RCBO vs RCD vs MCB: What’s the Difference?
An MCB handles overcurrent, an RCD handles earth faults, and an RCBO combines both in a single device — the differences decide how a consumer unit is laid out.
Three devices with similar-looking modules do very different jobs. Confusing them leads to boards that either don’t protect against the right faults or trip a whole floor of the house when one appliance develops a leak to earth. Knowing what each protects against is the key to a sensible consumer unit design.
This guide explains exactly what an MCB, an RCD and an RCBO each protect against, how split-load and all-RCBO boards are arranged, and why fitting individual RCBOs is now the usual way to avoid nuisance trips taking out unrelated circuits.
Key takeaways
- An MCB protects against overcurrent — overload and short-circuit — but not earth-leakage faults.
- An RCD protects against earth-leakage (residual) current and shock, but not against overload or short-circuit.
- An RCBO combines both functions in one module: overcurrent and residual-current protection together.
- A split-load board shares one RCD across several MCBs; an RCBO board gives each circuit its own protection.
- Individual RCBOs stop one earth fault tripping a whole bank of circuits, so a single fault is contained.
What each device protects against
An MCB (miniature circuit breaker) protects the cable against overcurrent — both slow overloads via its thermal element and short-circuits via its magnetic element. It does nothing about current leaking to earth, because it only measures the current in the line conductor passing through it.
An RCD (residual current device) does the opposite: it compares the current flowing out in the line with the current returning in the neutral, and trips if they differ by more than its rating — typically 30 mA for shock protection. It protects people against electric shock and reduces fire risk from earth leakage, but it provides no overload or short-circuit protection at all.
| Device | Overcurrent protection | Earth-fault / residual protection |
|---|---|---|
| MCB | Yes | No |
| RCD | No | Yes |
| RCBO | Yes | Yes |
Why you often need both
Because an MCB and an RCD protect against different faults, most modern circuits need both kinds of protection. A socket circuit, for example, needs overcurrent protection to stop the cable overheating and RCD protection to guard against shock — neither device alone covers both hazards.
That leaves two ways to provide both: put an RCD upstream of a group of MCBs, or use a single RCBO per circuit that does both jobs. How you combine them is what distinguishes a split-load board from an all-RCBO board.
Split-load boards vs RCBO boards
A split-load consumer unit uses one or two RCDs, each feeding a bank of MCBs. Every circuit on that bank shares the single RCD, which is economical but has a drawback: an earth fault anywhere on the bank trips the RCD and kills every circuit on it. A fault on one socket circuit can take out the lights, the fridge and the freezer at the same time.
An all-RCBO board replaces each MCB-plus-shared-RCD arrangement with an individual RCBO per circuit. Each circuit then has its own overcurrent and residual-current protection, completely independent of its neighbours. It costs more in devices but gives far better discrimination.
Fault containment matters on the certificate
When you record circuit details on a TradePlanr EIC or EICR, the protective device type is captured per circuit — so an all-RCBO board shows each circuit’s independent protection clearly, which is exactly what a reviewer wants to see.
Why RCBOs avoid nuisance trips
The big practical advantage of individual RCBOs is fault containment. On a shared RCD, the small earth-leakage currents of every appliance on the bank add up, so the combined standing leakage can sit close to the trip threshold and a single extra fault tips it over — nuisance-tripping the whole bank. Splitting the circuits onto separate RCBOs keeps each circuit’s leakage on its own device.
It also means a genuine fault only disconnects the affected circuit. Lose one RCBO and the freezer, the lights and the boiler keep running — a real benefit in a home or a rented property where a tripped board discovered hours later can spoil food or leave someone in the dark. This is why RCBO boards have become the default choice for new consumer units.
Standing leakage adds up
A shared 30 mA RCD trips at well below 30 mA of combined leakage, and modern electronics each leak a little. Spread enough circuits across one RCD and normal standing leakage alone can cause unexplained trips — individual RCBOs cure it.
Frequently asked questions
Can an RCD replace an MCB?
No. An RCD only detects earth-leakage current — it provides no protection against overload or short-circuit. A circuit still needs an MCB (or a device that includes overcurrent protection, such as an RCBO) to protect the cable. An RCD and MCB together cover both fault types, which is what an RCBO combines in one module.
Is an RCBO better than a split-load board?
For fault containment, yes. On a split-load board an earth fault trips the shared RCD and everything on that bank. With individual RCBOs, a fault only disconnects the affected circuit, so unrelated circuits keep running. RCBO boards cost more but are now the usual choice for new consumer units.
Why does my whole board trip when one appliance faults?
Because those circuits share a single RCD on a split-load board. Any earth fault on the bank — plus the combined standing leakage of every appliance on it — trips the one RCD, killing all the circuits it feeds. Moving to individual RCBOs isolates each circuit’s protection and stops it.
From guidance to action
Related guides
RCD Types: AC, A, F and B Explained
The difference between Type AC, A, F and B RCDs and when each one is required.
MCB Types B, C and D Explained
How Type B, C and D tripping curves differ, the multiples of In that trip them, and how to pick the right one.
RCD Keeps Tripping: Causes and Fixes
A structured method for tracing why an RCD trips — from nuisance tripping to real faults.
