RCD, RCBO, MCB and RCCB Explained: What Is the Difference?
What each device protects against, why several circuits can lose power together, and how to understand an electrician’s protection-upgrade quotation.
By EICR Certificates London · Updated 1 November 2026
RCD is the umbrella term for residual-current protection; an RCCB provides this without overcurrent protection, an MCB protects against overloads and short circuits, and an RCBO combines both functions.
These names describe overlapping protective functions, rather than four unrelated products. Understanding them helps you read a consumer-unit label, an Electrical Installation Condition Report (EICR) or an upgrade quotation. EICR Certificates London explains the practical distinction: what a device detects, which circuits it serves and what still needs checking before work is specified.
RCD, RCCB, MCB and RCBO at a Glance
The key differences are residual-current detection, overcurrent protection and how the devices are arranged across circuits.
| Term | Full name | Residual-current protection | Overload and short-circuit protection | Typical role |
|---|---|---|---|---|
| RCD | Residual current device | Yes | Depends on the device | Family name covering devices with a residual-current function |
| RCCB | Residual current operated circuit-breaker without integral overcurrent protection | Yes | No; requires coordinated protection elsewhere | Often supplies several circuits with their own MCBs |
| MCB | Miniature circuit-breaker | No | Yes | Protects an individual circuit against excessive current |
| RCBO | Residual current operated circuit-breaker with integral overcurrent protection | Yes | Yes | Commonly provides combined protection for one final circuit |
Shared versus individual circuit coverage describes the installation arrangement; it is not the definition of RCD.
- Residual-current detection responds to current imbalance.
- Overcurrent protection addresses overloads and short circuits.
- Neither function protects against every electrical hazard.
What Is an RCD and How Does It Work?
An RCD detects an imbalance between currents in the live conductors and disconnects the supply when its operating conditions are met. In a typical single-phase circuit, current travels out through line and returns through neutral. Current escaping by another route, such as through a circuit protective conductor to earth, can create an imbalance.
A fixed RCD protects the wiring and equipment downstream of its position. A plug-in version covers only what is connected after it, leaving the supplying installation outside that coverage. Devices rated at 30mA are used for additional protection against electric shock, but other ratings exist.
What Are RCD-Controlled Circuits?
RCD-controlled circuits are the circuits supplied through the RCD identified by the board’s labelling. A shared RCCB can disconnect several circuits together, while others have separate protection. These devices sit within the consumer unit; What Is a Consumer Unit explains its components. Labels help identify circuit groups but cannot confirm their condition or whether the chart is accurate.
RCCB vs RCD: Are They the Same Thing?
An RCCB is one type of RCD, so the terms overlap but are not interchangeable in every context. The shared switch commonly called an “RCD” in a domestic board is often an RCCB. It needs coordinated overcurrent protection because its current-carrying rating is not an overload trip setting. That rating is distinct from the residual-current operating rating, which concerns the imbalance that triggers disconnection.
MCB vs RCD: Which Faults Do They Respond To?
An MCB responds to overcurrent, while an RCD’s residual-current function responds to current imbalance. MCB means miniature circuit-breaker.
| Situation | Relevant protective response |
|---|---|
| Overload: excessive equipment load | An MCB responds when too much current flows for its operating characteristic. |
| Short circuit: unintended low-resistance connection | The resulting high current can operate the MCB. |
| Leakage to earth | Imbalance can operate an RCD even below the current needed to trip an MCB. |
An overcurrent device can also disconnect an earth fault where the circuit protective conductor and earth fault loop impedance allow the required operation. The shorthand “MCBs protect cables; RCDs protect people” therefore misses how these measures work together.
What Does RCBO Mean, and How Is It Different from an MCB?
An RCBO combines residual-current and overcurrent protection in one device, while an MCB provides overcurrent protection alone. Its full name is residual current operated circuit-breaker with integral overcurrent protection. One RCBO commonly serves each final circuit, such as a lighting or socket circuit.
- MCB: responds to excessive current but has no imbalance-detection function.
- RCBO: adds residual-current detection alongside overload and short-circuit operation.
- Upgrade: needs a suitable product, a compatible consumer unit and verification of the circuit.
Adding RCBOs to an existing board requires assessment and pre-upgrade circuit testing. Matching a device’s width or current rating does not establish compatibility; manufacturer requirements for the complete assembly matter.
RCD vs RCBO: What Changes for the Household?
In household quotations, ‘RCD versus RCBO’ usually means shared RCCB protection with MCBs compared with individual RCBO protection.
- Shared arrangement: the RCCB detects residual current across its group; individual MCBs provide overcurrent protection. RCCB operation disconnects that group.
- Individual arrangement: each RCBO performs both functions for its own circuit, which can limit disruption when a fault occurs.
For a hypothetical example, appliance leakage on a socket circuit could operate its RCBO while a separately protected lighting circuit stays on. If both circuits instead share an RCCB, its operation would disconnect both. Upstream devices or other faults can still interrupt several circuits, so individual protection cannot ensure continuous power.
Identical residual-current ratings alone do not establish equal suitability. Changing protection also leaves the underlying fault needing attention. See RCD and RCBO protection upgrades for assessment and fitting options.
Dual-RCD vs RCBO Boards
A dual-RCD board groups circuits behind two shared RCDs, whereas an RCBO board commonly gives each final circuit its own combined protection.
| Dual-RCD arrangement | Individual RCBO arrangement |
|---|---|
| Shared RCCB A → circuit group A. Shared RCCB B → circuit group B. Each group has MCBs for its circuits. |
RCBO A → circuit A. RCBO B → circuit B. Further circuits have their own devices. |
These are conceptual groupings, not connection diagrams. Leakage from several loads can accumulate at a shared device. Circuit subdivision helps reduce the extent of an interruption and separates those contributions, although it does not eliminate faults.
Ask these four questions when comparing quotations:
- Which circuits will receive individual RCBO protection?
- Can compatible devices be added to the existing board?
- What circuit inspection and testing are included?
- Is replacement of the consumer unit proposed, and why?
Where suitable devices cannot be added, consumer unit upgrade options provide a route to a different arrangement. The decision should follow assessment of the assembly and circuits; age alone does not establish that replacement is necessary.
Does an RCBO Also Provide Arc-Fault Protection?
An RCBO does not inherently provide arc-fault detection; that requires an AFDD function. An arc fault detection device recognises characteristic electrical signatures of dangerous arcing and disconnects the affected circuit. Products can combine AFDD and RCBO functions within a consumer unit. AFDDs Explained covers their purpose and applicability.
Function distinction: residual current, overcurrent and arc-fault detection address different conditions. An ordinary RCBO should not be assumed to detect every arcing connection, and evidence of thermal damage still needs investigation.
What Do These Devices Tell You About an EICR?
The name of a protective device cannot tell you whether an installation will receive a satisfactory EICR. Suitability, physical condition, circuit circumstances and measured results all matter. Assessment uses BS 7671:2018+A4:2026; an older installation is not automatically unsafe because requirements have changed.
A device’s test button provides a functional check, rather than professional verification of its performance or the surrounding installation. Instrument testing, including RCD trip time testing, contributes to the schedule of test results. Missing protection is assessed in context: see how EICR observation codes are assessed.
Discuss RCD or RCBO Protection for Your London Property
EICR Certificates London provides RCD and RCBO protection upgrades for London properties. Qualified, insured electricians assess the proposed work, with upgrades quoted for the required scope. Free, no-obligation quotes for remedial and repair work. Open 8am to 10pm, seven days a week.
Have your EICR observations, property postcode and any existing quotation ready when you discuss your protection upgrade, so the conversation can address the circuits involved and the work being proposed.
RCD and RCBO Questions
These answers cover additional devices, RCBO selection, replacement work and a common tripping question. The appropriate arrangement depends on the installation as a whole.
Do I Need an RCD if I Use an RCBO?
An RCBO already provides residual-current protection, so another RCD is not automatically needed for that same function on the same circuit. Upstream protection may still be necessary for a sub-main or other installation requirements, particularly with a TT earthing system. The designer must consider earthing, distribution circuits and coordination between devices before deciding what remains necessary; fitting RCBOs does not justify removing upstream protection.
Do I Need an MCB if I Have an RCBO?
An appropriately selected RCBO already provides overcurrent protection, so a separate MCB is not normally needed solely to duplicate that function on its final circuit. Protection supplying the distribution board serves a different part of the installation. Ratings, available fault current, equipment compatibility and coordination still need professional design, including protection against an overloaded circuit.
What Type of RCBO Do I Need?
The appropriate RCBO depends on the circuit, connected equipment, supply arrangement and consumer-unit compatibility. Knowing the acronym does not select a product. Residual-current type, such as Type A, concerns detectable current waveforms; overcurrent characteristics concern response to excessive current. Bidirectional suitability may also matter. RCD and RCBO Types Explained covers these specifications and markings, which must be checked against the application and manufacturer’s instructions.
Can I Replace an RCD Myself?
RCD replacement should be undertaken by someone competent to isolate safely, select a compatible device and verify the completed work. Matching the old switch is insufficient, whether the replacement is an RCCB or RCBO. Incoming parts may remain live despite the main switch being off. Arrange competent electrical work, including appropriate post-alteration inspection and instrument testing; safe isolation and verification require more than visual checks.
Why Can an RCD Trip Without the MCB Tripping?
Residual current can operate an RCD even when the circuit current is too low to operate its MCB. Imbalance and excessive current are different operating criteria. A trip alone cannot distinguish appliance leakage, wiring with low insulation resistance or a defective device; electrical fault finding may be needed. For persistent operation, read what to do when an RCD keeps tripping.