Definition of MCCB(Molded Case Circuit Breakers)

Basic concept of MCCB(Molded Case Circuit Breakers)

MCCB
MCCB

Molded Case Circuit Breaker (MCCB for short) is mainly used to provide protection for overload and short circuit in low voltage distribution system and motor protection circuit. Because of its reliability and stability, it has become a widely used product in industry.

MCCB(Molded Case Circuit Breakers) can automatically cut off the current when the current exceeds the trip setting. Molded shell refers to a plastic insulator used as the housing of the device to isolate the conductors and the grounded metal portion. Molded case circuit breakers usually contain a thermal-magnetic trip unit, while large-sized molded case circuit breakers are equipped with a solid-state trip sensor. The trip unit is divided into: thermal magnetic trip and electronic release. Commonly used, the rated current has the following types: 16 25 30 40 50 60 75 80 100 125 160 200 225 250 315 350 400 500 630 A.

 

MCCB(Molded Case Circuit Breakers) are also referred to as device type circuit breakers. All parts are sealed in a plastic housing. Auxiliary contacts, under voltage releases, and shunt releases are modular. Because of its very compact structure, molded case circuit breakers cannot be overhauled. Many of them use manual operation, large-capacity optional electric split. Due to the application of electronic over-current release, molded case circuit breakers can also be divided into two types: Class A and B. Class B has good three-stage protection characteristics, but due to price factors, the use of thermal magnetic release Class A products have a higher market share. Molded Case Circuit Breakers are equipped with contacts, arc extinguishing chambers, trip units and operating mechanisms all housed in a plastic case. They are generally not considered for maintenance and are suitable for use as branch circuit breakers. Over-current trip units have thermal magnets. Both type and electronic type, general thermal magnetic type MCCB is a non-selective type circuit breaker, and there are only two types of protection methods: overload long delay and short-circuit transient. The electronic type MCCB has overload long delay and short circuit. Delay, short-circuit instantaneous and ground fault four protection functions. Some of the new electronic molded case circuit breaker products also feature regional selective chaining. Most molded case circuit breakers are manually operated and some have motor operating mechanisms.

 

Working conditions of  MCCB(Molded Case Circuit Breakers)

Ambient air temperature
Maximum ambient air temperature +40°C;
Lower ambient air temperature -5°C;
The average ambient air temperature of 24h does not exceed +35°C.
Altitude: The altitude of the installation site does not exceed 2000m.
Atmospheric conditions: Atmospheric relative humidity does not exceed 50% at ambient air temperature of +40°C; higher relative humidity may be obtained at lower temperature; monthly average maximum relative humidity of wettest month is 90%, and the month’s The monthly average minimum temperature is +25°C, taking into account the condensation that occurs on the surface of the product due to temperature changes.
Pollution level: pollution level is 3.

 

Maintenance method of  MCCB(Molded Case Circuit Breakers)

working principle

The main contacts of the low-voltage circuit breaker are 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 causes the bimetal to bend, pushing the free tripping mechanism, and the main contact opens the main circuit.
When the circuit is under voltage, the armature of the undervoltage release releases the free trip mechanism and the main contact opens the main circuit.
When the shunt trip button is pressed, the shunt release armature pulls in and causes the free trip mechanism to act and the main contact opens the main circuit.

 

Classification of MCCB(Molded Case Circuit Breakers)

Low-voltage circuit breakers are classified according to different types of extinguishing medium. Circuit breakers that use air as an arc extinguishing medium are called air circuit breakers (air switches); circuit breakers that use inert gas as an extinguishing medium are called inert gas circuit breakers. (Inert gas switch); A circuit breaker that uses oil as an arc extinguishing medium, called an oil circuit breaker (oil switch).

The main parameters

(1) Rated voltage
The rated voltage on the circuit breaker nameplate refers to the rated voltage of the main contacts of the circuit breaker and is the voltage value that ensures the long-term normal operation of the contactor contacts.
(2) Rated current
The rated current on the nameplate of the contactor is the rated current of the main contact of the circuit breaker and is the current value that ensures the long-term normal operation of the contactor contactor.
(3) Trip current
The tripping current is the current setting value for operating the overcurrent release. When the circuit is short-circuited or the load is heavily overloaded and the load current is greater than the tripping current, the main contact of the circuit breaker is broken.
(4) Overload protection current and time curve
The overload protection current and time curves are inverse time characteristic curves. The greater the overload current, the shorter the time for the thermal release to act.
(5) Rated voltage of undervoltage release coil
The voltage rating of the undervoltage release coil must be equal to the line rated voltage.
(6) Rated voltage of shunt release coil
The rated voltage of the shunt release coil must be equal to the control control supply voltage.
(7) Rated limit short-circuit breaking capacity Icu
There are two kinds of breaking capacity indicators of circuit breakers: rated short-circuit breaking capacity Icu and rated running short-circuit breaking capacity Ics.
The rated limit short-circuit breaking capacity Icu is the limit parameter of the breaking capacity of the circuit breaker. After several short-circuit faults are broken, the breaking capacity of the circuit breaker will decrease.
Rated operating short-circuit breaking capacity Ics is a breaking indicator of the circuit breaker, that is, it can ensure normal operation after several short-circuit faults are broken.
For molded case circuit breakers, if Ics is greater than 25% Icu, even if Icu is qualified, most Ics of circuit breakers currently on the market are between (50% – 75%) Icu.
(8) Limiting current breaking capacity
Current limiting breaking capacity refers to the ability to limit the fault current when the circuit breaker trips when the circuit is short circuited. When the circuit is short-circuited, the contact of the circuit breaker opens quickly and generates an arc, which is equivalent to a rapidly increasing arc resistance in the line, thereby limiting the increase of the fault current, reducing the electromagnetic effect, electric effect and thermal effect of the short-circuit current. The adverse effects on circuit breakers and electrical equipment extend the life of the circuit breaker. The shorter the circuit breaker opening time, the better the current limiting effect, and the closer Ics is to Icu.
(9) Tripping characteristics of miniature circuit breakers
Breaker tripping characteristics are divided into A, B, C, D, K and other types, their respective meanings are as follows:
Type A tripping characteristics: The tripping current is (2 ~ 3) In, suitable for the protection of semiconductor electronic circuits, measuring lines with low-power transformers, or systems with long lines and short-circuit currents;
Type B tripping characteristics: The trip current is (3 ~ 5) In, suitable for household distribution system, household appliances protection and personal safety protection;
Type C tripping characteristics: The tripping current is (5 ~ 10) In, which is suitable for protecting power distribution lines and lighting circuits and motor circuits with high on-current;
D-type tripping characteristics: The trip current is (10 ~ 20) In, suitable for the protection of equipment with high inrush current, such as transformers, solenoid valves, etc.;
K-type tripping characteristics: 1.2 times the thermal tripping action current and 8-14 times the magnetic tripping action range, suitable for protection of motor circuit equipment, high impact current capability.

 

 China National Standard for MCCB(Molded Case 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 | AC high voltage circuit breaker line charging current switching test
GB 7675-1987 | Open-close capacitor bank test for AC high-voltage circuit breakers

 

 

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