RCBO vs RCD + MCB: Which to Use and When
The two approaches to earth fault + overcurrent protection — and how the choice affects nuisance tripping, panel size and cost.
The two approaches
RCBO
Residual Current Breaker with Overcurrent protection — MCB + RCD combined in one 1-module device.
- ✓ 1 DIN module per circuit
- ✓ Fault trips only that circuit
- ✓ Independent per circuit
- ✗ Higher cost per circuit
- ✗ Limited to 1-pole (for 1ph circuits)
RCD + MCB (2-tier)
One upstream 30mA RCD protecting multiple MCBs downstream.
- ✓ Lower cost per circuit
- ✓ Standard approach for large boards
- ✗ One fault trips all circuits on that RCD
- ✗ 2 modules per group + 1 MCB each
- ✗ Nuisance tripping risk
The nuisance tripping problem
Every circuit has a small amount of natural earth leakage current — through capacitive coupling in cables, EMC filters in appliances, and LED driver interference suppression. A typical circuit leaks 0.5–2 mA.
A 30mA RCD protecting 8 circuits can accumulate 8 × 2 mA = 16 mA of natural leakage — half the trip threshold — before any fault occurs. A single additional leaky appliance tips it over 30 mA and trips all 8 circuits simultaneously.
⚠️ Maximum circuits per 30mA RCD
Practical maximum: 6–8 circuits per 30mA RCD in residential. In commercial with more electronic loads, limit to 4–6 circuits per RCD to avoid nuisance tripping.
With individual RCBOs, a leaky appliance on one circuit trips only that RCBO — the other 7 circuits continue operating. This is the main argument for RCBOs in modern panels, especially with high-density LED and electronic loads.
DIN rail space comparison
| Configuration | For 12 circuits | DIN modules used |
|---|---|---|
| 12× RCBO 1P (1 module each) | 12 individual RCBOs | 12 modules total |
| 2× RCD (2P, 2 mod) + 12× MCB (1 mod) | 2 RCDs + 12 MCBs | 16 modules total |
| 3× RCD (2P, 2 mod) + 12× MCB (1 mod) | 3 RCDs + 12 MCBs (better selectivity) | 18 modules total |
| 1× RCD (2P, 2 mod) + 12× MCB (1 mod) | 1 RCD + 12 MCBs (not recommended) | 14 modules total |
RCBOs use slightly fewer DIN modules for the same protection level — but cost more per unit.
When to use each approach
Use RCBOs when:
- →Continuous availability is critical (server rooms, critical equipment)
- →High density of electronic loads (LED driver heavy circuits)
- →Commercial projects where a tripped circuit affects multiple tenants
- →Client specifically requires maximum selectivity
- →Panel is very full and DIN space is the constraint (RCBOs save space)
Use RCD + MCBs when:
- →Standard residential distribution board (cost-sensitive)
- →Circuits grouped logically (all bedroom sockets on one RCD, all kitchen on another)
- →Budget is the primary concern — significant cost saving at scale
- →Commercial panels with low-leakage loads (resistive heaters, simple lighting)
- →Large panels where per-circuit RCBOs would require many more modules
Selectivity (discrimination) in practice
For a large commercial panel (e.g. 3-phase, 32 circuits), a common design uses:
Main incomer: 300mA time-delayed RCD (S-type) — fire protection, 400ms delay
Sub-incomer per floor/zone: 100mA time-delayed RCD — zone isolation
Final circuits: 30mA instantaneous RCBO — personal protection, individual selectivity
This 3-tier approach ensures a fault on one circuit trips only the final RCBO. The 100mA RCD only trips if the 30mA RCBO fails (backup). The 300mA incomer RCD is last resort — prevents building-wide blackout.
Distribution board designed for your project
RCBO, RCD selectivity, DIN space optimisation — all part of our panel design service.
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