Miniature Circuit Breaker, Do You Really Know It?

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Miniature circuit breaker is one of the most widely used electrical protection device in the world. It is available in a wide variety of ratings that allows it to protect electrical installations of many types in residential, commercial and industrial buildings.

What is a Miniature Circuit Breaker and what are its Functions?
miniature circuit breaker, or MCB, is a compact electromechanical device that can work in circuits of both 60 Hz and 50 Hz. It has a rated current under 125 amperes, and has several functions such as:
·         Protecting electric circuits from overload conditions
·         Protecting against short circuits and other electric faults
·         Circuit switching, although this is not very frequent and isn’t the main function of the MCB
Miniature circuit breakers are available in a wide variety of current ratings, which allows them protect many different types of loads. They have a very important role in the prevention of fire and electric shock, by interrupting dangerous currents before they have a chance to melt the insulation of wiring.
Miniature Circuit Breaker Working Principle
The miniature circuit breaker is simply a thermal magnetic circuit breaker of compact size. It has the same mechanism used for electrical protection in all thermal magnetic circuit breakers:
·         The miniature circuit breaker has thermal protection against overcurrent conditions. It has a bimetallic contact that expands with the heat caused by electric current and is calibrated to trip the miniature circuit breaker when the rated current is exceeded. The thermal protection is not instantaneous to allow for low duration overcurrent, which is normal in several electric devices such as motors.
·         The miniature circuit breaker has magnetic protection against electric faults, where the current is higher and more dangerous. The MCB has an internal solenoid that produces a strong magnetic field when there is a high current. This magnetic field opens the metallic contact and disconnects the circuit. Unlike thermal protection, magnetic protection operates in an instant to prevent the dangerous fault currents from causing damage.
·         Miniature circuit breakers are also equipped with a manual interruption mechanism, allowing disconnection of an electric circuit for maintenance, retrofitting or installation of new equipment.
In addition to the miniature circuit breaker, there are alternative electrical protection devices such as the molded case circuit breaker (MCCB) and the fuse. Their general purpose is the same, but important differences exist between the three.
Difference between Miniature Circuit Breaker and Molded Case Circuit Breaker
The molded case circuit breaker (MCCB) is another type of electric protection device that is commonly used. Internally, the MCB and MCCB are both thermal magnetic circuit breakers. However, there are very important differences:
·         The miniature circuit breaker is much smaller than the molded case circuit breaker.
·         Miniature circuit breaker current ratings are below 125 amperes, while molded case circuit breakers have higher current ratings of up to 2,500 amperes.
·         Miniature circuit breaker trip settings are fixed during factory production, and can’t be adjusted. Molded case circuit breakers, on the other hand, often allow adjustment of the current rating and protection response.
·         The MCB is more affordable, thanks to its simplicity compared with the MCCB.
Difference between Miniature Circuit Breaker and Fuse
Fuses are an alternative electric protection device, with a very different operating principle compared with the miniature circuit breaker. The differences are the following:
·         The MCB disconnects the electric circuit when there is a fault or overload, without suffering damage. Normally, the MCB can be reutilized for protection multiple times and will only become damaged when it is subject to a very severe fault or if it wears out due to a long service time.
·         The fuse, on the other hand, can only protect the electric circuit one time. Instead of tripping, the fuse interrupts the circuit by burning out when there is excessive current. For this reason, a fuse must be discarded and replaced with a new one every time there is an electric fault.
In the long term, MCBs are more economical than fuses because the same device can provide protection for an extended period of time. Fuses, on the other hand, require constant replacement and are more dangerous for the user since the electric contacts have to be reconnected every time the fuse is replaced. With an MCB it is only a matter of switching the device back on once the cause of the electric fault has been found and eliminated.
Miniature Circuit Breaker Applications
The miniature circuit breaker has the advantage of cost and simplicity compared with the MCCB, as well as the advantage of durability compared with the fuse. Thanks to the benefits it offers, the MCB has a wide range of applications in electrical protection. The MCB is used in buildings of all types, including homes, commercial buildings and industrial plants. It is very common to find miniature circuit breakers protecting devices such as:
·         Lighting circuits
·         General purpose electric outlets
·         Data centers and computers
·         Kitchen devices
·         Laundry equipment
·         Air conditioners, heat pumps and fans
·         Small water pumps
·         Low horsepower industrial motors
·         Electric relay systems for automation and control
In each case it is very important to select the miniature circuit breaker correctly according to the electric current that is consumed by the device being protected. It is also important to consider the number of poles of the MCB, which must match with the number of live wires in an electric circuit.
Miniature Circuit Breaker Technical Specifications (Ratings and Types)
Miniature circuit breakers are characterized according to several technical specifications. It is very important to know these specifications, in order to select the correct MCB to protect an electric circuit:
·         Rated working voltage (Un) is the normal operational voltage for which the circuit breaker is designed.
·         Rated insulation voltage (Ui) is the voltage value for which the miniature circuit breaker is tested. The rated insulation voltage is much higher than the rated working voltage to provide a margin of safety.
·         Rated impulse withstand voltage (Uimp) is the maximum peak voltage a miniature circuit breaker can withstand without failure during an electric fault. The MCB may suffer permanent damage from a voltage spike that exceeds the rated impulse withstand voltage.
·         Rated current (In) is the current value under which the circuit breaker will allow continuous operation without tripping. If the rated current value is exceeded, the thermal protection mechanism will activate and disconnect the MCB.
·         Rated service short-circuit breaking capacity (Ics) is the fault current that aminiature circuit breaker can interrupt without suffering permanent damage. The MCB can be reused if the fault that was interrupted is below this breaking capacity.
·         Rated ultimate short-circuit breaking capacity (Icn) is the maximum fault current that circuit breaker can interrupt. The MCB can’t interrupt fault currents above this value. If the fault current is below the ultimate breaking capacity (Icn) but above the service breaking capacity (Ics), the MCB will be able to interrupt it, but it will suffer permanent damage and must be replaced.
·         The poles of a miniature circuit breaker simply refer to the number of parallel contacts, and they depend on the type of circuit. MCBs can have anywhere from one to four poles; single pole MCBs protect circuits with a single live conductor, while four pole MCBs can protect up to three phase conductors and a neutral conductor. In the case of the neutral conductor, it can be connected to a circuit breaker of multiple poles. However, it must never be connected individually to a single pole circuit breaker, because it creates the risk of interrupting the wrong conductor during a fault.
·         The types of miniature circuit breakers are B, C and D, and they describe the tripping behavior of the MCB. This topic is described in greater detail below.
·         The mechanical life of a circuit breaker refers to the number of times its switch can be operated manually before failure, while electrical life refers to the number of times the MCB can trip.

Miniature Circuit Breaker Types

There are three types of miniature circuit breakers, according to how they respond to fault currents:
·         Type B miniature circuit breakers trip when a fault current of 3 to 5 times the rated value is present. Type B miniature circuit breakers are used in applications where high currents are not expected, such as in circuits proving electric power to lamps or general purpose electric outlets. Sensitive equipment that can be easily damaged by high currents is normally protected by Type B miniature circuit breakers.
·         Type C miniature circuit breakers trip when a fault current 5 to 10 times the rated value is present. Type C circuit breakers are used in circuits where it is normal to have higher current values for short periods of time. For example, it is normal for electric motors driving fans or pumps to draw up to 8 times the rated current during startup. In these cases, a type B circuit breaker would trip, preventing the motor from starting! Therefore, it is necessary to use a type C circuit breaker.
·         Type D miniature circuit breakers trip with fault currents of 10 to 20 times the rated value, and are normally used in specialized industrial applications where very high currents are normal for short periods of time.
Hiring professional services is highly recommended when determining the types of miniature circuit breakers that will be used in an electrical installation. If MCBs are selected incorrectly, they will not be able to provide adequate protection!
Miniature Circuit Breaker Sizing
miniature circuit breaker must be sized according to the estimated current in the circuit that will be protected. MCBs are typically selected with a current rating that is slightly above the current drawn by the circuit.
For example, a motor with a full-load current of 16 amperes can be protected effectively by an MCB rated at 20 amperes. If the motor suffers an overcurrent condition, the MCB will trip and protect the motor from overheating. It is very important to size the MCB adequately:
·         If the MCB is undersized, for example at 10 amperes, the electric device will not be able to operate because the current exceeds the MCB rating and makes it trip.
·         If the MCB is oversized, for example at 40 amperes, the electric device will not be protected effectively. If there is an overcurrent of 30 amperes, a 16 ampere motor will surely be damaged. However, the MCB will not trip because it is rated at 40 amperes.
It is very important to select wiring with a capacity above the rated current of the MCB. Even if the MCB is selected correctly according to the load, the insulation of undersized wiring can melt and cause an electric accident.
How to Install a Miniature Circuit Breaker
The installation of miniature circuit breakers is very easy, since they are designed to be installed with a DIN rail. The procedure is very simple:
·         First of all, the circuit breaker must be in the OFF position to allow safe installation. There is risk of an electric accident if connection is carried out in the ON position!
·         The circuit breaker is installed on the DIN rail, it has a specially designed base for it.
·         Then, the wire that will feed power to the electric circuit must be cut and the ends must be inserted into the contacts of the MCB while it is kept OFF. The circuit breaker is equipped with a pair of screws that allow the wire connection to be tightened, preventing an accidental disconnection.
·         It will be necessary to connect from one to four wires depending on the type of electric circuit that will be protected, and the number of poles in the MCB.
·         Once connection is finished, the circuit breaker is turned ON. Now, it allows electric current to flow through the circuit and electrical devices can operate.
·         If there is an electric fault, the circuit breaker will automatically switch to the OFF position. The electric fault must be investigated before turning it back on, or otherwise it will trip again!
Miniature Circuit Breaker Advantages and Conclusions
The miniature circuit breaker is a very versatile electrical protection device that is available in a wide variety of ratings. Its main advantage is that it can be used multiple times for protection of an electric circuit and will only be damaged if there are very large voltage spikes or fault currents. The MCB also offers the following advantages:
·         Thanks to its compact size, it requires little space.
·         It is safe thanks to its insulation and screw mechanism to tighten electrical connections.
·         It can be easily mounted on a DIN rail, and multiple MCBs can be mounted in the same rail.
The miniature circuit breaker plays a very important role in the protection of most electrical circuits in homes, and plenty of electrical circuits in businesses. The miniature circuit breaker is available in various current ratings for both single-phase and three-phase circuits, and there are three different types designed for the protection of electric devices with different characteristics.
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