The model and specification of the hottest air swi

Selection of model, specification and breaking capacity of air switch circuit breaker

I. circuit breaker model and selection

low voltage circuit breaker is not only the power supply switch of the circuit, but also has multiple protection functions such as short circuit, overload and undervoltage. After breaking the fault current, it can restore the power supply without replacing parts. These advantages make it more and more widely used in various electrical systems

low voltage circuit breaker is an important protective device in the subway train control system and auxiliary system. The selection and application of low voltage circuit breaker are directly related to the reliability of subway train operation. If the protection setting value of low-voltage circuit breaker is too large, it will not play a protective role; On the contrary, if the protection setting value of low-voltage circuit breaker is too small, it will cause frequent tripping

when selecting a circuit breaker, the designer should not only determine the type and performance parameters of the circuit breaker according to the characteristics of the protected circuit, but also consider the restrictions on the installation position and overall dimension of the circuit breaker. How to correctly select and use low-voltage circuit breakers is a practical problem that system developers and designers must pay attention to and solve

1, basic knowledge of low-voltage circuit breaker

1.1, structure and working principle of low-voltage circuit breaker

the main contact of low-voltage circuit breaker is closed manually or electrically. 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 power supply

when the circuit is short circuited or seriously overloaded, the armature of the over-current release pulls in, making the free tripping mechanism act, and the main contact disconnects the main circuit

when the circuit is overloaded, the thermal element of the thermal release heats up, bending the bimetal, pushing the free tripping mechanism to act, and the main contact disconnects the main circuit

when the circuit is under voltage, the armature of the under voltage release releases, which also makes the free tripping mechanism act, and the main contact disconnects the main circuit

when the shunt release button is pressed, the armature of the shunt release is pulled in, making the free release mechanism act, and the main contact disconnects the main circuit

1.2, classification of low-voltage circuit breakers

low voltage circuit breakers are classified according to different extinguishing media. Circuit breakers that use air as arc extinguishing medium are called air circuit breakers (air switches); 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)

1.3, the low-voltage circuit breaker also has many main parameters of the missing circuit breaker

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, which is the voltage value to ensure the long-term normal operation of the contactor contact

(2) rated current

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

⑶ tripping current

tripping current is the current setting value that causes the overcurrent release to act. When the circuit is short circuited or the load is seriously overloaded, and the load current is greater than the tripping current, the main contact of the circuit breaker is disconnected

⑷ overload protection current and time curve

overload protection current and time curve are inverse time limit characteristic curves. The larger the overload current is, the shorter the action time of the thermal release is

(5) rated voltage of undervoltage release coil

the rated voltage of undervoltage release coil must be equal to the rated voltage of the line

⑥ rated voltage of shunt release coil

the rated voltage of shunt release coil must be equal to the control power supply voltage

⑺ rated limit short-circuit breaking capacity ICU circuit breaker has two indicators of breaking capacity: rated limit short-circuit breaking capacity ICU and rated operating short-circuit breaking capacity ICs. The rated limit short-circuit breaking capacity ICU is the limit parameter of the circuit breaker's breaking capacity. After breaking several short-circuit faults, the breaking capacity of the circuit breaker will decrease

the rated operating short-circuit breaking capacity ICs is a breaking index of the circuit breaker, that is, it can ensure its normal operation after breaking several short-circuit faults

for molded case circuit breakers, ICS is qualified as long as it is greater than 25% ICU. At present, most ics of circuit breakers in the market are between (50% - 75%) ICU

8 current limiting breaking capacity

current limiting breaking capacity refers to the ability to limit fault current when the circuit breaker trips in case of short circuit. When a short circuit occurs in the circuit, the contact of the circuit breaker will open quickly and produce an arc, which is equivalent to a rapidly increasing arc resistance in series in the circuit, thus limiting the increase of fault current, reducing the electromagnetic effect, electric effect and thermal effect of the short circuit current, which will have a negative impact on the circuit breaker and electrical equipment, and extending the service life of the circuit breaker. The shorter the breaking time of the circuit breaker, the better the current limiting effect, and the closer ICs is to ICU

⑼ tripping characteristics of miniature circuit breakers

tripping characteristics of circuit breakers are divided into a, B, C, D, K, etc., and their respective meanings are as follows:

type a tripping characteristics: the tripping current is (2 ~ 3) in, which is suitable for protecting semiconductor electronic circuits, measuring circuits with low-power power transformers, or systems with long lines and low short-circuit current

type B tripping characteristics: the tripping current is (3 ~ 5) in, which is suitable for household distribution system, household appliances protection and personal safety protection

c type tripping characteristics: the tripping current is (5 ~ 10) in, which is suitable for protecting distribution lines and lighting lines and motor circuits with high making current

d tripping characteristics: the tripping current is (10 ~ 20) in, which is suitable for protecting equipment with high impulse current, such as transformers, solenoid valves, etc

k type tripping characteristics: it has 1.2 times of thermal tripping action current and 8 ~ 14 times of magnetic tripping action range, which is suitable for protecting motor line equipment and has high impact current resistance

1.4, the main functions of the low-voltage circuit breaker

⑴ short circuit protection

short circuit protection refers to the tripping of the circuit breaker. To achieve reliable short-circuit protection, we must pay attention to the following two points:

① the rated operating short-circuit breaking capacity ics of the circuit breaker should be greater than the short-circuit current when the circuit is short circuited at the near end

② the setting value of circuit breaker tripping protection should be greater than the normal operating current of the load (including the load starting current) and less than the short-circuit current when the circuit is short circuited at the far end. In this way, the short-circuit protection can be realized reliably without affecting the normal operation of the circuit

⑵ overload delay protection

overload delay protection refers to that when the load current exceeds the limited range of electrical equipment, the circuit breaker can cut off the power supply according to the set delay time, so that the circuit and equipment can be effectively protected

the setting of overload delay time of circuit breaker should be matched according to the nature of the controlled load and overload characteristics, which should not be too long or too short. The delay time is too long, and the equipment cannot be effectively protected; The delay time is too short, which affects the normal operation of the equipment, such as: the motor cannot be started, the incandescent lamp cannot be lit, the capacitor cannot be charged, etc

⑶ leakage protection

the working principle of the electronic leakage protector is: the executive circuit receives the induced voltage signal on the secondary side of the zero sequence current transformer. When the leakage current reaches the set value, the transfer contact is driven to output the leakage protection signal, so that the release acts to cut off the power supply

the setting value of leakage tripping current of general terminal switch is 30mA, and the setting value of leakage tripping current of upper level branch switch is 300mA. The leakage device can reliably disconnect the grounding fault and prevent personal electric shock and phase to ground short circuit fault

⑷ isolation function

isolation function means that the leakage current after the circuit breaker is disconnected will not cause harm to people and equipment. After multiple short-circuit trips, the switch performance will decline and the leakage current will increase

for human body, the safe leakage current is less than 30mA; For circuits and electrical equipment, the safe leakage current is less than 300mA. If the leakage current exceeds 300mA and the leakage lasts for more than 2 hours, the insulation may be damaged, phase to ground short circuit may occur, and then fire may be caused

2, type selection of circuit breaker

2.1, determination of adaptability of circuit breaker environment

use environment mainly refers to: temperature (maximum temperature and minimum temperature), humidity (generally refers to the maximum relative humidity at 40 ℃), air pressure (use height below 1000 meters can not be considered), vibration and impact, salt spray, installation position, installation mode, installation size, etc

environmental adaptability is one of the reliability indexes of circuit breaker. If it is used under environmental mechanical conditions beyond the product standard, it may damage the relay. In actual use, the environmental adaptability performance index of the relay can be determined according to the actual environmental conditions of the contactor or the environmental conditions of a higher level

2.2, determination of the type of circuit breaker

⑴ the voltage level of the control system and auxiliary system of the subway train is not high, the power is not large, and the installation space is limited (guide rail installation). Miniature air circuit breakers (air switches) should be selected

⑵ in the control system of metro train, the objects of circuit breaker protection mainly include: electronic equipment, dc/dc power module, command switch (driver controller, button, selector switch), indicator light, relay, contactor, solenoid valve, heater, micro motor, etc. Most loads have impulse current, so miniature circuit breakers with C-type tripping curve should be selected

⑶ in the auxiliary system of metro train, the objects of circuit breaker protection mainly include: cooling fan, air conditioner, air compressor, air conditioner, emergency ventilation inverter, lighting, socket, etc. Most loads are AC asynchronous motors, and there is impulse current when starting. Miniature circuit breakers with C-type tripping curve should be selected

2.3, determination of circuit breaker parameters

2.3.1, determination of rated voltage

the rated voltage of the circuit breaker should be greater than or equal to the normal working voltage of the line and equipment

2.3.2, determination of the rated current. The rated current is determined by the following formula:

ib ≤ in ≤ iz

where: IB -- calculated load current of the line (a)

in -- rated current of the main contact (a) iz -- allowable continuous current carrying capacity of the conductor (a)

considering a certain margin, generally choose: in = 1.2ib~1.4ib

the rated current of the circuit breaker should neither be too small nor too large. If it is too small, it will cause frequent false tripping; If the load is too large, it will not trip when the load is overloaded, and the protection function will be lost

2.3.3， Determination of setting current of overcurrent release

ilmax

where: I2 -- setting current of overcurrent release (a)

ilmax - maximum load current (including motor starting current, charging current of input capacitor of power module, etc.)

ismin - Minimum short-circuit current of line (a) (short circuit of line away from circuit breaker end)

the setting current of the overcurrent release is less than the minimum short-circuit current of the line (the line is short circuited away from the circuit breaker end), ensuring that a short circuit occurs anywhere in the circuit, which can cause the circuit breaker to trip

the action current setting value of overcurrent trip corrosion resistance (Experiment in solution) and other devices can be fixed or adjustable. When adjusting, it is usually the lever of the current release

2.3.4, determination of breaking capacity

ICU> ICs> ismax

where: ismax - Maximum short-circuit current of the line (a) (short circuit near the circuit breaker end of the line)

ICs - rated operation