Choosing the Right Circuit Breaker (MCB)
The Miniature Circuit Breaker (MCB) is the central protection device in every electrical installation. It protects cables from overload and short circuit and is mandatory in every circuit. Selecting the correct trip characteristic and rated current is essential for a safe and functional installation.
Important to Know
An MCB primarily protects the cable – not the connected device or the person. For personal protection, an additional RCD (Residual Current Device) is required.
What is a Miniature Circuit Breaker?
A Miniature Circuit Breaker (MCB) is a thermo-magnetic protective switch. It combines two tripping mechanisms in one device.
The thermal release (bimetal) responds to slow overload, while the magnetic release (coil) trips instantly on short circuit. This reliably protects the cable in both fault scenarios.
Trip Characteristics Comparison
The trip characteristic determines at what multiple of the rated current the magnetic instantaneous release trips. Different types are needed depending on the application.
Characteristic B
Magnetic tripping: 3 to 5 times rated current
Standard application in residential and office buildings. Suitable for resistive loads such as lighting, sockets and heaters.
Characteristic C
Magnetic tripping: 5 to 10 times rated current
For loads with moderate inrush currents such as motors, fluorescent lamps, air conditioning units and transformers.
Characteristic D
Magnetic tripping: 10 to 20 times rated current
For loads with high inrush currents such as large motors, welding machines, X-ray equipment and UPS systems.
Characteristic K
Magnetic tripping: 8 to 14 times rated current
Special application for transformers and motors with particularly high starting currents in industrial environments.
Rated Currents and Cable Assignment
The rated current of the MCB must match the current carrying capacity of the connected cable. Here are the most common assignments for copper cables installed in conduit:
| Rated Current | Typical Use | Min. Cable Size (Cu) |
|---|---|---|
| 6 A | LED lighting circuits | 1,5 mm² |
| 10 A | Lighting, lightly loaded socket circuits | 1,5 mm² |
| 13 A | Socket circuits (UK/IE standard) | 1,5 mm² |
| 16 A | Socket circuits (DE standard), individual appliances | 2,5 mm² |
| 20 A | Instantaneous water heater (single-phase), wallbox | 2,5 mm² |
| 32 A | Cooker, instantaneous water heater (three-phase), 22kW wallbox | 6 mm² |
Selectivity and Back-Up Protection
Selectivity means that in the event of a fault, only the nearest protective device trips rather than shutting down the entire installation. This is particularly important for supply reliability.
Selectivity Tips
- The upstream MCB should have at least 1.6 times the rated current of the downstream one
- Refer to manufacturer selectivity tables – not all combinations are selective
- When in doubt, use selective main circuit breakers (SLS) as upstream protection
Working Together with RCDs
In modern electrical installations, MCBs are always used in combination with RCDs (Residual Current Devices). Proper coordination of both devices is important.
Best Practices
- Always install the RCD upstream of the MCBs (in the energy flow direction)
- Rated current of the RCD should be equal to or greater than the sum of downstream MCBs (considering diversity factor)
- Maximum 6 MCBs per RCD for optimal selectivity and availability