Electrical Knowledge

Molded Case Circuit Breaker

Publish Time: 2022-07-27 11:14:18     Author: DELIXI

The molded case circuit breaker can automatically cut off the current when it exceeds the tripping setting. Plastic case refers to the use of plastic insulators as the outer shell of the device to isolate the conductors and ground the metal parts. Molded case circuit breakers usually contain thermal and magnetic jump units, while larger models are equipped with solid state jump sensors. The tripping unit is divided into thermal magnetic tripping and electronic tripping. The commonly used rated current is 16, 25, 30, 40, 50, 63, 80, 100, 125, 160, 200, 225, 250, 315, 350, 400, 500, 630A.

Molded case circuit breakers are also known as device circuit breakers. All components are sealed in plastic enclosures. Auxiliary contacts, undervoltage tripping and shunt tripping are modular. Because the structure is very compact, the plastic case circuit breaker is basically impossible to overhaul. It adopts manual operation, large capacity can choose electric splitting and closing. Due to the application of electronic overcurrent trip, the plastic case circuit breaker can also be divided into two types: Class A and Class B. Class B has good three-stage protection characteristics, but due to the price factor, the market share of class A products using thermal magnetic trip is higher. Molded case circuit breaker is to contact, arcing chamber, trip and operating mechanism has been packed in a plastic shell, generally do not consider maintenance, applicable to branch protection switch, overcurrent trip with thermal magnetic and electronic type two kinds, the general thermal magnetic molded case circuit breaker is a selective circuit breaker, only overload long time delay and short circuit instantaneous protection way, The electronic plastic case circuit breaker has four protection functions: overload long delay, short circuit short delay, short circuit instantaneous and ground fault. Some of the newly introduced products of electronic plastic case circuit breaker also have the function of area selective interlocking. Most moulded circuit breakers are operated manually, but some are operated with electric motors.

The main contacts of low-voltage circuit breakers are closed manually or electrically. After the main contact is closed, the free trip mechanism locks the main contact in the closing position. The coil of the overcurrent trip and the thermal element of the thermal trip are connected in series with the main circuit, and the coil of the undervoltage trip is connected in parallel with the power supply.

When the circuit is short-circuited or seriously overloaded, the armature of the overcurrent tripper pulls in, making the free tripper act, and the main contact disconnects the main circuit.

When the circuit is overloaded, the hot element of the thermal trip heats up and bends the bimetal sheet, pushing the free trip mechanism into action, and the main contact breaks the main circuit.

When the circuit is undervoltage, the armature of the undervoltage tripper is released, which also makes the free tripper act and the main contact disconnects the main circuit.

When the shunt tripping button is pressed, the shunt tripping device arbit pulls in, making the free tripping mechanism act and the main contact disconnects the main circuit.

Low-voltage circuit breaker is classified according to the different extinguishing medium, using air as the arc extinguishing medium circuit breaker, known as the air circuit breaker (air switch); The circuit breaker using inert gas as arc extinguishing medium is called inert gas circuit breaker (inert gas switch); The circuit breaker using oil as the arc extinguishing medium is called oil circuit breaker (oil switch).

Main Parameters

⑴ Rated voltage

The rated voltage on the nameplate of the circuit breaker refers to the rated voltage of the main contact of the circuit breaker and is the voltage value to ensure the normal operation of the contactor contact for a long time.

(2) Rated current

The rated current on the contactor nameplate is the rated current of the main contact of the guide, which is the current value to ensure the long-term normal operation of the contactor contact.

⑶ Trip current

Trip current is the current set value that makes the overcurrent trip act. When the circuit is short-circuited or the load is seriously overloaded and the load current is greater than the trip current, the main contact of the circuit breaker will break.

(4) Overload protection current, time curve

The curve of overload protection current and time is the inverse time characteristic curve. The larger the overload current is, the shorter the operation time of the thermal trip is.

⑸ Rated voltage of the undervoltage trip coil

The rated voltage of the undervoltage trip coil must be equal to the rated voltage of the line.

⑹ shunt trip coil rated voltage

The rated voltage of the shunt trip coil must be equal to the voltage of the control supply.

⑺ Rated limit short circuit breaking capacity Icu

There are two kinds of breaking capacity indicators for circuit breakers: rated limit short circuit breaking capacity Icu and rated operation short circuit breaking capacity Ics.

Rated limit short circuit breaking capacity Icu is the limit parameter of breaker breaking capacity. After several short circuit faults are broken, breaker breaking capacity will decrease.

Ics for rated operation short circuit breaking capability is a breaking index of a circuit breaker, that is, after breaking several short circuit faults, it can still ensure its normal operation.

For molded case circuit breakers, Ics is qualified as long as it is greater than 25% of Icu, and the majority of Ics for circuit breakers on the market are between (50%-75%) icus.

⑻ Current limiting and breaking capability

Current limiting and breaking capability refers to the ability to limit the fault current when the circuit breaker trips when a short circuit occurs. Open circuit when the short circuit, the circuit breaker contacts quickly, produce arc, equivalent to a string into a rapid increase in a circuit of arc resistance, limiting the fault current increase, reduce the short circuit current of electromagnetic effect, electric effect and thermal effect of circuit breaker and adverse effects of electrical equipment, prolong the service life of circuit breaker. The shorter the breaker disconnect time, the better the current limiting effect, and the closer the Ics is to the Icu.

⑼ Tripping characteristics of the micro circuit breaker

The tripping characteristics of circuit breakers are classified into A, B, C, D, and K, and their meanings are as follows:

Type A tripping characteristics: tripping current is (2 ~ 3) In, suitable for protecting semiconductor electronic lines, measuring lines with small power supply transformers, or systems with long lines and small short circuit current;

Type B tripping characteristics: tripping current is (3 ~ 5) In, suitable for household power distribution system, household appliances protection and personal safety protection;

Type C tripping characteristics: tripping current is (5 ~ 10) In, suitable for protecting distribution lines and lighting lines and motor circuits with high turn-on current;

Type D tripping characteristics: tripping current is (10 ~ 20) In, suitable for protecting equipment with high impact current, such as transformers, solenoid valves, etc.

Type K tripping characteristics: with 1.2 times of thermal tripping action current and 8 ~ 14 times of magnetic tripping action range, suitable for protecting motor line equipment, with high impact current resistance.


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