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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.
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How We Calculate This
Frequently Asked Questions
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.
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.
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.
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.
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.
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Last updated: April 2026
Verified against UK standards · estimates only, confirm with your supplier.