What is the difference between an MCB and a fuse?

Both MCBs and fuses provide overcurrent protection, but they work differently and have different characteristics. A fuse operates by melting a fusible element when the current exceeds a threshold — once blown, it must be replaced. An MCB uses an electromagnetic trip (for short-circuit protection) and a bimetallic strip (for overload protection) and can be reset by hand after the fault is cleared. MCBs are preferred in modern installations because they are resettable, have more predictable trip characteristics, and allow the cause of the trip to be investigated before resetting.

What does MCB curve type B, C or D mean?

The MCB curve type defines the instantaneous magnetic trip threshold — the multiple of rated current at which the device trips almost instantaneously (within milliseconds) to handle short-circuit conditions. Type B trips instantaneously at 3 to 5 times rated current, making it suitable for domestic circuits with resistive loads (lighting, sockets). Type C trips at 5 to 10 times rated current, suitable for commercial circuits with moderate inrush loads (fluorescent lighting, small motors). Type D trips at 10 to 20 times rated current, used for high inrush loads such as transformers, large motors, and welding equipment.

When is an MCCB used instead of an MCB?

MCCBs are used where the fault current or load current exceeds the ratings available in MCB format, or where adjustable trip settings are required. Standard domestic MCBs are available up to 125A and have fixed breaking capacities typically up to 10kA. MCCBs are available from 16A to 1,600A or more, with breaking capacities up to 150kA, and often feature adjustable thermal and magnetic trip settings. They are common as main incoming protective devices in commercial distribution boards, sub-main isolators, and industrial motor control.

Does an MCB provide RCD (earth-fault) protection?

No. A standard MCB provides overcurrent protection only — it will trip on overload and short-circuit conditions but cannot detect earth faults (current leaking to earth through a person or fault path). RCD protection requires a separate RCCB, an RCBO (which combines MCB and RCD), or a socket-outlet RCD (SRCD). BS 7671 18th Edition requires 30mA RCD additional protection for socket-outlet circuits rated up to 20A, so MCBs alone are not sufficient for those circuits.

What breaking capacity do I need for a domestic consumer unit?

For most domestic consumer units in the UK, a breaking capacity of 6kA is sufficient. UK prospective fault currents at the meter position are typically in the range of 1kA to 6kA for most domestic supplies. However, on large commercial sites, near substations, or on TN-S systems with low earth loop impedance, fault currents can exceed 6kA. Always measure the prospective fault current (PFC) at the incoming supply and select devices with a breaking capacity at least equal to or greater than the measured PFC.

Can I mix MCB brands in a consumer unit?

Generally, no. MCBs in a consumer unit must be compatible with the busbar system and the board manufacturer's specifications. Mixing manufacturers within the same board is not recommended and can void certifications and warranties. Some manufacturers produce "compatible" MCBs that fit rival boards, but these should only be used if explicitly confirmed as compatible. When replacing individual MCBs, always use the same manufacturer and range as the existing board, or replace the entire board.

What is the difference between a 6kA and 10kA MCB?

These figures refer to the rated short-circuit breaking capacity — the maximum fault current the device can safely interrupt without damage or hazard. A 6kA MCB can safely clear fault currents up to 6,000 amperes, while a 10kA MCB can clear up to 10,000 amperes. For domestic installations with measured prospective fault currents below 6kA (which covers the vast majority of UK homes), 6kA MCBs are adequate. On commercial supplies, substation-adjacent sites, or where PFC measurements exceed 6kA, 10kA or higher devices must be specified.

WIRING GUIDE

Types of Circuit Breakers UK: MCB, MCCB, RCCB and RCBO Explained

A complete practical guide to circuit breaker types used in UK electrical installations — MCB, MCCB, RCCB and RCBO explained with curve types, breaking capacity requirements, BS 7671 compliance, and guidance on selecting the right device for every application.

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11 min readUpdated 2026-05-18Andrew Moore, Founder of Elec-Mate

Written and reviewed by Andrew Moore, founder of Elec-Mate, against BS 7671:2018+A4:2026, IET Guidance Note 3 and the IET On-Site Guide.

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Key Takeaways

1MCBs (Miniature Circuit Breakers) provide overcurrent protection for individual circuits in domestic and light commercial consumer units — they do not provide earth-fault (RCD) protection.
2MCCBs (Moulded Case Circuit Breakers) are used in larger commercial and industrial installations where higher fault current ratings and adjustable trip settings are required.
3RCCBs (Residual Current Circuit Breakers) provide earth-fault protection only — they must always be used with separate MCBs for complete circuit protection.
4RCBOs combine MCB and RCD functions in one device, providing both overcurrent and 30mA earth-fault protection for a single circuit.
5MCB curve type (B, C or D) determines the instantaneous trip threshold — Type B for domestic, Type C for commercial/light industrial, Type D for high inrush loads.
6Breaking capacity must exceed the prospective fault current at the point of installation — always measure with a PFC meter before specifying devices.

01 · Wiring Guide

What Is a Circuit Breaker?

A circuit breaker is an automatically operated electrical switch designed to protect a circuit from damage caused by excess current, typically resulting from an overload or a short circuit. Unlike a fuse, which must be replaced after it operates, a circuit breaker can be reset — either manually or automatically — to resume normal operation after the fault has been identified and cleared.

Overload protection — a circuit breaker will trip after a sustained period of overcurrent (e.g. too many appliances on one circuit), protecting cables from overheating. The thermal element of the breaker heats up proportionally to the excess current and trips when the temperature threshold is reached.
Short-circuit protection — the electromagnetic element of a circuit breaker trips almost instantaneously (within milliseconds) when a short-circuit current is detected, preventing damage to wiring and equipment.
Not RCD protection — standard circuit breakers (MCBs and MCCBs) do not detect earth-fault currents. RCD protection must be provided separately unless an RCBO is used. See the RCD types guide for full details.

In UK electrical installations governed by BS 7671, the main types of circuit breaker used are MCBs, MCCBs, RCCBs, and RCBOs. Each has specific applications and characteristics that determine where it should be used.

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02 · Wiring Guide

MCB — Miniature Circuit Breaker

The MCB (Miniature Circuit Breaker) is the standard protective device for individual final circuits in UK domestic and light commercial consumer units. MCBs are compact single-module devices that provide both overload and short-circuit protection for a single circuit.

Current ratings — MCBs for domestic use are typically available in 6A, 10A, 16A, 20A, 25A, 32A, 40A, 50A, 63A, and 100A. Standard final circuits use: 6A for lighting, 32A for ring final circuits (sockets), 32A to 40A for cookers, 16A or 20A for dedicated circuits.
Breaking capacity — standard domestic MCBs have a breaking capacity of 6kA, which is sufficient for most UK domestic supplies. Higher breaking capacity MCBs (10kA) are available for commercial applications or where prospective fault current is higher.
No RCD function — MCBs provide overcurrent protection only. For circuits requiring 30mA additional protection (socket outlets, bathrooms, outdoor circuits), either an RCBO must replace the MCB, or an upstream RCCB must be used.
BS EN 60898 — UK MCBs are manufactured to BS EN 60898, which defines the performance requirements, trip characteristics, and marking for miniature circuit breakers for household and similar installations.

03 · Wiring Guide

MCCB — Moulded Case Circuit Breaker

An MCCB (Moulded Case Circuit Breaker) is a larger, more powerful circuit breaker used in commercial, industrial, and large residential installations. MCCBs can handle higher current ratings, have greater breaking capacities, and often feature adjustable trip settings that make them suitable as main incoming protection or sub-main protection in large distribution systems.

Higher current ratings — MCCBs are available from around 16A up to 1,600A or more, making them suitable as main switches for commercial distribution boards, sub-main circuit protection for large loads, and incoming protection for industrial installations.
Adjustable trip settings — unlike fixed MCBs, MCCBs often have adjustable thermal and magnetic trip settings. This allows the protection characteristic to be tailored to the connected load, improving selectivity and reducing nuisance tripping.
Higher breaking capacity — MCCBs are available with breaking capacities from 16kA up to 150kA, essential for installations near transformer secondary sides where fault currents can be very high.
Three-phase versions — MCCBs are commonly available in three-pole (three-phase) and four-pole (three-phase plus neutral) configurations, making them essential for three-phase commercial and industrial distribution.

04 · Wiring Guide

RCCB — Residual Current Circuit Breaker

An RCCB (Residual Current Circuit Breaker) provides earth-fault protection for a group of circuits. It does not provide overcurrent protection and must always be used in conjunction with individual MCBs. In a split-load consumer unit, an RCCB typically protects half the board's circuits from earth faults while individual MCBs on each way provide overcurrent protection.

Earth-fault protection only — the RCCB detects residual current (the difference between live and neutral currents) and trips when the imbalance exceeds the rated threshold. It does not respond to overload or short-circuit conditions.
Nuisance tripping — because one RCCB may protect 8 or more circuits, a fault on any single circuit trips all protected circuits. This is a significant disadvantage compared to individual RCBOs.
Sensitivity ratings — RCCBs are available at 30mA (personal protection), 100mA, and 300mA (fire protection). For domestic socket-outlet circuits, 30mA is required by BS 7671 18th Edition.

For a detailed explanation of all RCD types including SRCDs and RCDMs, see the RCD types UK guide.

05 · Wiring Guide

RCBO — Combined MCB and RCD in One Device

An RCBO (Residual Current Breaker with Overcurrent protection) is a single-module device that combines the functions of an MCB and a 30mA RCD. Each RCBO provides both overcurrent protection and earth-fault protection for one individual circuit, eliminating the need for a separate upstream RCCB for that circuit.

Per-circuit isolation — a fault on one circuit trips only that RCBO, leaving all other circuits unaffected. This eliminates the nuisance tripping problem inherent in RCCB-based designs and is a major benefit for domestic properties.
Higher cost per device — an RCBO costs three to five times more than an equivalent MCB. However, when the cost of the RCCB is removed from the calculation, an all-RCBO board is typically only £150 to £300 more expensive than a split-load RCCB/MCB board for a typical domestic installation.
RCD type selection — RCBOs come in Type AC, Type A, and Type B variants. Type A is now recommended as standard for domestic installations. Type B is required for certain EV charging circuits and equipment with smooth DC residual current components.

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06 · Wiring Guide

MCB Curve Types: B, C and D Explained

The MCB curve type defines the instantaneous magnetic trip threshold — the multiple of the rated current at which the device trips almost instantaneously to clear a short circuit. Choosing the correct curve type is essential: too sensitive and the breaker nuisance-trips on inrush currents; not sensitive enough and the breaker fails to provide adequate protection against shock and fire.

Type B — 3 to 5 × In — trips instantaneously at 3 to 5 times rated current. Used for domestic circuits with resistive and slightly inductive loads: lighting circuits, socket-outlet circuits, immersion heaters, electric showers. The standard choice for most domestic final circuits.
Type C — 5 to 10 × In — trips instantaneously at 5 to 10 times rated current. Used for commercial circuits with moderate inrush loads: fluorescent and LED lighting banks, small motors, power supplies, and HVAC equipment. Common in commercial and light industrial distribution boards.
Type D — 10 to 20 × In — trips instantaneously at 10 to 20 times rated current. Used for high inrush loads: large motors (star-delta starting), transformers, welding equipment, and UPS systems where the inrush current at switch-on would otherwise trip a Type B or C breaker.
Impact on earth loop impedance — higher curve types require a lower earth loop impedance (Zs) to ensure the breaker trips within the required disconnection time under fault conditions. Always verify that the measured Zs meets the maximum Zs value for the MCB type and rating in use.

07 · Wiring Guide

Breaking Capacity and Prospective Fault Current

Breaking capacity is the maximum fault current a circuit breaker can safely interrupt without damage, expressed in kiloamperes (kA). Selecting a device with insufficient breaking capacity can result in catastrophic failure during a fault — the device may be unable to clear the fault, leading to arc flash, fire, or explosion.

Measure before specifying — always measure the prospective fault current (PFC) at the supply origin before specifying protective devices. For domestic consumer units this is typically done at the meter tails / incoming terminals. Record the result on the Electrical Installation Certificate.
6kA for most domestic — the vast majority of UK domestic supplies will have a PFC below 6kA, making standard 6kA MCBs adequate. However, properties close to a substation, on TN-S systems, or on large commercial developments may exceed this.
Commercial installations — 10kA MCBs are standard for commercial distribution boards. For installations at or near transformer secondaries, MCCBs with breaking capacities of 25kA, 36kA, or higher may be required. The network operator's technical standards and site measurements determine the requirement.

08 · Wiring Guide

Selecting the Right Circuit Breaker for Your Installation

Circuit breaker selection involves assessing the load current, the prospective fault current, the earth loop impedance, the circuit's disconnection time requirements, and any specific load characteristics such as inrush current. The following practical guidance covers the most common UK scenarios.

Domestic consumer unit replacement — specify an all-RCBO board with Type A, 30mA, 6kA RCBOs. Use Type B curve for all final circuits. This eliminates nuisance tripping, provides BS 7671 18th Edition compliant additional protection, and simplifies certification.
Commercial distribution board — specify MCCBs for incoming protection (with appropriate breaking capacity based on measured PFC), Type C MCBs for general circuits, and Type D for motor circuits. Add 30mA RCDs or RCBOs for socket-outlet circuits as required by BS 7671.
EV charging circuit — use a Type A or Type B RCBO (not Type AC). Check the charge point manufacturer's documentation — some units have built-in Type B RCD protection, in which case a Type A upstream RCBO is acceptable.
Retrofit where Zs is marginal — if measured Zs is close to the maximum for the existing MCB, consider replacing with an RCBO of the same rating and curve type to add RCD protection without changing the overcurrent characteristic. Do not use a lower-rated MCB to improve Zs compliance — always consult the tables in BS 7671.

09 · Wiring Guide

For Electricians: Certification and Testing

Consumer unit replacements and new circuit installations are notifiable work in England and Wales under Part P of the Building Regulations. Electricians registered with a competent person scheme (NICEIC, NAPIT, ELECSA, or similar) can self-certify without involving building control.

EIC and Test Results Required

Every consumer unit replacement requires an Electrical Installation Certificate (EIC) and a complete Schedule of Test Results including PFC measurement, Zs measurements for each circuit, and RCD test results where applicable. Use the Elec-Mate EIC app to complete certification on site with no evening admin.

EICR Assessment

During an EICR, circuit breaker type, rating, and breaking capacity must be verified against the installation's requirements. MCBs without RCD protection on circuits requiring 30mA additional protection are a C2 observation. Devices with insufficient breaking capacity relative to the measured PFC are a C2 or C1 finding depending on the severity.

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Frequently Asked Questions About Circuit Breaker Types

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Types of Circuit Breakers UK: MCB, MCCB, RCCB and RCBO Explained

Key Takeaways

What Is a Circuit Breaker?

Get the BS 7671 A4:2026 Cheat Sheet — free

MCB — Miniature Circuit Breaker

MCCB — Moulded Case Circuit Breaker

RCCB — Residual Current Circuit Breaker

RCBO — Combined MCB and RCD in One Device

Try Elec-Mate free for 7 days

MCB Curve Types: B, C and D Explained

Breaking Capacity and Prospective Fault Current

Selecting the Right Circuit Breaker for Your Installation

For Electricians: Certification and Testing

EIC and Test Results Required

EICR Assessment

Complete consumer unit certificates on your phone

Frequently Asked Questions About Circuit Breaker Types

What is the difference between an MCB and a fuse?

What does MCB curve type B, C or D mean?

When is an MCCB used instead of an MCB?

Does an MCB provide RCD (earth-fault) protection?

What breaking capacity do I need for a domestic consumer unit?

Can I mix MCB brands in a consumer unit?

What is the difference between a 6kA and 10kA MCB?

Continue reading

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