Miniature circuit breakers (MCBs) and moulded case circuit breakers (MCCBs) are the two main devices that protect electrical installations against overcurrent and short circuits. Both work on a similar principle, but they differ in the current they can carry, their adjustment options and where they are used. In this article we explain both devices, how they differ and how to connect the line and load terminals of an MCB correctly.
What Is an MCB?
A miniature circuit breaker (MCB) is a switch that protects the circuit it is connected to against overload and short circuit. It is connected in series with the circuit and opens automatically when the current exceeds the rated value.
Unlike old-style fuses, an MCB does not need replacing after it trips – once the fault is cleared, simply push the lever back up. MCBs are the most common protective device in homes and small businesses.
How Does an MCB Work?
- Thermal protection: A bimetallic element that heats up during a sustained overload opens the circuit.
- Magnetic protection: The high current of a short circuit trips the breaker via a coil within milliseconds.
MCBs are typically made in ratings from 0.5 A to 63 A. Tripping curves such as B, C and D determine which type of load the breaker suits: curve B is used for lighting and socket circuits, and curve C for motor and inductive loads.
In a domestic installation each circuit has its own breaker, so a fault in one room trips only that circuit’s breaker and the rest of the home stays powered.
What Is an MCCB?
A moulded case circuit breaker (MCCB) is a thermal-magnetic protective switch designed for higher-current circuits. MCCBs are typically made in ratings from 16 A to 1600 A and have a much higher short-circuit breaking capacity than MCBs.
On many models the thermal and magnetic trip settings are adjustable, so protection can be tailored precisely to the circuit and load. MCCBs are used in main distribution boards, power factor correction panels, motor feeders and industrial facilities.
Differences Between MCBs and MCCBs
| Feature | MCB | MCCB |
|---|---|---|
| Current range | Typically 0.5 – 63 A | Typically 16 – 1600 A |
| Short-circuit breaking capacity | Low to medium (e.g. 6 kA, 10 kA) | High (e.g. 25 kA – 100 kA and above) |
| Adjustment | Fixed | Adjustable on many models |
| Mounting | DIN rail | Screw-mounted in the panel or with a special adapter |
| Applications | Domestic, office and small business circuits | Main distribution, industry, high-power loads |
How Should the Line and Load Terminals of an MCB Be Connected?
The upper terminals of an MCB are usually marked 1, 3, 5 and N, and the lower terminals 2, 4, 6 and N. The basic rule is that if a phase enters at terminal 1, the same phase must leave from terminal 2. The same pairing applies to 3–4 and 5–6.
For most MCBs it makes no technical difference whether the supply is connected at the top or the bottom; either can be used depending on the panel layout. However, to avoid confusion when the breaker is in the open (0) position, it is recommended to adopt a single supply direction throughout the installation. If the manufacturer’s catalogue states otherwise, always follow the manufacturer’s instructions.
The neutral must be connected on the same principle as the phases, without mixing up the incoming and outgoing neutral terminals. All terminal screws must be tightened to the torque specified by the manufacturer.
Choosing the Right Product and Safety
Whichever device you choose, use certified products that comply with the relevant standards (e.g. IEC/EN 60898-1 for MCBs and IEC/EN 60947-2 for MCCBs). Non-compliant products do not provide protection – on the contrary, they increase the risk of fire and electric shock. Panel installation and connections must always be carried out by a qualified electrician or electrical engineer.
Choosing the right breaker is also critical for electric heating systems. You will find relevant examples in our cable underfloor heating guide.
Frequently Asked Questions
Is an MCB the same as an RCD?
No. An MCB protects against overcurrent and short circuits, while a residual current device (RCD) detects current leaking to earth and protects people against electric shock.
Are MCCBs used in homes?
Generally not. In homes, MCBs are sufficient for main and circuit protection. MCCBs are mainly used in main building distribution boards and in industry.
Conclusion
MCBs are a practical solution for low- and medium-current circuits, while MCCBs are chosen for applications requiring high current, high breaking capacity and adjustable protection. Contact us for help choosing the right protective devices and designing panels for your project.
