MCB Time-Current Curves
Understanding Type B, C, and D MCB tripping characteristics
BS EN 60898-1
Two Protection Mechanisms
Thermal Element
Bimetal strip heats and bends under overload. Provides inverse time characteristic - higher current = faster trip.
Magnetic Element
Electromagnetic solenoid trips instantly on high fault currents. The trip range defines the MCB type (B, C, or D).
MCB Types Compared
3-5 × In
Trips between 3 and 5 times rated current. Most common for domestic installations.
Typical applications:
- • Domestic circuits
- • Lighting circuits
- • Socket outlets
- • Small heaters
- • Most general loads
5-10 × In
Trips between 5 and 10 times rated current. Handles moderate inrush currents.
Typical applications:
- • Small motors
- • Fluorescent lighting banks
- • Air conditioning
- • IT equipment with inrush
- • Commercial installations
10-20 × In
Trips between 10 and 20 times rated current. For very high inrush equipment.
Typical applications:
- • Large motors
- • Transformers
- • Welding equipment
- • X-ray machines
- • UPS systems
| Current | Type B | Type C | Type D | Note |
|---|---|---|---|---|
| 1.13 × In | < 1 hour | < 1 hour | < 1 hour | Conventional non-tripping current |
| 1.45 × In | < 1 hour | < 1 hour | < 1 hour | Conventional tripping current |
| 2.55 × In | 1-60 sec | 1-60 sec | 1-120 sec | Thermal trip range |
| 3 × In | Instant* | > 0.1 sec | > 0.1 sec | *Magnetic may operate |
| 5 × In | < 0.1 sec | Instant* | > 0.1 sec | *Magnetic may operate |
| 10 × In | < 0.1 sec | < 0.1 sec | Instant* | *Magnetic may operate |
| 20 × In | < 0.1 sec | < 0.1 sec | < 0.1 sec | All types trip instantly |
× In = multiples of rated current
Selection Guide
| Scenario | Type | Reason |
|---|---|---|
| Domestic socket circuit | Type B | Resistive loads, no significant inrush. Standard 32A ring or 20A radial. |
| LED lighting circuit | Type B or C | Some LED drivers have inrush. Type C if nuisance tripping occurs. |
| Fluorescent lighting bank | Type C | Magnetic ballasts create inrush current on switch-on. |
| Small motor (< 3kW) | Type C | Motor starting current is typically 6-8× running current. |
| Large motor (> 3kW) | Type D | High inrush current on DOL start. Consider soft starter. |
| Transformer circuit | Type D | Magnetising inrush can be 10-15× rated current. |
| UPS or inverter | Type C or D | Capacitor charging creates inrush. Check manufacturer spec. |
| IT equipment room | Type C | Switch-mode PSUs have inrush. May need diversity consideration. |
Nuisance Tripping
If an MCB trips on equipment start-up, upsizing the type (B→C or C→D) is often the solution. However, this affects fault protection - always verify the maximum Zs is still within limits for the new type.