Miniature circuit breaker is a switching appliance for connection, carrying and breaking, referred to as micro break or MCB (Miniature Circuit Breaker).Its main function is to switch the circuit, and can open the circuit when the circuit current exceeds the limit current.Typically, a miniature circuit breaker can be replaced by a disconnect switch in series with a matching fuse, the only difference being that when the fuse blows, the core needs to be replaced; whereas when the micro-breaker is overloaded and disconnected, the switch only needs to be pushed up to be restored.

Working Principle of Miniature Circuit Breakers

Typically, the MCB is used as a switch to open or close a circuit (manual switch) under normal operating conditions.

In the event of a short circuit or overload condition, the device will automatically trip.This opens the current in the load circuit and has a safe environment to correct the problem.A visual indication of tripping can be seen when the operating knob is automatically tripped to the OFF position.

Automatic operation/tripping of the MCB can be obtained in two ways: magnetic tripping and thermal tripping.

During an overload, the current flowing through the bimetal causes the temperature of the bimetal to rise.The heat accompanying the increased temperature within the bimetal then causes a deflection in the bimetal, due to the expansion of the metal.The deflection releases the trip latch and separates the contacts.

In the case of a short circuit or severe overload, electromagnetic tripping is used more often.Under normal operating conditions, the slug is held in place by a light spring.This is because the magnetic field generated by the coil is not strong enough to attract the latch.When there is a problem in the circuit, current flows through the circuit and the magnetic field strength generated by the coil is strong enough to pull on the spring holding the latch in the proper position, which moves the latch and activates the trip mechanism.

Most MCBs use a combination of thermal and electromagnetic trip mechanisms.In both operations, an electric arc is formed when the contacts begin to separate.The arc is then forced through an arc runner and into an arc separator.The arc separator is also known as the arc extinguishing chamber.In this, the arc is formed as a series of arcs and at this point, the arc energy is absorbed and cooled by the interrupter chamber.As a result, the arc extinguishing chamber extinguishes the arc.

Miniature short circuit breaker classification

Miniature circuit breakers are mainly classified into AC Micro Breakers (AC MCB) and DC Micro Breakers (DC MCB).If you look at MCBs only by their physical parameters, the difference between AC MCBs and DC MCBs is simply that AC MCBs will have the terminals labelled LOAD and LINE terminals, whereas DC MCBs will have a positive (+) or negative (-) sign on the terminals.And in fact, it is more difficult to disconnect DC current with a DC MCB than with an AC MCB. AC MCB goes off with the help of AC current crossing the zero point while DC MCB goes off with the help of mechanical interruption or cooling.DC MCB opens faster as compared to AC MCB.The two types of micro-breaks are subdivided into polarised (Polarity/Polarized) and non-polarised (Non-polarity/Non-polarized) respectively.

Polarity AC MCB:

If the manufacturer has marked the MCB with (Input/Input/Line), the MCB must be placed with the power supply on the "Input/Input/Line" terminals and the load on the "Load/Load" terminals for the MCB toThe MCB will operate normally.If incorrectly connected, the MCB will probably not provide the intended protection for the circuit when required.

Non-polarity AC Microbreak (Non-polarity AC MCB):

For Non-polarity AC MCBs, the manufacturer does not make special instructions on the terminals, and the user can connect the power supply on either side and the load on the other side.

Polarity DC MCBs:

Polarity DC MCBs are marked with the symbols "+" and "-".If the Polarity DC MCB is not connected or wired correctly, problems may occur and the MCB will not be able to cut off the current and extinguish the arc in the case of an overload or short circuit, which may cause the breaker to burn out.

Polarised DC miniature circuit breakers use small magnets to pull the arc out of the contacts and into the interrupter chamber.If the current flowing through the unit is in the opposite direction, the magnets will guide the arc out of the interrupter chamber and into the MCB mechanism, damaging the equipment.

Non-polarity DC MCB (Non-polarity DC Micro Break):

Non-polarity DC MCBs are not marked with the " + " and " - " symbols.These MCBs can be safely used as load isolators and protect circuits in the event of fault currents, regardless of the direction of current flow in the circuit.Particularly in circuits with storage batteries, where current often needs to be redirected as required, non-polarised DC micro-breaks are effective in providing protection.

Main parameters of miniature circuit breaker

Miniature circuit breakers have three main characteristics in addition to the conventional rated voltage and rated current: rated overload current, rated short-circuit current, and release characteristic curve.

Rated overload current:

When there is too much load in the circuit, it will lead to the actual current exceeding the current that the cable can withstand, at this time, the MCB is required to cut off the power supply, to prevent overheating or even fire of the components and terminals etc. connected in series in the circuit.

Rated short-circuit current:

When a fault occurs somewhere in the circuit, the circuit will be short-circuited, and the current will be hundreds or thousands of times higher than the usual current, and the MCB needs to disconnect the power supply in a very short time to avoid damage to the electrical appliances or fire and other accidents.Therefore, MCBs are required to have the ability to break the fault current under the rated short-circuit current, and to ensure that the action is completed within the specified time limit.

The characteristic curve of disconnection:

Fig. 4 MCB decoupling characteristic curve

Type B MCB

This type of MCB trips immediately when the circuit current reaches three to five times the rated current.These MCBs are typically used for small inductive or resistive loads with very small switching surges.They are therefore suitable for residential and light commercial use.

Type C MCB

This type of MCB trips when the circuit current reaches five to ten times the rated current.These MCBs are typically used to switch highly inductive loads with high inrush, such as in fluorescent lamps and small motors.MCBs are not permitted to operate at inrush currents, so choose Type C MCBs, which have higher short-circuit current values. they are therefore suitable for use in industrial and highly inductive commercial installations.

D type MCB

This type of MCB trips immediately when the circuit current reaches ten to twenty times the rated current.They are typically used for very high inductive loads where high inrush currents are very frequent.They are suitable for specific and specialised industrial and commercial applications.Some common examples of these equipment applications include X-ray machines, UPS systems, large wire-wound motors and industrial welding equipment.

Other types of release characteristic curves such as Type A, Type K, Type Z, etc. are not covered here.