Circuit Breaker Selector

Select the correct MCB or RCBO for your electrical circuit according to BS 7671

Circuit Breaker Selector

Select the correct MCB/RCBO for your circuit

Amps

Maximum current the circuit will carry

Cross-sectional area of the circuit cable

MCB Types Explained (B, C, D)

Miniature Circuit Breakers (MCBs) are categorized by their tripping characteristics, specifically how much instantaneous overcurrent triggers the magnetic trip mechanism.

Type B MCB

Magnetic Trip: 3-5 times rated current (3-5 × In)
Best For:
  • Lighting circuits
  • General socket outlets in domestic installations
  • Resistive loads (heaters, immersion heaters)
  • Long cable runs (higher earth loop impedance)
  • Circuits with low starting currents
Why Use Type B: Lower trip threshold means better protection for circuits with predominantly resistive loads. Suitable for typical domestic installations where high inrush currents are not expected.

Type C MCB

Magnetic Trip: 5-10 times rated current (5-10 × In)
Best For:
  • Inductive loads (motors, transformers)
  • Fluorescent and LED lighting with drivers
  • Commercial socket circuits
  • Air conditioning units
  • Equipment with moderate inrush current
Why Use Type C: Most common in commercial installations. Can handle inrush currents from inductive loads without nuisance tripping. Default choice for many installations.

Type D MCB

Magnetic Trip: 10-20 times rated current (10-20 × In)
Best For:
  • Large motors with high starting current
  • Transformers (especially during energization)
  • Welding equipment
  • X-ray machines
  • Industrial equipment with very high inrush
Why Use Type D: Only use where very high starting currents are expected. Higher trip threshold means less sensitive protection - earth fault loop impedance must be low enough to achieve disconnection within required time.

Choosing the Right Type

Rule of thumb:

  • Domestic installations: Type B for most circuits
  • Commercial installations: Type C for most circuits
  • Industrial installations: Type C or D depending on load

Important: MCB rating (In) must be ≥ design current (Ib) and ≤ cable current capacity (Iz). Also verify that earth fault loop impedance allows disconnection within required time (typically 0.4s or 5s depending on TN or TT system).

RCD Types Explained (AC, A, B, F)

Residual Current Devices (RCDs) detect imbalance between live and neutral currents, providing protection against electric shock and fire. Different types detect different fault waveforms.

Type AC RCD

Protection: Detects AC sinusoidal residual currents only
Status: Considered outdated, not recommended for new installations
Applications: Very limited - only pure resistive loads
Warning: May not trip with modern electronic equipment producing DC residual currents

Type A RCD (Current Standard)

Protection: AC + pulsating DC residual currents
Status: Current minimum standard for most installations
Applications:
  • Socket outlet circuits
  • Modern domestic installations
  • Equipment with rectifiers/electronic components
  • EV chargers (minimum requirement)

Type B RCD (Enhanced Protection)

Protection: AC + pulsating DC + smooth DC
Status: Recommended for EV chargers and renewable energy
Applications:
  • EV charging installations (preferred)
  • Photovoltaic (solar) systems
  • Frequency converters
  • UPS systems
  • Medical equipment

Type F RCD

Protection: Type A + high frequency immunity
Features: Delayed tripping to prevent nuisance trips
Applications:
  • Variable speed drives
  • Frequency inverters
  • Industrial equipment

When is RCD Protection Required? (BS 7671)

  • ✓ Mandatory: All socket outlets up to 20A (30mA RCD)
  • ✓ Mandatory: Bathroom circuits (30mA RCD)
  • ✓ Mandatory: Outdoor socket outlets and equipment (30mA RCD)
  • ✓ Mandatory: Cables buried less than 50mm deep (30mA RCD)
  • ✓ Mandatory: EV charging circuits (30mA Type A minimum)
  • ✓ Recommended: All circuits for additional protection

Reference: BS 7671:2018+A2:2022 Section 411.3.3 and 722.531.2 (EV charging)