RCBO versus separate RCD + MCB — the practical trade-off

The 531-rcd guide covers how to distinguish a residual-current device (RCD) from an RCBO on a diagram or in a distribution board. This article covers the question that comes before that: when should you choose an RCBO per circuit, and when a shared RCD upstream of several MCBs?

Two arrangements, the same protection goal

RCBO (residual current circuit breaker with overcurrent protection): residual-current protection (earth fault) and overcurrent protection (overload/short circuit) combined in one device, per individual circuit.
Shared RCD upstream of several MCBs: one RCD monitors the combined leakage current of several circuits, while each circuit has its own, separate MCB for overcurrent protection.

Both arrangements meet the same protection requirement (residual-current and overcurrent protection per circuit), but differ fundamentally in what happens during a fault, and in cost and panel space.

Difference 1 — how outage spreads during an earth fault

With an RCBO per circuit, an earth fault trips only the affected circuit — other circuits remain live. With a shared RCD upstream of, say, four to eight MCBs, an earth fault in one of those circuits trips the entire shared RCD, taking down all circuits behind it simultaneously — including the fault-free circuits. In a residential installation this could mean, for example, that an earth fault in a garden-lighting circuit also takes down the router, the fridge, and the lighting on other, healthy circuits.

Difference 2 — cost and panel space

A shared RCD with separate MCBs is generally cheaper per circuit than an RCBO per circuit, and the shared RCD itself takes up relatively little extra panel space relative to the number of circuits it monitors. RCBOs per circuit are more expensive in total (each circuit gets its own, complete device) and typically take up more width per circuit in the board than a separate MCB alone.

Difference 3 — fault diagnosis and replacement

After an RCBO trips, the faulty circuit is identified immediately (the tripped device points to the circuit). After a shared RCD trips, every connected circuit must be tested individually to find the faulty one, because the RCD itself does not indicate which of the downstream circuits caused the fault. In addition, for a faulty device: an RCBO where one of its two functions (residual-current or overcurrent) fails must be replaced entirely; with the separate arrangement, the faulty function can be replaced on its own.

Note: this article covers the arrangement choice (one device per circuit versus shared), not the type of residual-current protection (AC/A/F/B) — see the RCD type classification guide for that separate decision. Both an RCBO and a separate RCD are available in types AC/A/F/B.

Practical relevance

When designing a distribution board, it must be explicitly weighed how many circuits behind one shared RCD is acceptable — the more circuits shared, the lower the cost per circuit, but the greater the impact (number of circuits tripped simultaneously) during an earth fault in one of them. For critical or independent loads (for example a fridge, a server room, or a circuit that must not trip together with others), an RCBO per circuit is often the better choice, even though it costs more.

Common mistakes

Putting all circuits behind one shared RCD "to save cost", without assessing which circuits need to keep functioning independently of each other during an earth fault.
Assuming an RCBO trip requires the same diagnostic process as a shared RCD trip — an RCBO points directly to the faulty circuit, a shared RCD does not.
Confusing the arrangement choice (RCBO versus shared) with the type choice (AC/A/F/B) — these are two independent decisions that must both be made separately.