Select the correct MCB, RCBO, or RCD for your electrical circuit. Covers Type B, C, D curves with AFDD and SPD guidance per BS 7671:2018+A2:2022
Select the correct MCB, RCBO, or RCD for your circuit - BS 7671:2018+A2:2022 compliant
Miniature Circuit Breakers (MCBs) are classified by their instantaneous magnetic trip characteristics per BS EN 60898-1. The three common types - B, C, and D - differ in how much overcurrent is needed to trigger the fast magnetic trip mechanism. Choosing the correct type prevents both nuisance tripping and inadequate protection.
The standard choice for domestic installations. Type B provides the most sensitive magnetic trip, activating between 3 and 5 times the rated current. This makes it ideal for circuits where high inrush currents are not expected.
The default choice for commercial and light industrial installations. Type C handles moderate inrush currents from inductive loads without nuisance tripping, while still providing adequate fault protection.
A specialist choice for heavy industrial equipment with very high inrush currents. Type D is rarely used in domestic installations. Its high trip threshold means the earth fault loop impedance must be very low to ensure disconnection within 0.4 seconds.
Critical: The MCB rating (In) must satisfy the coordination rule: Ib ≤ In ≤ Iz, where Ib is the design current and Iz is the cable current-carrying capacity after derating. Also verify that Zs allows disconnection within the required time (0.4s for final circuits, 5s for distribution).
An RCBO (Residual Current Breaker with Overcurrent protection) combines an MCB and RCD in a single device. It provides both overcurrent protection and earth leakage detection for an individual circuit, which is the modern best practice for consumer unit design.
The professional consensus has shifted toward all-RCBO configurations. Benefits include:
Traditional dual RCD boards split circuits into two banks. A fault on one circuit trips the RCD for the entire bank, potentially affecting 6-8 circuits.
BS 7671 Reg 531.3.2 states that standing earth leakage current should not exceed 30% of the RCD rated residual current. For a 30mA RCD, this means no more than 9mA of combined leakage from all circuits on that RCD. With modern LED drivers, dimmers, and electronic equipment, this budget is easily exceeded on shared RCDs - another strong reason to use individual RCBOs.
Arc Fault Detection Devices (AFDDs) use microprocessor technology to analyse the current waveform and identify the electronic signature of a dangerous arc fault. Standard MCBs and RCDs cannot detect series arc faults, which are a significant cause of electrical fires.
AFDDs continuously monitor the circuit waveform using digital signal processing. They can distinguish between normal arcs (switch contacts, motor brushes) and dangerous arc faults (damaged cable, loose connections). When a dangerous arc pattern is detected, the AFDD disconnects the circuit within milliseconds. Modern AFDD/RCBO combination devices provide overcurrent, earth leakage, and arc fault protection in a single module.
BS 7671 Regulation 531.3.3 requires the RCD type to be specifically compatible with the waveforms generated by the connected equipment. Using an incorrect RCD type can result in the device being “blinded” by DC residual currents, preventing it from tripping during a genuine fault.
Reference: BS 7671:2018+A2:2022 Section 411.3.3, 531.3.3, and Amendment 3:2024 for bidirectional device requirements.
These are the maximum measured earth fault loop impedance values for 0.4 second disconnection time. The 80% temperature correction rule has been applied (i.e., these values are 80% of the BS 7671 Table 41.3 tabulated values, accounting for the increase in conductor resistance from ambient to operating temperature).
| Rating (In) | Type B Max Zs (Ω) | Type C Max Zs (Ω) | Type D Max Zs (Ω) |
|---|---|---|---|
| 6A | 6.13 | 3.06 | 1.53 |
| 10A | 3.68 | 1.84 | 0.92 |
| 16A | 2.30 | 1.15 | 0.57 |
| 20A | 1.84 | 0.92 | 0.46 |
| 25A | 1.47 | 0.73 | 0.37 |
| 32A | 1.15 | 0.57 | 0.29 |
| 40A | 0.92 | 0.46 | 0.23 |
| 50A | 0.73 | 0.36 | 0.18 |
| 63A | 0.58 | 0.29 | 0.14 |
Reference: BS 7671 Table 41.3 with 80% temperature correction applied (NICEIC/IET guidance). Values shown are for 0.4s disconnection at ambient temperature measurement.
Zs is typically measured during initial verification when the installation is at ambient temperature (approximately 20°C). However, the BS 7671 limits are set for when conductors are at their full operating temperature (70°C for PVC). Since copper resistance increases with temperature, a value measured at 20°C must not exceed 80% of the tabulated maximum to ensure compliance at operating temperature.
Typical MCB/RCBO specifications for standard UK residential circuits. These are starting points - always verify for your specific installation conditions.
| Circuit | MCB Rating | Type | Cable | RCD Required |
|---|---|---|---|---|
| Lighting | 6A | B | 1.0-1.5mm² | Recommended |
| Ring Final (Sockets) | 32A | B | 2.5mm² | Mandatory (30mA) |
| Radial Sockets (20A) | 20A | B | 2.5mm² | Mandatory (30mA) |
| Immersion Heater | 16A | B | 2.5mm² | Recommended |
| Boiler | 6-16A | B | 1.5-2.5mm² | Recommended |
| Cooker (up to 10kW) | 32A | B | 6mm² | If cooker control has socket |
| Shower (8.5kW) | 40A | B | 6mm² | Mandatory (bathroom) |
| Shower (9.5kW+) | 50A | B | 10mm² | Mandatory (bathroom) |
| EV Charger (7.4kW) | 32A | B | 6mm² SWA | Mandatory (30mA Type A min) |
| Hot Tub (32A) | 32A | C | 6mm² SWA | Mandatory (outdoor) |
| Outbuilding Sub-main | 32-63A | B | 6-10mm² SWA | Mandatory (30mA at sub-board) |
Important: These are typical domestic values. Always calculate cable size for your specific installation considering cable length, installation method, ambient temperature, grouping, and thermal insulation contact. Use the calculator above for accurate MCB/RCBO selection.
The fundamental safety principle of BS 7671 circuit protection is the coordination between the design current, the protective device rating, and the cable capacity. This ensures the cable is always protected by the MCB.
Maximum current the circuit carries under normal operating conditions
Rated current of the MCB/RCBO (must be ≥ Ib)
Current-carrying capacity of the cable after applying all derating factors
Scenario: 9.5kW electric shower, 6mm² twin and earth cable clipped direct
Check: 41.3A (Ib) ≤ 50A (In) ≤ 64A (Iz) - PASS
Note: 6mm² cable (Iz = 47A) would fail because In (50A) > Iz (47A). The cable must be upgraded to 10mm² to satisfy coordination.
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