What is the main advantage of RCBOs over MCBs with shared RCDs?

RCBOs provide individual circuit protection — if one circuit develops a fault, only that circuit trips. With a shared RCD protecting multiple MCBs, a fault on any single circuit trips the RCD and kills power to all circuits on that side of the board, including lighting circuits.

Are RCBOs required by BS 7671?

BS 7671 18th Edition Amendment 2 does not mandate RCBOs specifically, but it does require RCD protection for most circuits (30mA for sockets, mobile equipment and cables in walls). You can achieve this with shared RCDs or individual RCBOs. However, Regulation 314.1 recommends that a single fault should not cause loss of the entire installation.

How much more do RCBOs cost than MCBs?

A single RCBO typically costs £15-25, while an MCB costs £5-8 and a 2-pole RCD costs £25-40. For a 10-circuit board, full RCBOs cost roughly £150-250, while 10 MCBs plus 2 RCDs cost around £100-130. The premium is modest for the improved fault discrimination you get.

Do RCBOs reduce nuisance tripping?

Yes, significantly. With shared RCDs, cumulative earth leakage from multiple healthy circuits can add up and cause nuisance tripping. Each RCBO only monitors its own circuit, so small normal leakage currents from appliances on other circuits are irrelevant. This is especially beneficial where LED lighting drivers or electronic equipment cause minor leakage.

Can I mix RCBOs and MCBs in the same consumer unit?

Yes. A common approach is to use RCBOs for critical circuits (freezer, alarm, smoke detectors) and MCBs under shared RCDs for less critical circuits. Dual RCD split-load boards can also have individual RCBOs retrofitted in place of MCBs on either side.

RCBO vs MCB + RCD — Which Protection?

Compare per-circuit RCBO protection against shared MCB and RCD arrangements for UK consumer units under BS 7671.

Side-by-Side Comparison

Feature	RCBO (per circuit)	MCB + Shared RCD
Cost per circuit	£15-25	£5-8 MCB + share of £25-40 RCD
Fault isolation	Only faulted circuit trips	All circuits on that RCD trip
Nuisance tripping	Minimal — monitors one circuit	Higher risk — cumulative leakage
Board space	1 module per circuit	1 module per MCB + 2 for RCD
Fault finding	Tripped RCBO identifies circuit	Must isolate each MCB to find fault
BS 7671 compliance	Fully compliant	Fully compliant
10-circuit board cost	£150-250 total	£100-130 total

When to Choose RCBOs

Critical circuits that must stay live — freezers, alarms, medical equipment
Commercial or HMO installations where loss of supply affects multiple occupants
Installations with high earth leakage equipment (LEDs, IT equipment, VFDs)
Where the client wants maximum reliability and easy fault identification
New-build projects where the marginal cost increase is small relative to total spend

When MCB + RCD Works Fine

Budget-conscious domestic rewires with standard loads
Simple installations with few circuits (e.g. garage or outbuilding)
Where split-load with 2 RCDs provides adequate discrimination
Temporary or non-critical installations

Related Calculators

Use the Circuit Breaker Sizing Calculator to determine correct ratings, or the Consumer Unit Sizing Calculator to plan your board layout.

How We Calculate This

Frequently Asked Questions

Last updated: April 2026

All calculations are estimates. Verify with your supplier.

RCBO vs MCB + RCD — Which Protection?

Side-by-Side Comparison

When to Choose RCBOs

When MCB + RCD Works Fine

Related Calculators

How We Calculate This

Frequently Asked Questions

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