RCBO vs RCD vs MCB: Complete Comparison Guide
Understand the differences between MCBs, RCDs, RCBOs, and AFDDs. Selection guide with BS 7671 requirements, comparison tables, and circuit protection recommendations.
What Is the Difference Between an MCB, RCD, and RCBO?
Understanding the three core protective devices used in UK electrical installations and what each one protects against.
An MCB (Miniature Circuit Breaker) provides overcurrent protection only. It trips on overload and short circuit but does NOT detect earth faults. Standard ratings are 6A, 10A, 16A, 20A, 32A, 40A, and 50A. MCB types B, C, and D define the magnetic (instantaneous) trip threshold for different load characteristics.
An RCD (Residual Current Device) provides earth fault protection only. It detects an imbalance between the line and neutral conductors — indicating current is leaking to earth. An RCD does NOT protect against overload or short circuit. Typically rated at 30mA sensitivity for personal protection, or 100mA/300mA for fire protection.
An RCBO (Residual Current Breaker with Overcurrent protection) combines an MCB and an RCD in a single device. It provides both overcurrent AND earth fault protection. Each circuit gets its own independent protection, meaning a fault on one circuit does not affect any other.
Quick Rule of Thumb
MCB, RCD, RCBO Comparison Table
Protective device comparison
BS EN 60898, BS EN 61008, BS EN 61009| Feature | MCB | RCD | RCBO |
|---|---|---|---|
| Overcurrent protection | Yes | No | Yes |
| Earth fault protection | No | Yes | Yes |
| Short circuit protection | Yes | No | Yes |
| Typical width | 1 module | 2 modules | 1-2 modules |
| Relative cost | Low | Medium | Higher |
| Independent per circuit | Yes | No (shared) | Yes |
| BS standard | BS EN 60898 | BS EN 61008 | BS EN 61009 |
An RCBO replaces the need for a separate MCB and shared RCD on each circuit.
What Are the Different Types of RCD?
RCDs are classified by the type of fault current they can detect. Choosing the correct type is a BS 7671 requirement.
RCD types and applications
BS 7671 Regulation 531.3.3| Type | Detects | Application | BS 7671 Status |
|---|---|---|---|
| Type AC | AC fault currents only | Legacy installations | NOT permitted in new UK installations |
| Type A | AC + pulsating DC | Most domestic and commercial circuits | Minimum requirement per Reg 531.3.3 |
| Type F | Type A + composite faults from frequency-controlled equipment | Circuits with VFDs, inverter-driven motors, some heat pumps | Recommended for inverter loads |
| Type B | AC + pulsating DC + smooth DC | EV chargers without RDC-DD, medical equipment | Required where smooth DC leakage is possible |
| Type S (selective) | Same as base type, with intentional time delay (~150ms) | Upstream main switch RCD for discrimination | Used to prevent total loss of supply |
Type AC is no longer acceptable for new or modified installations in the UK.
Type A Is the Minimum
What Are the Different MCB Types (B, C, D)?
MCB types define the magnetic (instantaneous) trip threshold as a multiple of the rated current.
MCB type characteristics
BS EN 60898-1| MCB Type | Magnetic Trip Range | Typical Applications |
|---|---|---|
| Type B | 3-5 x In | Domestic resistive loads, lighting circuits, socket circuits |
| Type C | 5-10 x In | Small motors, LED driver arrays, air conditioning, commercial lighting |
| Type D | 10-20 x In | Large motors, transformers, welding equipment, X-ray machines |
In = rated current of the MCB. Type B is standard for domestic installations.
Type B MCBs are used in the vast majority of domestic installations because domestic loads are predominantly resistive (heaters, kettles, ovens) or have low inrush currents. Type C is specified where inrush currents from motors or large LED arrays could cause nuisance tripping of Type B. Type D is reserved for heavy industrial loads with very high inrush characteristics.
What Is an AFDD and When Is It Required?
Arc Fault Detection Devices represent the newest layer of circuit protection, detecting dangerous electrical arcing.
An AFDD (Arc Fault Detection Device) detects dangerous series and parallel arc faults — arcing caused by damaged cables, loose connections, or crushed conductors. These arc faults produce heat that can ignite surrounding materials, but they often do not draw enough current to trip an MCB and may not create enough earth leakage to trip an RCD.
AFDD recommendation summary
BS 7671 Regulation 421.1.7| Location | AFDD Status | Reference |
|---|---|---|
| Buildings with sleeping accommodation | Recommended | Reg 421.1.7 |
| Locations with combustible construction (timber-framed) | Recommended | Reg 421.1.7 |
| Premises with irreplaceable goods (museums, heritage) | Recommended | Reg 421.1.7 |
| HMOs (Houses in Multiple Occupation) | Recommended | Reg 421.1.7 |
| Standard domestic (non-combustible) | Not yet mandatory | May be required by building control |
AFDDs are available as standalone devices or combined AFDD/RCBO units.
Future-Proofing
How to Choose the Right Protection Device
A step-by-step approach to selecting the correct protective device for each circuit.
Step 1: Calculate the design current (Ib) of the circuit and select an MCB rating (In) where In is greater than or equal to Ib.
Step 2: Determine whether the circuit requires RCD protection. Per Regulation 411.3.4, RCD protection is required for: socket outlets rated up to 32A, bathroom circuits, outdoor circuits, cables concealed in walls at less than 50mm depth without mechanical protection, and lighting circuits in domestic premises.
Step 3: If RCD protection is needed, choose between an individual RCBO per circuit (recommended) or MCBs behind a shared RCD. RCBOs provide independent protection and better fault isolation.
Step 4: Select the correct RCD type: Type A as a minimum for all circuits, Type F for circuits with variable frequency drives, and Type B where smooth DC earth leakage is possible.
Step 5: Assess whether an AFDD is recommended — particularly for circuits in premises with sleeping accommodation, combustible construction, or fire risk per Regulation 421.1.7.
Verify Breaking Capacity
Split-Load Board vs Full RCBO Board
Comparing the two most common consumer unit configurations in UK domestic installations.
Board configuration comparison
BS 7671 and industry best practice| Feature | Split-Load (Dual RCD) | Full RCBO Board |
|---|---|---|
| Cost | Lower initial cost | Higher cost per way |
| Fault behaviour | One fault trips half the circuits | Only the faulty circuit trips |
| Nuisance tripping | Higher risk from cumulative leakage | Minimal — each circuit independent |
| Fault finding | Harder — multiple circuits affected | Easy — faulty circuit is obvious |
| Circuit flexibility | Limited by RCD grouping | Any circuit type on any way |
| Industry trend | Being phased out | Increasingly standard for new installations |
Most major manufacturers now recommend full RCBO boards for new domestic installations.
Nuisance Tripping
The split-load (dual RCD) configuration was standard for many years due to lower cost. However, as RCBO prices have decreased and electricians recognise the fault-finding and discrimination benefits, full RCBO boards have become the preferred configuration for new installations and consumer unit upgrades.