MCBs, RCDs, RCBOs and Consumer Units: A Practical Guide for Electricians

Circuit protection is one of the most critical elements of any electrical installation. The 18th Edition of BS 7671 has tightened requirements significantly — RCBOs are now standard on most circuits in new domestic consumer units, SPD (surge protection device) is required on most new installations, and the old split-load consumer unit with a single RCD covering half the circuits is effectively obsolete for new work. This guide covers the circuit protection devices in stock at APM: Axiom MCBs, RCDs, RCBOs, consumer units, and isolators, alongside CED HRS rewirable fuses for older installations.

MCBs, RCDs, and RCBOs: The Difference

Why RCBOs are now the default: Under 18th Edition, fitting individual RCBOs to each circuit means that an earth fault on a shower circuit only trips that one RCBO — not the entire upstairs ring, lighting, and everything else on that side of a split-load board. This is the required approach for most new domestic consumer unit installations. RCBOs also satisfy the SPD bonding requirements more cleanly than split-load arrangements.

MCB Type B vs Type C

All Axiom MCBs stocked at APM are Type B. The type refers to the magnetic (instantaneous) trip threshold:

• Type B: Trips at 3–5× rated current. For resistive loads: lighting circuits, ring mains, immersion heaters, general socket circuits. The correct type for the vast majority of domestic circuits.
• Type C: Trips at 5–10× rated current. For motor loads and equipment with high inrush current: pumps, air conditioning, compressors. Only specify Type C where the load specifically requires it — Type B gives better fault discrimination on resistive circuits.

All Axiom MCBs have 6kA breaking capacity — correct for domestic and light commercial installations where the prospective short-circuit current (PSCC) does not exceed 6kA. Check the PSCC at the distribution board before selecting MCBs; installations with high PSCC (close to the network supply point) may require 10kA devices.

Axiom MCBs — Type B, 6kA, Single Pole

• AXIOM 6A SP MCB Type B — lighting circuits (final circuit up to 6A load)
• AXIOM 10A SP MCB Type B — lighting circuits, small socket circuits
• AXIOM 16A SP MCB Type B — 2.5mm² ring circuit spur, 16A dedicated circuit (shower pump, small cooker)
• AXIOM 20A SP MCB Type B — 20A radial socket circuit (BS 7671 Table 4A2)
• AXIOM 32A SP MCB Type B — 32A ring main, 7.2kW electric shower
• AXIOM 40A SP MCB Type B — 40A cooker circuit, large shower, EV supply (where overload protection is provided)
• AXIOM 50A SP MCB Type B
• AXIOM 63A SP MCB Type B — sub-main and sub-distribution applications

Axiom RCDs

Double-pole 30mA RCDs for use as main protection in split-load consumer units, or for general fault protection on a sub-board where individual RCBOs are not being used. A 30mA RCD provides the minimum protection level required by BS 7671 for socket circuits and bathroom circuits.

• AXIOM 40A DP RCD 30mA — 2-module. For sub-boards and split-load consumer units protecting circuits up to 40A total load
• AXIOM 80A DP RCD 30mA — 2-module. For consumer units with higher total load, or where the incoming supply is 80A

Note: RCDs are 2-module devices. Plan the busbar space in the consumer unit accordingly — a 40A RCD protecting 6 MCBs occupies 2 ways, leaving 6 ways for MCBs in an 8-way board.

Axiom RCBOs — Type A, 1-Module, Single Pole

Type A RCBOs detect both AC residual current and pulsating DC residual current. BS 7671 18th Edition Amendment 2 (effective from October 2022) requires Type A RCBOs on circuits that may carry DC-component leakage — this includes EV charging points, inverter-controlled appliances, LED drivers, and circuits with variable speed drives. Type A is now the practical default choice for new domestic consumer units.

Axiom mini RCBOs are 1-module (single pole) — each occupies only one way in the consumer unit, the same as an MCB. This allows a full 12-way consumer unit to be fitted with 12 individual RCBOs, giving independent fault protection on every circuit.

• AXIOM RCBO 6A SP 1 Module Type A
• AXIOM RCBO 16A SP 1 Module Type A
• AXIOM RCBO 20A SP 1 Module Type A
• AXIOM RCBO 32A SP 1 Module Type A

Consumer Units

Axiom 12-Way 100A Consumer Unit

• Axiom Consumer Unit 12 Way 100A Main Switch

12 MCB ways. 100A DP main switch. White dual-RCD metal enclosure. BS EN 61439-3 compliant. Suitable for new domestic installations under BS 7671 18th Edition. Accepts Type B MCBs or Type A mini RCBOs — or a combination. The 18th Edition requires SPD on all new consumer unit installations; an SPD can be fitted to one way of this board. Note: MCBs and RCBOs are sold separately.

Axiom Metal Consumer Units — Sub-Boards and Outbuildings

• Axiom Metal Consumer Unit 4 Way 100A Main Switch — for small sub-distributions: garages, sheds, annexes. 4 ways for lighting, sockets, and one or two dedicated circuits. Fed from the main consumer unit via a 4-way isolator or dedicated MCB. Metal enclosure for robustness in non-domestic environments.
• Axiom Metal Consumer Unit 8 Way 100A Main Switch — for larger outbuildings and workshops with multiple circuits. 8 ways: lighting, general sockets, dedicated machine supply, EV charging, security, and two spare ways for future extension.
• Axiom Metal Consumer Unit Blank — blanking plate for unused MCB ways. Required by BS 7671 to prevent access to live busbars through vacant slots.

Isolators

100A DP Isolators

• AXIOM 100A DP Isolator — 2-pole double-throw isolator for use as a main switch or for isolating sub-boards. Fits standard 35mm DIN rail. For domestic main isolators, solar PV disconnects, and outbuilding sub-boards.
• Axiom 100A DP Isolator with Safety Enclosure — pre-mounted in an IP-rated enclosure for surface mounting where DIN rail is not being used. Suitable for outbuilding or meter position installation.

4-Pole Rotary Isolators

For 3-phase machinery disconnection in workshops and light industrial premises. BS EN 60947-3 compliant. The rotary handle locks in the OFF position for safe maintenance isolation (lockout/tagout).

• Axiom 25A 4-Pole Rotary Isolator 11kW 415V
• Axiom 32A 4-Pole Rotary Isolator 15kW 415V

RCD Protected Socket

Where a socket outlet needs integral RCD protection — for example a garage socket, garden socket, or bathroom shaver supply — an RCD socket provides 30mA protection within the socket faceplate without using a consumer unit way.

• Axiom 2G 13A 30mA RCD Switched Socket — passive/latching type. Provides individual socket-level RCD protection. Suitable for use in garages and outbuildings per BS 7671 Regulation 701.411.3.

CED HRS Rewirable Consumer Unit Fuses

HRS (High Rupture Capacity/Semi-enclosed rewirable) fuses are still found in older consumer units installed before the widespread adoption of MCBs. They are not appropriate for new installations but are a common service call item — a blown rewirable fuse must be replaced with the correct rating and fuse wire, never a substitute current rating.

• CED HRS Consumer Unit Fuse 5A — lighting circuits
• CED HRS Consumer Unit Fuse 15A — older immersion heater and ring circuits
• CED HRS Consumer Unit Fuse 20A
• CED HRS Consumer Unit Fuse 30A — ring main and 30A cooker circuits in old installations

18th Edition Compliance Checklist

For any new consumer unit installation under BS 7671 18th Edition (including Amendment 2, 2022):

1. Metal enclosure required. Consumer units in domestic premises must be metal-clad (Regulation 421.1.201).
2. SPD required. A Type 2 surge protection device must be installed at the origin of every new installation, or where the consumer unit is replaced (Regulation 443.4).
3. RCD/RCBO protection. All socket outlets up to 32A in domestic premises must have RCD protection (30mA). Type A RCBOs recommended for circuits with non-linear loads (EV chargers, inverter appliances).
4. Blanking plates. All unused ways must be blanked.
5. Main switch rating. The 100A main switch must be rated at or above the maximum demand of the installation.

All Axiom circuit protection devices and CED HRS fuses are in stock at APM's Acton trade counter — available for same-day collection.

APM Plumbing & Electrical | 24 Western Avenue, Acton, London W3 7TZ | 020 8702 8080 | apmi.uk