Three devices, three jobs
- MCB — a circuit breaker. Protects the cable against overload and short circuit. Blind to leakage: a person touching a live conductor draws far less current than the breaker needs.
- RCD — a residual current device. Compares the current flowing out with the current coming back; if the difference exceeds its rating, typically 30 mA, it disconnects. Blind to overload: it does not care how much current flows, only that it comes back.
- RCBO — both in one module. Overload, short circuit and leakage for a single circuit.
Neither an MCB nor an RCD can do the other's job, which is why every board needs both functions in some arrangement.
Shared RCD or one RCBO per circuit?
This is the real decision, and it is about how much goes dark when something fails.
A shared RCD sits ahead of a group of MCBs. It is cheaper, takes less width, and is the classic split-load consumer unit. The price is collateral damage: leakage anywhere in the group disconnects the whole group. A failing kettle takes out the freezer, the router and half the lights.
An RCBO per circuit confines the fault to the circuit that caused it. Everything else keeps running, and fault-finding takes minutes instead of an evening. It costs more per circuit and usually one module width per circuit.
Why leakage adds up
Every appliance with electronics leaks a small current to earth through its filters — a fraction of a milliamp each, entirely normal. Put a dozen such circuits behind one 30 mA RCD and the standing leakage can reach half the trip threshold before anything is even wrong. Add a damp morning outdoors and the RCD trips with no fault to find.
This is the practical argument for splitting: not elegance, but keeping the accumulated leakage per device low. If you keep a shared RCD, spread the circuits across at least two of them and never put all the lighting behind one.
Circuits that deserve their own device
- Freezer and fridge — nobody wants to discover a trip after a holiday.
- Home office, network equipment, heating controls.
- Outdoor sockets, garden and garage circuits, which are the most likely to leak.
- Anything with a big motor or heating element that may age badly.
Type matters as much as the letters
An RCD only detects the waveform it is built for. Electronics, inverters, EV chargers and some appliances produce leakage that an ordinary type AC device cannot see. Which type belongs where is a topic of its own — see RCD types.
Common mistakes
- Fitting an RCD and assuming the circuit is now fully protected — overload protection still has to come from an MCB or an RCBO.
- Sharing a neutral between two circuits behind different RCDs, which trips them at random.
- Replacing an RCBO with an MCB "because it kept tripping". The tripping was the message.
What PlanMyPanel does
The generator groups ordinary circuits under shared RCDs, splits those groups so the standing leakage stays low, and gives dedicated RCBOs to circuits where a trip would be expensive. Every group is then checked against the rules of your selected profile, and anything unusual is flagged in the project.