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Domestic Miniature Circuit Breaker (MCB) – What’s inside em?




A circuit breaker is an electrical safety device designed to protect an electrical circuit from damage caused by an overcurrent or short circuit. Its basic function is to interrupt current flow to protect equipment and to prevent the risk of fire. Unlike a fuse, which operates once and then must be replaced, a circuit breaker can be reset (either manually or automatically) to resume normal operation.


A circuit breaker is often referred to as an “Over Current Protection Device” (OCPD).



Circuit breakers are made in varying sizes, from small devices that protect low-current circuits or individual household appliances (domestic MCB), to large switchgear designed to protect high voltage circuits feeding an entire city.


The DIN rail-mounted thermal-magnetic miniature circuit breaker is the most common style in modern domestic consumer units and commercial electrical distribution boards throughout Europe. ‘DIN’ stands for "Deutsches Institut für Normung", which means "German institute for Standardization"


MCB’s are distributed with a ‘trip’ rating, which establishes and indicates the level/magnitude of fault current the MCB will operate/trip. Common MCB trip ratings are, 1, 2, 4, 6, 10, 16, 20, 25, 32, 40, 50, 63 Amperes.


A MCB is a “thermal-magnetic” device which refers to the two (2) modes of fault current detection and switching arrangement of the MCB.


Thermal detection and switching is designed into the function of the MCB to manage a low overcurrent fault circumstance.


The manifestation of a low overcurrent causes the bimetallic strip/sheet within the MCB to heat, bend, and trip. This releases a switch which moves the electrical contact (moving contact in the diagram) isolating the overcurrent (fault) circumstance.


Electromagnetic switching is designed into the function of the MCB to manage a high overcurrent (short circuit) fault circumstance. In this circumstance the magnetic field generated by the increased current flowing through a coil of wire generates sufficient magnetic force to displace the moving contact to isolate the fault current circumstance.



In both modes of switching, when the moving contact displaces to isolate a fault current, an electrical arc (a blue/white hot electrical gas discharge) generally manifests. The inclusion of an ‘arc extinguishing chamber’ within the MCB provides for safe/efficient/quick extinction of the arc.


About the author, …


Eur Ing Dr Robert Brown (Robert) is the Executive Director of Fraser George and Associates Limited and is a Consultant and Forensic Engineer in the fields Electrical Electronic and Control Engineering.


Robert is an accomplished professional Expert Witness having prepared and presented many court compliant reports and presented oral evidence within the High Court, Crown Courts and County Courts.


For further information please contact Robert via;


Email,…robert.brown@frasergeorge.com or robertbrown@robertbrown.uk.com


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