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. MCBs to BS EN 60898 go up to 125 A with breaking capacities commonly of 6 kA or 10 kA. 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). Regulation 411.3.3 of BS 7671 requires 30 mA RCD additional protection for socket-outlets rated up to 32 A (with a documented risk-assessment exception outside dwellings), so an MCB alone is not enough on those circuits.

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

A 6 kA MCB in a domestic consumer unit is normally fine even where the prospective fault current is higher, because of the conditional rating. Regulation 536.4.201 accepts a consumer unit built with the manufacturer's own devices that has passed the 16 kA conditional short-circuit test of BS EN 61439-3 Annex ZB, for a single-phase supply up to 100 A. The service fuse limits the energy the MCB has to break. That only holds if every device in the board is the board manufacturer's specified part. Outside that case, the breaking capacity of each device must be at least the prospective fault current at its point of installation.

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?

They are the rated short-circuit capacity (Icn under BS EN 60898): the largest fault current the device can interrupt and stay safe. A 6 kA MCB clears up to 6,000 A, a 10 kA MCB up to 10,000 A. In a domestic consumer unit with the manufacturer's own devices, the 16 kA conditional rating usually covers the supply (Regulation 536.4.201). In commercial distribution boards, or where devices are not covered by a conditional rating, choose 10 kA or higher wherever the prospective fault current at the board exceeds 6 kA.

WIRING GUIDE

Circuit Breaker Types UK: MCB, MCCB, RCCB and RCBO

What each device protects against, how Type B, C and D curves differ, the maximum Zs for each from BS 7671 Table 41.3, and how to choose between 6 kA and 10 kA.

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11 min readUpdated 2026-10-24Andrew 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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The short answer

What is a circuit breaker?

A circuit breaker is a resettable switch that opens automatically when too much current flows. A thermal element trips on sustained overload; a magnetic element trips within milliseconds on a short circuit. UK installations use four types: MCB, MCCB, RCCB and RCBO. Only RCCBs and RCBOs detect earth leakage.

MCBs to BS EN 60898 come in Type B (3–5 × In), C (5–10 × In) and D (10–20 × In).

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Key takeaways

01MCBs (Miniature Circuit Breakers) provide overcurrent protection for individual circuits in domestic and light commercial consumer units — they do not provide earth-fault (RCD) protection.
02MCCBs (Moulded Case Circuit Breakers) are used in larger commercial and industrial installations where higher fault current ratings and adjustable trip settings are required.
03RCCBs (Residual Current Circuit Breakers) provide earth-fault protection only — they must always be used with separate MCBs for complete circuit protection.
04RCBOs combine MCB and RCD functions in one device, providing both overcurrent and 30mA earth-fault protection for a single circuit.
05MCB curve type (B, C or D) sets the instantaneous trip threshold: B for most domestic circuits, C for moderate inrush loads, D for transformers and large motors. Higher curves need a lower Zs.
06Breaking capacity must cover the prospective fault current where the device is installed. In a domestic consumer unit using the maker's own devices, the 16 kA conditional rating does this (Regulation 536.4.201).

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

Circuit Breaker Types at a Glance

Four devices cover almost every UK installation. The first question is what each one actually protects against.

Device	Overcurrent	Earth leakage	Standard
MCB	Yes	No	BS EN 60898
MCCB	Yes, often adjustable	Only with an add-on module	BS EN 60947-2
RCCB	No	Yes	BS EN 61008
RCBO	Yes	Yes	BS EN 61009

In a home the choice is usually between RCBOs on every circuit and a split-load board with MCBs behind one or two RCCBs. MCCBs appear in commercial and industrial distribution, where currents and fault levels are higher.

03 · 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 are rated 6 kA. In a consumer unit that uses the manufacturer's own devices, the board's 16 kA conditional rating covers a higher fault level (Regulation 536.4.201). 10 kA MCBs are the norm in commercial distribution boards.
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.

04 · 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.

05 · 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 30 mA (additional protection), 100 mA and 300 mA (fire protection and discrimination). Socket-outlets rated up to 32 A need 30 mA protection under Regulation 411.3.3.

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

06 · 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, A, F and B. Regulation 531.3.3 limits Type AC to fixed equipment known to have no DC component, so Type A is the practical default. Type F suits inverter-driven appliances, and Type B covers smooth DC residual current (some EV chargers and three-phase drives).

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07 · 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 — the higher the curve, the lower the maximum Zs. For a 32 A device, BS 7671 Table 41.3 gives 1.37 Ω for Type B, 0.68 Ω for Type C and 0.34 Ω for Type D (0.4 s). Compare the measured value against the table for the device actually fitted.

Maximum earth fault loop impedance from BS 7671 Table 41.3 (0.4 s, Cmin 0.95) for the common ratings:

Rating	Type B	Type C	Type D
6 A	7.28 Ω	3.64 Ω	1.82 Ω
16 A	2.73 Ω	1.37 Ω	0.68 Ω
20 A	2.19 Ω	1.09 Ω	0.55 Ω
32 A	1.37 Ω	0.68 Ω	0.34 Ω
40 A	1.09 Ω	0.55 Ω	0.27 Ω

These are limits at conductor operating temperature. A reading taken cold is compared against 80% of the value. The full set is on the maximum Zs values page.

08 · 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 — measure or determine the prospective fault current (PFC) at the origin and at each distribution board. PFC is the greater of the prospective short-circuit current and the prospective earth fault current. Record it on the Electrical Installation Certificate.
Domestic: the 16 kA conditional rating — a consumer unit to BS EN 61439-3, fitted only with the manufacturer's specified devices and on a single-phase supply up to 100 A, is covered to 16 kA (Regulation 536.4.201). Where the distributor declares 16 kA, Appendix 14 says PFC need not be measured at the origin of a domestic installation; most electricians still measure and record it.
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.

09 · 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 — an all-RCBO board with Type A, 30 mA RCBOs from the board manufacturer, Type B curve on the final circuits. One fault trips one circuit, every circuit gets additional protection, and the 16 kA conditional rating stays valid because the devices match the board.
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 — Regulation 722.531.3.101 needs a Type B RCD, or a Type A or F RCD plus 6 mA DC fault detection (RDC-DD). Most domestic charge points have the RDC-DD built in, so a Type A RCBO is the usual choice. Check the charger's documentation before fitting.
Where Zs is marginal — an RCBO of the same rating and curve adds 30 mA protection, and Regulation 411.4.204 then lets the RCD provide fault protection against the much higher Table 41.5 limits. A lower-rated device is only an option if it still carries the circuit's design current.

10 · 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 the inspector checks each device's type, rating and breaking capacity against the circuit and the measured PFC. Missing 30 mA protection on sockets in an older installation is usually coded C3, rising to C2 where a socket could reasonably supply equipment outdoors. Zs above the Table 41.3 limit for the device fitted, with no RCD to rely on, is a C2.

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Frequently Asked Questions About Circuit Breaker Types

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