If you’re new to the electrical industry, understanding MCBs is an essential starting point.
An MCB, or miniature circuit breaker, is an automatically operated electromechanical device designed to protect a circuit from overcurrent conditions and short-circuit faults. Unlike fuses, which require replacement after operation, an MCB can be reset once the fault has been cleared. MCB is a commonly used generic term, but it does not appear in BS 7671 or the relevant product standards, where the correct term is circuit breaker.
Circuit Breakers
There are two product standards for circuit breakers: BS EN 60898-1 and BS EN 60947-2. Circuit breakers to BS EN 60898-1, commonly referred to as MCBs, have fixed characteristics with ratings up to and including 63A and are suitable for operation by ordinary persons. After breaking a short circuit, you must be able to reset the circuit breaker.
The second standard, BS EN 60947-2, covers circuit breakers with adjustable characteristics and ratings up to 1600A. These are to be used by instructed persons and may not be suitable for reuse after breaking a short circuit.
Short Circuit Breaking Capacity
To meet BS EN 60898-1, the Icn rated short-service capacity must be printed on the front of the MCB, for example 10kA. This represents the maximum short-circuit current the device can safely interrupt twice and still resume normal service when reset.
There are additional short circuit breaking requirements at 75% of the Icn and at a multiple of the rated current. For circuit breakers to BS EN 60947-2, two breaking capacities are defined: Icu, the ultimate breaking capacity, and Ics, expressed as a percentage of Icu, typically between 25% and 100% selected by the manufacturer. Following a short-circuit trip, the instructed person must follow the company maintenance policy before resetting the circuit breaker.
Energy Limiting MCBs
BS EN 60898-1 classifies B and C characteristic MCBs up to and including 63A into energy-limiting classes 1 or 3. In the event of a short circuit, class 3 devices offer superior performance by reducing the amount of let-through energy, or I²t, thereby protecting downstream cables and components.
Tripping Curves
Tripping characteristic curves issued by the manufacturer illustrate the relationship between the current flowing through an MCB and the time taken for the breaker to trip. These curves ensure correct protection against both overcurrent and short-circuit conditions.
MCBs to BS EN 60898-1 are classified according to their tripping characteristics, commonly known as B, C and D curves. General tripping characteristics for B and C curve MCBs can be found in BS 7671 Appendix 3. D curves are not recognised for use in domestic applications.
- Type B: 3 to 5 times rated current, commonly used on ring main circuits.
- Type C: 5 to 10 times rated current, commonly used on LED lighting circuits.
- Type D: 10 to 20 times rated current, commonly used on motors and transformers.
- Type Z: 2 to 3 times rated current, commonly used on semiconductors.
- Type K: 8 to 12 times rated current, commonly used where motor inrush is expected.
Overcurrent and Short Circuit Release
MCBs have two independent releases: thermal release, also called delayed release, and magnetic release, also called instantaneous release.
Thermal Release
Tripping in the delayed overload range occurs via a bimetal element that deflects as current flows through the MCB. The time taken to trip depends on the magnitude of the overcurrent. If the current remains below the rated current of the MCB, no tripping occurs. Once the tripping current is exceeded, delayed tripping takes place.
Magnetic Release
The current flowing through the MCB generates a permanent magnetic field in the coil. When a short circuit occurs, a large magnetic field is created due to the high current flowing through the MCB. This causes the latch to be triggered. Because of the rapid tripping, the electromagnetic release is referred to as instantaneous release.
Conclusion
MCB is a commonly used generic term that does not appear in BS 7671 or the relevant product standards, where the correct term is circuit breaker.
Circuit breakers are designed either to BS EN 60898-1, for operation by ordinary persons, or to BS EN 60947-2, for use by instructed persons. The former are commonly referred to as MCBs and the latter are often referred to as MCCBs.
Written by Martin Plumbridge – TMIET, Technical Manager, Doepke UK.