RCD vs RCBO vs MCB: Differences Explained for UK Homes

The difference at a glance

The simplest way to tell these devices apart is to look at the type of fault they are designed to interrupt. The names are not interchangeable, even though they are frequently grouped together as breakers in everyday conversation.

• MCB (miniature circuit breaker): disconnects a circuit when an overload or short circuit produces excessive current. It does not detect residual current by itself.
• RCD / RCCB (residual current device / residual current circuit breaker): detects an imbalance between outgoing and returning current. A standard RCCB does not provide its own overload or short-circuit protection.
• RCBO (residual current breaker with overcurrent protection): combines residual-current and overcurrent protection, normally on an individual final circuit.

One important detail is often missed: RCD is the broad family name. An RCCB is an RCD without built-in overcurrent protection, while an RCBO is also a type of RCD, with overcurrent protection added. The IET's consumer-unit and protective-device guidance makes this distinction clear.

30 mA is the residual operating current; it is not the current the device can carry.

What an MCB protects, and what it misses

An MCB is primarily there to protect a circuit and its wiring from excessive current. A sustained overload can arise when the demand on a circuit is too high; a short circuit can produce a much larger fault current. The breaker must be selected to suit the cable, circuit conditions and prospective fault current, not simply the appliances someone plans to plug in.

Consider a socket circuit carrying more current than its design allows. The MCB may operate to prevent damaging overheating, but it cannot recognise a small current leaking to earth merely because it is a potentially dangerous leakage. That is why an MCB alone does not provide the additional residual-current protection required in many modern domestic circumstances.

If the same breaker keeps operating, repeatedly switching it back on is not a repair. Our guide to repeated breaker trips deals with the causes and safe next steps. Increasing an MCB rating to stop trips, without assessing the circuit, can remove a critical part of its protection.

Two devices that look alike and fit the same width are not necessarily compatible.

How an RCD or RCCB detects current leakage

An RCD compares the current flowing out on the live conductor with the current returning on the neutral. If enough current flows by another route, the imbalance can cause it to disconnect the supply. In a domestic consumer unit, an RCCB often provides residual-current protection to a group of circuits, each also protected against overcurrent by its own MCB.

You may see 30 mA marked on a device used for additional protection. That is its rated residual operating current, expressed in milliamps. It is not the amount of current the device can continuously carry, and it does not mean that every possible electric shock is prevented. An RCD cannot detect every dangerous situation; for example, contact between live and neutral can leave the outgoing and returning currents balanced.

An RCCB may also carry a marking such as 63 A or 80 A. This is a current-carrying rating, not an overload trip threshold. The RCCB needs coordinated overcurrent protection, as the IET's guidance on RCCB ratings and overload protection explains. A larger number printed on the device must never be treated as a reason to replace another component without proper design checks.

What an RCBO adds to the protection

An RCBO combines two protective functions in one device: it can operate for residual-current faults and for overcurrent faults within its specified characteristics. In many consumer units, one RCBO serves one final circuit, such as kitchen sockets or upstairs lighting. That arrangement can help keep unrelated circuits energised if a fault is confined to a single circuit.

However, an RCBO is not an automatic diagnosis tool. When it trips, the cause may be leakage, overload, a short circuit, connected equipment or a fault in the installation. The position of the switch alone cannot establish which condition caused it. An electrician may need to test the circuit and device to determine what happened.

Neither an RCBO nor any other protective device can compensate for damaged cable insulation, poor connections, an incorrect earthing arrangement or an unsuitable circuit design. Protection is specified as part of the installation, rather than selected from a photograph or just the price of a breaker.

Why one fault sometimes switches off several rooms

Imagine a consumer unit where a single RCCB protects several MCB circuits. If a faulty outdoor light causes enough residual current to operate that RCCB, the sockets and lighting supplied through the same device may lose power together. The individual MCBs do not necessarily move, because the condition that caused the trip was leakage rather than excessive current on those circuits.

Now imagine that each circuit has its own correctly specified RCBO. A leakage fault confined to the outdoor-lighting circuit would ordinarily disconnect that circuit rather than all the circuits previously sharing one RCCB. The result is less disruption, although upstream protection, installation design and the precise fault can still affect what switches off.

This is one practical advantage of individual-circuit RCBO protection. It does not mean every older split-load consumer unit is unsafe or that an immediate replacement is necessary. An electrician needs to assess the whole installation before advising whether an upgrade is worthwhile.

How to understand common markings on the devices

The letters and numbers on a device describe different properties. They are useful when discussing an installation with an electrician, but they are not instructions for changing or testing components yourself. Labels vary by manufacturer and some devices have more than one rating.

B32 on an MCB is not the same as Type B on an RCD

A marking such as B32 on an MCB typically indicates a Type B overcurrent operating characteristic and a 32-ampere rated current. The B in that marking describes how the breaker responds to overcurrent; it is unrelated to a Type B residual-current device. Confusing those two meanings can lead to a completely wrong discussion about protection.

30 mA and 63 A on an RCCB describe different things

A label showing 30 mA identifies the rated residual operating current. A marking such as 63 A identifies the device's rated current capacity. Neither figure, on its own, tells you whether the installation has adequate overload protection, whether circuits have been divided suitably or whether the device is the correct type for the connected loads.

RCD Types AC, A, F and B relate to residual-current behaviour

Some modern appliances and electronics can produce residual-current waveforms that differ from a simple sinusoidal AC fault. RCD Type AC, A, F and B refer to the types of residual current a device is designed to recognise, not to its current rating or the circuit-breaker trip curve. The IET's explanation of RCD types gives the technical background. Device type must be selected for the load and applicable current requirements; it should not be guessed from the letters alone.

Choosing between shared RCDs and individual RCBOs

There is no universal rule that says an RCBO should replace every MCB and RCCB simply because it combines two functions. The right arrangement depends on the consumer unit, its manufacturer-approved devices, circuit loads, earthing, the number and importance of circuits, likely residual currents, available space and applicable standards. Particular equipment may need a carefully selected type of residual-current protection.

Shared RCCB and MCB layouts can provide the required protective functions when correctly designed and maintained. Individual RCBOs often allow more separation between circuits, which can reduce inconvenience from a single leakage fault. A quotation for a new board should explain which arrangement is proposed and why, rather than treating every property as identical.

For new electrical installation work, the current national wiring standard is BS 7671:2018+A4:2026. The IET's current-edition guidance confirms that the previous transition period ended on 15 October 2026. That does not mean all existing consumer units must be replaced automatically on that date; it means work needs to be assessed against the requirements relevant to the installation and the work proposed.

What the pattern of tripping can tell you

The device that has operated may help an electrician decide where to investigate, but it does not prove the cause. A single MCB trip suggests checking for overcurrent or short-circuit conditions on that circuit. A shared RCCB trip points towards residual current somewhere among the circuits it protects. An RCBO trip could involve either protective function.

Make a safe note of what lost power and whether the problem followed rain, use of a particular appliance or recent electrical work. Do not remove the consumer-unit cover or touch exposed wiring to identify the fault. If a device trips again after resetting, or there is heat, buzzing, scorching, moisture or a burning smell, stop using the affected equipment and arrange professional assessment.

For a detailed explanation of leakage-related trips, read why an RCD keeps tripping. If a protective device continues to operate, TRACEFORM's RCD tripping service can assess the device, connected loads and wiring without relying on guesswork. If there is smoke, fire or an immediate danger to life, leave the property and call 999.

Testing, maintenance and safe limitations

An RCD or RCBO with a test button should be operated in line with the manufacturer's instructions and the notice provided for the installation. Household user-test guidance commonly specifies an interval no longer than six months. Pressing the test button can interrupt the electricity supply, so account for alarms, refrigeration and any essential equipment before testing.

The test button checks a particular part of the device's operation; it does not prove the whole wiring system is safe or establish whether the device meets all its required operating times. Inspection and electrical testing by a competent person are still needed when faults are suspected or when the installation is due for periodic assessment. The Electrical Safety First wiring-regulations guidance also distinguishes the protective functions and the use of RCDs, including RCBOs.

Do not swap devices between manufacturers, fit a higher-rated breaker to prevent tripping, bypass residual-current protection or open a live consumer unit. Compatibility is not guaranteed merely because two products look alike or fit the same width. Any replacement must be assessed, installed and verified by an electrician competent to carry out the work.

When to ask an electrician about an upgrade

An assessment is worthwhile when repeated trips affect several rooms, a consumer unit shows signs of overheating or damage, you are planning additional circuits, or you are considering replacing an older board. The right outcome may be targeted repair, correction of a circuit fault, a protection change or a full consumer-unit replacement. It should not be decided solely by the age of the enclosure.

TRACEFORM's consumer unit faults service is the appropriate starting point when the board is already behaving unusually. For a planned improvement, the consumer unit replacement service explains how existing circuits, protective devices, testing and certification are considered together. Ask for the proposed protective arrangement to be explained in plain English before work starts.