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Automatic circuit breakers or miniature circuit breakers are protective and switching circuit elements designed to protect the electrical installation against overloads and short circuits. If the current passing through the circuit exceeds a certain level of the rated (declared) currents of the breaker or a short circuit occurs in the circuit, the miniature circuit breaker breaks the electrical connection and protects the circuit after itself.
Figure-1: Miniature Circuit Breaker with 4 Pole
The main parts of the internal mechanism of miniature circuit breakers are shown in Figure-2. The current is applied from the input terminal. Then, the current is transferred through flexible copper from the input terminal to a structure called bimetal and transferred from the bimetal to the magnetic coil thorugh another flexible copper, from the coil to the moving contact again with flexible copper, as a result of the contact between the moving contact and the fixed contact, the current is transferred to the output terminal via the fixed contact. Briefly, in an miniature circuit breaker, the current follows this way. The current is checked by means of thermal and magnetic protection devices while following this way in the breaker. Thermal protection (overcurrent protection) is carried out by bimetal and magnetic protection (short circuit protection) by the coil.
Bimetal is formed by combining different metals with different elongation coefficients. With this structure, bimetal starts to warm up as the breaker starts to be loaded over a certain level above the nominal current .The metal with a high elongation coefficient in bimetal is heated, expands towards the metal with a small elongation coefficient and bends and takes the form of brackets “(” .With the change of form of bimetal, the mechanical opening mechanism is triggered and the breaker opens the circuit. This protection made over bimetal is called thermal protection. Thermal protection occurs when the current of the breaker passes a certain level above the nominal current of the breaker.
In case of a high current short circuit in the electrical circuits, the electrical installation is protected by the magnetic protection device of the miniature circuit breakers. In the internal mechanism of the breaker, the current passes over the magnetic coil with a flexible copper connection after the bimetal structure. With the formation of a short circuit in the circuit, this high current also passes over the coil in the breaker. When the short circuit current reaches the level to load the coil as a magnetic field, the shaft inside the coil’s core is drawn by magnetizing. With the movement of the shaft, the opening device is triggered mechanically and the moving contact is separated from the fixed contact. In this way, the breaker goes into the open position and the circuit is de-energized. The magnetic protection device provides protection by acting on the 3, 5, 10 times of the nominal (enounced) current of the breaker.
In miniature circuit breakers, two-point tripping is provided, namely thermal and magnetic protection. The opening mechanism of the breaker is triggered by bimetal in overcurrent, in short circuit current, it is triggered by the shaft in the coil.
There are arc chambers in the breaker internal mechanism to extinguish the arc coming out during the breaking as soon as possible. The arc chamber is located within the working area of the moving contact. The arc formed between the movable and fixed contact during breaking is extinguished by extending it through the magnetic field by the sequenced metal plates on the arc chamber.
Figure-2: Internal Mechanism of Miniature Circuit Breaker
Miniature circuit breakers are named in B, C and D types according to different opening characteristics. Type B circuit breakers are used for lighting and resistive loads in homes and similar places. Type B breakers open the circuit through the magnetic coil in case of a short circuit current of 3 to 5 times the rated currents. Type C and D breakers are for industrial areas and are used in inductively loaded circuits (motor, transformer circuit, etc.). Due to the sudden inrush currents occurring in these circuits, a tolerance must be shown. C and D type breakers perform magnetic protection 5-10 times and 10-20 times respectively of the rated currents.
| Miniature Circuit Breaker Type | Characteristic | Place of Use | |
| B | I1 (t>1h) | 1.13 x In | Home and similar places /Lighting, resistive loads |
| I2 (t<1h) | 1.45 x In | ||
| I3 (t>0.1s) | 3 x In | ||
| I4 (t<0.1s) | 5 x In | ||
| C | I1 (t>1h) | 1.13 x In | Industrial areas /Inductive loads |
| I2 (t<1h) | 1.45 x In | ||
| I3 (t>0.1s) | 5 x In | ||
| I4 (t<0.1s) | 10 x In | ||
| D | I1 (t>1h) | 1.13 x In | Industrial areas /Inductive loads |
| I2 (t<1h) | 1.45 x In | ||
| I3 (t>0.1s) | 10 x In | ||
| I4 (t<0.1s) | 20 x In | ||
Table-1: Miniature Circuit Breakers Magnetic Opening Characteristic Table
The current value is written on the circuit breakers without the symbol (A). With the expressions like B16, C25, the type of the circuit breaker and the rated current value are specified. These factors should be considered in the selection of circuit breakers. According to the standards, miniature circuit breakers are generally produced in nominal current values of 1A, 2A, 3A, 4A, 6A, 10A, 16A, 20A, 25A, 32A, 40A, 63A, 80A, 100A and 125A. These current values on the circuit breakers indicate the currents that the circuit breakers will carry continuously. The rated current passing through the circuit to which it is connected must be below the circuit breaker rating current. When choosing a circuit breaker, the load characteristics (resistive, inductive) should be analyzed first and the circuit breaker type (B, C, D) should be chosen correctly accordingly. Practically, the circuit breaker rated current is selected approximately 1,2 times the nominal current drawn by the load connected to the circuit.
The benefits and conveniences of electricity for human life are too many to count. In daily life, devices used in residences, factories, hospitals and many different fields operate with electrical energy. While electrical energy provides these conveniences to people, it can also cause great disasters due to imprudence and carelessness. In order to protect human health, electrical installation, structure and devices from negative consequences of electricity, protection elements are used in circuits.
Miniature circuit breakers, one of the protection elements, protect the subsequent installation and load against overcurrent and short circuit conditions. However, they cannot protect human life. In order to protect human life against direct and indirect contacts, residual current circuit breakers are used in the circuits. Miniature circuit breakers do not provide protection against residual currents at very small amperage levels that may pose a risk to human life. Rated current values of miniature circuit breakers are selected according to the amount of current drawn by the load in the circuit. The circuit breaker will not open unless the current passing through the circuit exceeds a certain level of rated current of the breaker. Therefore, miniature circuit breakers protect the connected installations and devices against over currents and short-circuit currents. The electrical or physical damage caused by high short circuit currents in the installation and load is prevented. Damage caused by overcurrent or short circuit currents in cable insulations or load insulation may pose a risk of fire. These fault currents are detected and the circuit is cut in a short time with these protection elements.
Miniature circuit breakers perform thermal and magnetic protection functions in circuits. In residential or industrial areas; It can be used as main or distribution circuit breaker in lighting, heater and motor circuits. Depending on the assembly of the miniature circuit breakers in the panel, energy can be input from the upper or lower terminals. The use of miniature circuit breakers is as follows ;
The point that should be considered in the input and output connections, from which terminal phase input is made, phase output should be made from the corresponding terminal.
In neutral circuit breakers, the neutral terminal is marked by the letter “N”.
Neutral terminal can be manufactured unprotected according to customer demand. For this reason, it should be ensured that the neutral line in the circuit is connected to the neutral terminal specified in the circuit breaker.
Figure-3: Connection of Terminals
Direct Current (DC) is used in many fields such as automation circuits, communication devices, dc electric motors, coating rectifiers in plants, solar power plants, solar roof systems. In DC energy, current does not change direction in (+) and (-) alternans as in AC energy. For this reason, DC current is more difficult to cut by circuit breakers. DC current that does not exceed 0 (zero) point creates an arc that is difficult to break by circuit breakers. Due to the AC current passing through the 0 (zero) point and constantly changing direction, the arc formed during cutting in AC current is extinguishedmore quickly. Magnetic routers are used in DC circuit elements to extinguish the arc formed during the breaking operations performed by circuit breakers because the DC current does not change direction. The arc formed by this magnetic routers is pushed towards the separators (arc chambers). Thus, the arc is provided to be extinguished in less time without damaging the contact surface.
Federal miniature circuit breakers manufactured for AC systems can be used at 60V DC voltage at rated currents. It is used in systems such as 60V DC requirement, solar energy, generator and control circuits. In automation circuits, the input (output) and output (output) voltages of the controller (such as PLC) are usually 24 V DC. For this reason, standard AC miniature circuit breakers can also be used in DC circuits in many areas. For the needs above this working voltage, DC miniature circuit breakers with 250V DC working voltage per pole are included in the product range.