Inside a Crabtree C50 Circuit Breaker.

Crabtree C50 MCBs operate entirely on a magnetic principle, unlike most other circuit breakers there is no thermal trip element.

External view

Crabtree C50 circuit breaker Crabtree C50 5 amp circuit breaker These circuit breakers are made from moulded brown bakelite. Power connects to the top of the breaker, a single screw secures it to a solid metal busbar.

Circuit wiring connects to a clamp on the lower terminal.

Device rating is shown on the end of the handle - this example is a 5 amp.

Side text

Crabtree C50 BS3871 bakelite circuit breaker Crabtree C50 MCB, Made In England One side has moulded text with Crabtree, Type C-50, 250/440V, Made in England, Patent Applied For.

The other side is plain and has a small label with BS3871, 415V, 3ph, Cat M3.


Crabtree C50 Here the cover has been removed. The various components are held in place by the mouldings in the base and cover.

Grease on the pivot of the handle has spread iver the surrounding plastic.


Crabtree C50 The centre contains the operating handle and spring loaded trip mechanism.

Contacts can be seen at the top right, with three arc supression plates surrounding the contact gap.

The magnetic coil at the left side carries the full load current. When this is exceeded it causes a magnetic field which pulls the trip bar towards it, releasing the trip mechanism and disconnecting the supply.

Time delay

Crabtree C50 The metal component inside the magnetic coil is a brass tube containing a steel plug and spring. In normal operation the spring keeps the steel plug at one end of the tube, which is partially covered with another steel piece.

With a moderate overload the magnetic field is not sufficient to trip the mechanism but will cause the steel plug to travel inside the tube towards the opposite end. Oil inside the tube slows the movement of the steel plug.

As the steel plug reaches the end of the tube, it completes the magnetic circuit and increases the magnetic force which pulls the trip bar disconnecting the circuit.

Video with explanation of how this circuit breaker operates

View this video on Youtube.