Question 1

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

Accepted Answer

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.

Question 2

Are RCBOs required by BS 7671?

Accepted Answer

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.

Question 3

How much more do RCBOs cost than MCBs?

Accepted Answer

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.

Question 4

Do RCBOs reduce nuisance tripping?

Accepted Answer

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.

Question 5

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

Accepted Answer

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.

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

RCBO vs MCB + RCD — Which Protection?

Side-by-Side Comparison

When to Choose RCBOs

When MCB + RCD Works Fine

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