show prospective fault current >6kA

Calculate the Prospective Fault Current (PFC) at the consumer unit using Ze (external earth loop impedance from DNO) plus the impedance of the supply tails. If PFC exceeds 6kA, use 10kA devices or a limiting fuse upstream.

RCD Protection

RCD Types

• Type AC: Detects sinusoidal AC earth fault current only. No longer adequate for circuits supplying electronic loads (Regulation 531.3.3 — only permitted for specific circumstances)
• Type A: Detects AC and pulsating DC earth fault current. Required for circuits supplying electronic loads, inverters, EV chargers, PV inverters, Class A appliances
• Type F: Type A plus detection of high-frequency fault currents from variable-speed drives, soft starters
• Type B: AC + pulsating DC + smooth DC fault current. Required for circuits supplying certain EV chargers and medical equipment with DC fault current risk

For domestic installations: Type A is the standard minimum. Always use Type A or better when supplying circuits with switch-mode power supplies, EV chargers, inverters, or similar.

RCD Sensitivity

• 30mA (IΔn = 30mA): The standard sensitivity for socket circuits, circuits in bathrooms, circuits supplying Class I luminaires. Trips when earth leakage reaches 30mA — provides personal protection (50% of 30mA = 15mA through body is maximum safe level)
• 100mA or 300mA: Fire protection on circuits where 30mA nuisance tripping would be unacceptable — not for personal protection; do not use as the sole RCD on socket circuits
• 10mA: Enhanced personal protection — medical, agricultural, swimming pool circuits

RCBOs

An RCBO (Residual Current Breaker with Overcurrent protection) combines a Type B or C MCB and an RCD in a single 18mm DIN-rail device. Advantages:

• One device per circuit — trip only removes one circuit
• Independently selectable current rating and RCD sensitivity
• Simplifies the consumer unit — no need for split RCD busbars
• Allows mixing of Type A and Type AC protection per circuit

The width (typically 18–24mm) means an RCBO takes more DIN rail space than an MCB. A 16-way MCB board needs to become a 24-way RCBO board if full RCBO coverage is required. Plan enclosure size accordingly.

Consumer Unit Sizing: Worked Example

Planning a consumer unit for a 3-bedroom semi-detached house with standard circuits. All circuits to use 30mA Type A RCBOs:

Total: 11 circuits + 3 spares = 14-way minimum. Use a 16-way or 18-way consumer unit to allow for future growth. Add Type 2 SPD module in one way.

Main Switch Sizing

The main switch must be rated for the maximum demand of the installation. For domestic premises, the DNO typically provides a 100A service fuse at the meter. The consumer unit main switch should be 100A double-pole.

For larger installations (heat pump + EV charger + electric shower + cooker): check that diversity does not result in the 100A fuse being exceeded during simultaneous operation. If it does, upgrade the service fuse with the DNO, or use load management (demand side management) to prevent overload.

Installation: Practical Steps

Isolation and Safe Working

1. Isolate supply at the meter tails (DNO cut-out is sealed — do not open)
2. Use a voltage indicator and proving unit to confirm isolation on both supply tails
3. Apply lock-off if working on a live installation with others present
4. Do not work on the DNO supply side of the meter without DNO authorisation

Supply Tails

The meter tails between the DNO meter and the main switch must be:

• Minimum 16mm² (100A main switch) — most DNOs require 25mm² or 35mm²
• Sleeved or enclosed for protection — no exposed live conductor
• Length kept as short as possible — PFC increases with tail length
• Double-insulated or in conduit where passing through consumer unit enclosure

Earthing and Bonding

• Connect main earthing conductor from MET (Main Earth Terminal) in consumer unit to the DNO earth or TT earth electrode
• Main protective bonding: 10mm² green/yellow minimum to gas service, water service, structural steel — within 600mm of entry point where practicable
• See Earth Bonding and Equipotential Bonding guide (article #18) for bonding requirements

DIN Rail and Busbar Installation

1. Install DIN rail at correct position in enclosure
2. Fit main switch — phase and neutral connections to supply tails
3. Fit phase busbar (copper or aluminium DIN rail busbar) to link all RCBO/MCB phases
4. Fit neutral busbar — separate neutral for each RCBO required (RCBOs do not use a common neutral busbar)
5. Snap in RCBOs/MCBs
6. Connect circuit conductors — ensure correct identification (line: brown, neutral: blue, CPC: green/yellow)

Cable Entry and Identification

• Use cable glands or grommets at enclosure entry points to protect cable sheathing
• Label every way in the consumer unit with circuit description (e.g., "Kitchen sockets", "Upstairs lighting")
• Record max demand, Zs, Ik max, and test results in the Electrical Installation Certificate

Testing Before Energisation

Before connecting supply, complete the following tests (refer to Electrical Installation Testing guide, article #122):

1. Continuity of protective conductors: Every CPC from consumer unit to final circuit outlet
2. Continuity of ring final circuits: Verify ring (not spur) topology — end-to-end continuity with cross-test
3. Insulation resistance: All circuits at 500V DC — minimum 1MΩ (typically >200MΩ on a new installation)
4. Polarity check: Correct connection of L, N, and CPC at all accessories
5. Earth electrode resistance (TT systems): Measure RA to verify compliance with disconnection requirements

After energisation:

1. Earth fault loop impedance (Zs): Measure at furthest point of each circuit — verify Zs ≤ maximum permitted for the RCBO/MCB rating and type
2. RCD/RCBO operation: Test with calibrated RCD tester — trip time at 1×IΔn (30mA): ≤300ms; at 5×IΔn (150mA): ≤40ms
3. Functional check: Operate all switches, test each circuit under load
4. Prospective fault current: Measure PFC at consumer unit — must not exceed device breaking capacity

Certification

Consumer unit installation or replacement is Notifiable Work under Part P of the Building Regulations (England and Wales). This means:

• The work must be carried out by a Part P Competent Person (NICEIC, NAPIT, ELECSA, SELECT, or similar) who can self-certify, or
• A Building Notice must be submitted to the local authority before work begins, and the work inspected and tested by a third party after completion

On completion, issue the customer:

• Electrical Installation Certificate (EIC) with Schedule of Inspections and Schedule of Test Results
• Building Regulations Compliance Certificate (if self-certifying)
• Operating manuals for consumer unit and RCD test button instructions

See also: Building Regulations Part P guide (article #162) and Electrical Installation Testing (article #122).

Common Faults and Troubleshooting

• RCBO trips immediately on reset: Persistent earth fault on that circuit — test insulation resistance, isolate loads one by one
• RCBO trips on current (overload) but not RCD: Overloaded circuit — check total load, add additional circuit or uprate cable
• Nuisance RCD tripping at night: Leakage current from refrigerators, freezers, or LED drivers. Move these to dedicated 30mA circuits or use 10mA Type A RCDs on sensitive circuits
• Main switch trips: PFC exceeded or sustained overload — check diversity, measure PFC, verify main switch rating
• High earth loop impedance on one circuit: Poor CPC connection or undersize CPC — trace and repair

Recommended Products from APM

APM stocks a range of consumer units, MCBs, and RCBOs suitable for domestic split-load board installations:

Related Articles

• Earth Bonding and Equipotential Bonding (article #18)
• RCD Tripping: Diagnosing Causes and Fault-Finding on 18th Edition Boards (article #133)
• 18th Edition Wiring Regulations: Key Changes (article #90)
• Surge Protection Devices (SPDs) and AFDDs (article #96)
• Electrical Installation Testing (article #122)
• Building Regulations Part P (article #162)
• Fused Connection Units and Switched Spurs (article #159)
• Junction Boxes and Electrical Enclosures (article #131)