Summary

"RCD" and "RCBO" get used almost interchangeably on site, which causes real confusion for customers and even some newer electricians. They are not the same device, and the difference matters for both safety and fault-finding. An RCD on its own is a shock-protection device only — it has no idea whether a circuit is overloaded or short-circuited, and relying on it without a separate MCB or fuse leaves the circuit's cables unprotected against overcurrent damage. An RCBO does both jobs in one unit, per circuit, which is why modern consumer unit design has moved decisively away from RCD-plus-shared-MCBs arrangements toward full RCBO boards.

The confusion is compounded by consumer unit layout. A "split-load" board has two RCDs, each covering a bank of circuits, with each circuit also having its own MCB downstream of the shared RCD. That board has RCDs AND MCBs, but not RCBOs — a nuisance trip or genuine earth fault on any one circuit in a bank takes out every other circuit sharing that RCD, plunging half the house into darkness to find one faulty appliance. An RCBO board has one combined device per circuit, so a fault on the shower circuit trips only the shower circuit.

This article sets out exactly what each device does, how they differ in construction and function, when BS 7671 requires RCD protection at all, and the practical trade-offs — cost, discrimination, fault-finding — between split-load RCD boards and full RCBO boards for new work and consumer unit upgrades.

Key Facts

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Feature RCD (standalone) MCB (standalone) RCBO
Detects earth leakage (shock protection) Yes No Yes
Detects overload/short-circuit No Yes Yes
Protects Multiple circuits sharing the device (typically) One circuit One circuit
Product standard BS EN 61008-1 BS EN 60898-1 BS EN 61009-1
Fault isolates to Whole bank of circuits sharing the RCD Single circuit (overcurrent only) Single circuit (both fault types)
Typical use Paired with a bank of MCBs on a split-load board Overcurrent protection where earth-leakage is handled elsewhere Per-circuit protection on a full RCBO board
Approx. unit cost £15–£30 (2-pole, 63A/80A, 30mA) £8–£18 £25–£50
Nuisance-trip impact Affects every circuit in that bank N/A (overcurrent only) Isolated to one circuit

Detailed Guidance

What an RCD actually does — and doesn't do

An RCD works by comparing the current flowing out through the line conductor with the current returning through the neutral conductor, using a toroidal current transformer. In a healthy circuit these are equal and cancel to zero. If current is leaking to earth — through a person, a damaged cable, or a faulty appliance — the two currents no longer match, and once the imbalance exceeds the device's rated residual operating current (commonly 30mA for shock protection), it disconnects the circuit.

Crucially, an RCD has no mechanism for detecting overcurrent. If a circuit is overloaded (too many appliances drawing more current than the cable is rated for) or suffers a line-to-neutral short circuit, a standalone RCD will not necessarily trip — the line and neutral currents can still be balanced even while grossly excessive. This is why an RCD is never fitted alone on a final circuit; it is always paired with a fuse or MCB providing separate overcurrent protection, either as two discrete devices or combined in an RCBO.

What an RCBO adds

An RCBO houses both protection mechanisms in one module: the same residual-current detection as a standalone RCD, plus the thermal-magnetic tripping mechanism of an MCB (a bimetallic strip for sustained overload, and a solenoid/electromagnetic trip for high fault current/short circuit). The result is a single device, occupying one way on the consumer unit, that fully protects one circuit against both electric shock (earth leakage) and cable/equipment damage (overcurrent).

Because each circuit has its own RCBO, a fault on any one circuit — a nuisance trip from a faulty appliance, a genuine earth fault, an overload — disconnects only that circuit. Every other circuit in the property remains live. This directly satisfies the design principle in Regulation 314.1: circuits should be arranged to avoid danger and minimise inconvenience when a fault occurs, and to facilitate safe inspection, testing and maintenance.

Split-load boards vs full RCBO boards

Both arrangements are compliant with BS 7671 provided they're correctly designed — the choice is a specification decision, not a compliance one, in most standard domestic scenarios.

Split-load board (RCD + MCB per circuit, shared RCD per bank):

Full RCBO board:

See consumer units and consumer unit upgrade for full consumer unit specification guidance, including SPD and metal-enclosure requirements that apply regardless of RCD/RCBO choice.

When 30mA RCD (or RCBO) protection is required

Regulation 411.3.3 requires additional protection by a 30mA RCD for:

In practice, on a modern domestic rewire or consumer unit replacement, this results in 30mA protection being applied to effectively every final circuit — whether delivered via a split-load board's shared RCDs or via individual RCBOs.

Fault-finding: RCD/RCBO trip vs MCB trip

On site, the first diagnostic question after any trip is which type of device has operated, because it points to a different fault class:

Device tripped, no power
│
├─ RCD/RCBO tripped (test button will not reset until fault cleared,
│  or resets but trips again under load)
│  → Earth leakage fault: damaged cable, wet outdoor fitting,
│    faulty appliance insulation, or (rarely) genuine RCD failure
│  → Use half-split isolation method to locate:
│    isolate half the loads on the circuit, reset, repeat
│
├─ MCB tripped (mechanical trip, no earth-fault indication)
│  → Overload: too many appliances / high-draw appliance startup
│  → OR short circuit: line-to-neutral fault, damaged flex,
│    failed appliance component
│  → Check connected load before resetting; inspect for damage
│
└─ Both a shared RCD AND downstream MCBs tripped simultaneously
   (split-load board)
   → Isolate every circuit in that bank, reset RCD alone first
   → Reintroduce circuits one at a time via their MCBs to
     identify the faulty circuit

On a full RCBO board this process is far faster: the single tripped device identifies the faulty circuit directly, with no isolation exercise required across other circuits. See rcbo tripping and rcd tripping for detailed fault-tree diagnosis on nuisance vs genuine trips.

Discrimination between an incoming device and downstream RCBOs

Where an installation has an incoming RCD (e.g. at the origin, ahead of an RCBO board, or on a TT earthing system requiring an upfront RCD for fault protection), discrimination (selectivity) must be considered so the incoming device doesn't trip before the correct downstream RCBO on a fault. This typically uses a time-delayed (S-type) device upstream with a higher rated residual current than the downstream 30mA RCBOs, so the downstream device always operates first for a fault on its own circuit. Get this wrong and every fault anywhere in the property takes out the entire installation at the origin — defeating the purpose of per-circuit RCBOs entirely.

Frequently Asked Questions

Can I fit an RCBO instead of an MCB in an existing split-load board?

In many cases yes — a single-circuit RCBO can often replace one MCB on an existing split-load board to give that specific circuit independent RCD protection, provided the board manufacturer supports RCBO modules of the correct type for that consumer unit range and there's a compatible way available. This is a common upgrade for one problem circuit (e.g. an outdoor circuit causing nuisance trips that keep taking out the whole RCD bank) without replacing the entire board. Always confirm compatibility with the specific consumer unit manufacturer before ordering the part.

Is a full RCBO board a legal requirement?

No. BS 7671 does not mandate RCBOs specifically — it requires the outcome (30mA additional protection on qualifying circuits, correctly designed discrimination, and circuit division per Regulation 314.1), which a well-designed split-load board can also achieve. RCBOs are the industry's preferred method of delivering that outcome because of the fault-isolation and diagnostic benefits, not because the regulations name them specifically.

Why did my electrician recommend RCBOs over a cheaper split-load board?

Because a shared-RCD split-load board means any earth fault or nuisance trip on one circuit disconnects every other circuit sharing that RCD — commonly half the house. RCBOs isolate to a single circuit, which most customers value once it's explained (nobody wants the fridge-freezer circuit to go dead because a garden light tripped the shared RCD overnight). The material cost premium (typically £200–£400 across a full board) is usually small relative to the total consumer unit change cost — see consumer unit replacement pricing guide for current pricing.

Does an RCBO protect against the same things as an RCD plus an MCB?

Functionally, yes — an RCBO combines exactly the same two protective functions (earth-leakage detection and overcurrent protection) that a separate RCD-plus-MCB arrangement provides. The difference is packaging and scope: a standalone RCD on a split-load board is shared across a bank of circuits, each with its own MCB, whereas an RCBO gives one circuit its own dedicated instance of both protections in a single device.

Regulations & Standards