Basic concept of Miniature circuit breaker(MCB)

Miniature circuit breaker (MCB)
Miniature circuit breaker (MCB)

Miniature circuit breaker is suitable for AC 50/60Hz rated voltage 230/400V, rated current to 63A line overload and short circuit protection, can also be used as normal line switching of infrequent operation.
Miniature circuit breakers are mainly used in various locations such as industrial, commercial, high-rise and residential houses.
This product should meet the GB10963.1, IEC60898 standards.

It is used for single-phase and three-phase short circuit, overload, and overvoltage protection up to 125A, including unipolar 1P, 2-pole, 3-pole, and 4-pole.

Miniature Circuit Breaker, abbreviated as MCB (Micro Circuit Breaker), is a kind of terminal protection device that is most widely used in the construction of electrical terminal distribution equipment. It is used for single-phase and three-phase short circuit, overload, and overvoltage protection up to 125A, including unipolar 1P, 2-pole 2-pole, 3-pole 3-pole, and 4-pole 4-pole.
Circuit Breaker: A mechanical switching device that can turn on, load, and break currents under normal circuit conditions, as well as under certain non-normal circuit conditions, to turn on, carry a certain amount of time, and break current.

 

Working principle

Miniature circuit breakers consist of operating mechanisms, contacts, protection devices (various trip units), and arc extinguishing systems. Its main contacts are either manually operated or electrically closed. After the main contact is closed, the free tripping mechanism locks the main contact in the closing position. The coil of the overcurrent release and the thermal element of the thermal release are connected in series with the main circuit and the coil of the undervoltage release is connected in parallel with the supply. When the circuit is short-circuited or severely overloaded, the armature of the over-current release pulls in, causing the free tripping mechanism to act, and the main contact opens the main circuit. When the circuit is overloaded, the thermal element of the thermal release heats up to bend the bimetal and push the free tripping mechanism. When the circuit is under voltage, the armature of the undervoltage release releases. The free trip mechanism is also actuated.

 

Product selection

In civil building design, low-voltage circuit breakers are mainly used for overload, short-circuit, over-current, voltage loss, under-voltage, grounding, leakage, dual-supply automatic switching and protection, operation of the motor when the motor is not frequently started. Principles In addition to complying with the basic principles of the use environment characteristics of low-voltage electrical equipment (see Industrial and Civil Distribution Design Manual), the following conditions should also be considered:
1) The rated voltage of the circuit breaker shall not be less than the rated voltage of the line;
2) The rated current of the circuit breaker and the current rating of the overcurrent release shall not be less than the calculated current of the line;
3) The rated short-circuit breaking capacity of the circuit breaker is not less than the maximum short-circuit current in the circuit;
4) Selective distribution circuit breakers need to consider the short-delayed short-circuit breaking capacity and delay protection level coordination;
5) The rated voltage of circuit breaker undervoltage release is equal to the rated voltage of the circuit;
6) When used for motor protection, the selection of circuit breakers must take into account the starting current of the motor and make it not active during the starting time; design calculations refer to “Industrial and Civil Distribution Design Manual”;
7) Select circuit breakers should also consider the selective cooperation of circuit breakers and circuit breakers, circuit breakers and fuses.

 

Design Points

(1)The coordination ofcircuit breakers shall take into account the instantaneous release action value of the upper circuit breaker and shall be greater than the maximum expected short-circuit current at the outlet end of the lower circuit breaker, if the impedance value of the circuit element is small when the two-stage circuit breaker is short-circuited. To make the difference in the short-circuit current value small, the upper circuit breaker can be selected with a short-delay release.
(2) Current-limiting circuit breakers will trip within a few milliseconds when the short-circuit current is greater than or equal to the setting value of their instantaneous trip. Therefore, the lower-level protection circuit breakers should not use circuit breakers for selective protection.
(3) When a circuit breaker with a short delay is set at the maximum delay, its ability to turn off will be reduced. Therefore, in the selective protection loop, the short delay switching capacity of the circuit breaker should be considered.
(4) It should also be considered that the short-circuit delay return characteristic of the upper circuit breaker and the time characteristic of the lower circuit breaker operating characteristic should not intersect, and the short delay characteristic curve and the transient characteristic curve should not intersect.
(5) When using circuit breakers and fuses in conjunction with the upper and lower levels, the ampere-second characteristic of the circuit-breaker should be compared with the ampere-second characteristic curve of the fuse so that the protection selectivity can be provided in the event of a short-circuit current.
(6) When the circuit breaker is used for the protection of distribution lines, circuit breakers with long time-delay action over-current releases should be used. When a single-phase ground fault occurs at the end of the line, the short-circuit current is not less than instantaneous or short-time delay overcurrent of the circuit breaker. 1.5 times the setting current of the trip unit.

Miniature circuit breaker model meanings generally include: enterprise feature code, product code, design number, shell frame rated current, trip type, number of poles, such as SSB65-C63/3P, HSB1-D63/2P, SBTB1/2-63 , BYM-63/C63/4P C type 2P.

 

Miniature Circuit Breaker Operating Conditions

1), the ambient air temperature limit does not exceed +40 °C, the lower limit is not less than -5 °C, and the average temperature of 24h does not exceed +35 °C;
Note 1: The lower limit is -10 °C or -25 °C operating conditions, the user must declare to the manufacturer at the time of ordering;
Note 2: The user must consult with the manufacturer if the upper limit value exceeds +40°C or the lower limit is lower than -25°C.
2) The altitude of the installation site does not exceed 2000m;
3) Atmospheric relative humidity does not exceed 50% when the ambient air temperature is +40°C. Relatively high relative humidity may be allowed at lower temperatures, eg 90% at +20°C, occasionally due to temperature changes Condensation should take appropriate measures;
4) Pollution level: 2;
5) Installation categories: Category II and III;
6) The external magnetic field of the installation site should not exceed 5 times of the geomagnetic field;
7), generally installed vertically, any direction of tolerance 2 °;
8) There should be no significant impact and vibration at the installation site.

 

Miniature circuit breaker parameters

1) Rated working voltage (Ue): AC 230V/400V (1P), 400V (2P, 3P, 4P).
2) Rated insulation voltage (Ui): 600V.
3) Rated impulse withstand voltage (Uimp): 4KV.
4) Rated current (In): 1A, 3A, 6A, 10A, 16A, 20A, 25A, 32A, 40A, 50A, 63A.
5), rated short-circuit breaking capacity (Icn): 6A-40A: 6000A, 50A-63A: 4500A.
6), running short-circuit breaking capacity (Ics): 6A-40A: 6000A, 50A-63A: 4500A.
7) Number of poles: a. Unipolar (1P); b. Dipole (2P); c. Tripolar (3P); d. Quadrupole (4P).
8) Instantaneous trip type: a. Type C (5ln-10ln); b. Type D (10ln-16ln).
9) Baseline ambient temperature: 30°C.
10) Grid distance (mm): 6A-40A: 50mm, 50A-63A: 45 mm.
11) Shell protection grade: IP20.
12) Lifetime: a. Electrical life: no less than 4,000 times; b. Mechanical life: no less than 20,000 times.

 

Application of miniature circuit breakers

Miniature circuit breaker has the advantages of advanced structure, reliable performance, strong breaking capacity, and beautiful appearance. It is mainly used in places with AC 50HZ or 60HZ, rated voltage below 400V and rated operating current below 63A. For overloading and short-circuit protection of lighting, distribution lines and equipment in office buildings, residential buildings and similar buildings, it can also be used for infrequent on-off operation and conversion of the line. It is mainly used in various places such as industrial, commercial, high-rise, and residential houses.

 

Miniature circuit breakers

The miniature circuit breaker consists of the following major parts:
1. Breakable contact coupling. It consists of a fixed contact and a movable contact.
2. A device composed of a metal cylinder or an insulating cylinder, in which a breakable contact coupling and arc extinguishing device and a transmission device are mounted.
3. Drives that move the contacts.
4. Arc extinguishing equipment.
When a small circuit breaker is used to break the circuit, the movable contact of the miniature circuit breaker is separated from the fixed contact by a mechanical method. When closing, the movable contact and the fixed contact are closed by the opposite mechanical motion. When the load circuit is switched on and off, an arc is generated between the fixed contact and the movable contact. The arc generated during the breaking process is much more serious than the closing process. When the breaking current is large, especially when the short circuit is opened, the arc is very large, and it is often very difficult to disconnect the circuit.

 

Characteristics of miniature circuit breaker

1) The rated short circuit breaking capacity is high, and some rated current levels can reach 10KA or more.
2) Dual wiring function, convenient connection of standard busbars and soft wires.
3), with finger-touch protection combination terminal blocks, higher security.
4) With the operation of energy storage mechanism, the contacts are quickly closed, which overcomes the adverse effects caused by the speed of the manual operation of the manpower and improves the service life of the product.
5), a variety of accessories to choose from, and modular, flexible combination, the user is very easy to install.
6) The shell and some of the functional parts are made of imported high-flame-retardant, high-temperature-resistant, impact-resistant plastics.
7) High current limiting coefficient and superior performance to price ratio.

 

China National Standard of Miniature Circuit Breakers

GB 10963-1989 | Circuit breakers for household and similar premises
GB 14048.2-1994 | Low-voltage switchgear and controlgear devices
GB 16916.1-1997 | Residual current operated circuit-breakers without overcurrent protection for household and similar uses (RCCB) Part 1: General rules
GB 16916.21-1997 | Residual current operated circuit-breakers without internal overcurrent protection (RCCB) for household and similar uses – Part 2.1: Applicability of the general rules for RCCBs with independent operation functions and line voltage
GB 16916.22-1997 | Residual current operated circuit-breakers without internal overcurrent protection (RCCB) for household and similar uses – Part 2.2: Applicability of general rules for RCCB with regard to operating function and line voltage
GB 16917.1-1997 | Residual current operated circuit-breakers with overcurrent protection (RCBO) for household and similar use Part 1: General rules
GB 16917.21-1997 | Residual current operated circuit-breakers with overcurrent protection for household and similar uses (RCBO). Part 2.1: Applicability of the general rules to RCBOs with independent operation functions and line voltage
GB 16917.22-1997 | Residual current operated circuit-breakers with overcurrent protection (RCBO) for household and similar uses – Part 2.2: Applicability of the general rules for RCBO with regard to operating function and line voltage
GB 1984-1989 | AC high voltage circuit breaker
GB 4876-1985 | Line charging current switching test for AC high voltage circuit breakers
GB 7675-1987 | Open-close capacitor bank test for AC high-voltage circuit breakers

 

 

Author Details
Sorry! The Author has not filled his profile.