Three devices, different jobs
- An RCCB (residual-current circuit breaker) detects only residual current — leakage to earth — and has no overload or short-circuit protection, so it always needs an MCB or fuse.
- An RCBO combines an RCCB and an MCB in one device for one circuit: it trips on leakage, overload and short circuit.
The key difference is the scope of disconnection: a leak under a shared RCCB switches off every circuit behind it, while an RCBO switches off only its own circuit.
RCD is an umbrella term for residual-current protection, including RCBOs. Here RCCB means a device without built-in overcurrent protection, MCB means a miniature circuit-breaker, and RCBO combines both functions.
Circuit-breaker · MCB
Protects wiring against overload and short circuit. A small leakage current may be far too low to trip it.
B16 means curve B and 16 A rated current. That marking alone does not identify every function of a device.
Residual-current breaker · RCCB
Responds to an imbalance between outgoing and returning currents. It has no built-in overload or short-circuit protection and needs a coordinated breaker or fuse.
40 A is its rated current under specified conditions; 30 mA is residual-current sensitivity. 40 A does not mean it trips an overload itself.
Combined protection · RCBO
Combines residual-current and overcurrent protection. In this example each RCBO protects one outgoing circuit.
B16 + 30 mA combines the breaker curve and rating with leakage sensitivity. Residual type A/AC/F/B is separate. Both RCCBs and RCBOs can have TEST buttons.
| Built-in protection | MCB | RCCB | RCBO |
|---|---|---|---|
| Suitable residual-current waveform | No | Yes | Yes |
| Overload | Yes | No | Yes |
| L–N short circuit | Yes | No | Yes |
Contents
What switches off when a fault occurs?
Choose an event
Which circuit?
The selected circuit develops leakage of a waveform and magnitude sufficient to operate residual-current protection.
Shared RCCB + MCBs
- 1. Washing machineNo power
- 2. RefrigeratorNo power
- 3. LightingNo power
The common RCCB trips and all three circuits lose power. Their MCBs may remain closed.
One RCBO per circuit
- 1. Washing machineNo powerTripped
- 2. RefrigeratorPower available
- 3. LightingPower available
The faulty circuit RCBO trips. The other two circuits retain power in this arrangement.
Educational functional diagram, not wiring instructions. Lines show protection relationships; L, N, PE, the incomer and other devices are omitted. Protection is assumed correctly selected and coordinated. A real short circuit or non-selective arrangement can also trip upstream protection. Trip times are not simulated.
Compare the two arrangements
Extent of the outage
Shared RCCB + MCBs
One leakage fault disconnects the entire group. Normal leakage from multiple appliances also accumulates.
One RCBO per circuit
Residual-current protection is separate per circuit. This helps keep healthy circuits powered but cannot prevent shared causes of outages.
DIN-rail space
Shared RCCB + MCBs
Example for three single-phase circuits: a 2-module RCCB + three 1-module MCBs = 5 modules.
One RCBO per circuit
Three 2-module RCBOs = 6 modules; compact 1-module versions = 3. Use actual series, poles and widths, allowing for the incomer and other equipment.
Cost
Shared RCCB + MCBs
Sharing one RCCB often reduces equipment cost. Compare the actual bill of materials, installation and maintenance.
One RCBO per circuit
A combined device per circuit can cost more. There is no universal price difference: series, residual type and ratings matter.
Finding the fault
Shared RCCB + MCBs
A common RCCB trip requires identifying the faulty circuit. Its MCB handle usually does not identify a leakage fault.
One RCBO per circuit
The tripped device identifies the circuit, not necessarily the cause. Dedicated leakage indicators are model-dependent; consult the manual.
Three mistakes to avoid
Treating a 40 A RCCB as a 40 A MCB
Its rating is an allowable current, not an overload trip threshold. RCCB overload and short-circuit protection must be coordinated; three 16 A MCBs alone do not guarantee protection of a 40 A RCCB.
Mixing neutrals from separate protected circuits
Each circuit needs the intended L and N path through its residual protection. Joining neutrals after different RCCBs or RCBOs can cause unwanted trips and an incorrect arrangement. Have an electrician verify the wiring.
Assuming an RCBO protects against everything
It does not replace PE, surge protection or arc-fault protection, or make touching live parts safe. An upstream RCD may still be required; verify compatibility and selectivity.
Choosing for your distribution board
- 1.
Identify circuits that should not fail together: lighting, refrigeration, communications and workspaces. Separation limits a single fault, but is not backup power.
- 2.
Compare complete assemblies: current, B/C/D curve, residual type, IΔn, breaking capacity, poles, width, busbars and cost. Correctly selected RCCB + MCB protection also provides both functions.
- 3.
Have an electrician coordinate cables, incomer, earthing, neutrals, selectivity and RCCB protection. Carry out the required inspection and tests after installation.
Frequently asked questions
Does an RCBO protect better than an RCCB plus MCB?
Both correctly selected arrangements can provide the required functions. Here the main difference is shared versus individual residual protection and the extent of an outage, not simply one housing versus two.
Can I replace an MCB with an RCBO of the same rating?
Matching the number is insufficient. Cable, curve, residual type and sensitivity, breaking capacity, poles, neutral and board compatibility need checking. It changes the protection arrangement.
Does an RCBO need another MCB after it?
Its circuit already has built-in MCB functions. Incoming and backup protection, manufacturer conditions and short-circuit coordination remain part of the design.
Can I tell them apart by appearance?
Size and TEST are not decisive. Check the full model, markings and documentation: RCCBs typically follow IEC/EN 61008, RCBOs IEC/EN 61009.
Is one RCD for the whole panel enough?
One main RCD trips everything on a single leak: lights, fridge and sockets go dark together. Splitting the circuits between several RCDs or giving key circuits RCBOs limits the outage. What is required depends on national rules; your electrician decides the final arrangement.
Sources and further reading
- Schneider Electric — RCCB and RCBO
- Schneider Electric — Form factors of RCDs
- IET — Division of installation (Wiring Matters 91)
- ABB — DS301C and DS201: module widths
- Schneider Electric — Coordination of RCDs
- How an RCD detects leakage, step by step
- RCD types AC, A, F and B compared
- MCB ratings and B, C, D trip curves
- Selective RCDs: main and group RCD coordination