How to choose molded case circuit breaker

2022-08-20
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How to choose molded case circuit breaker

1 Application of molded case circuit breaker

molded case circuit breaker is widely used in industry and agriculture, transportation, mining, civil construction, national defense and other departments. It plays a great role in power transmission and distribution, circuit control and protection. It is a widely used product. Because users do not understand the characteristics and technical requirements of molded case circuit breakers deeply or comprehensively, some concepts are easy to be confused with each other, and some biases and misunderstandings often occur in practical applications. This paper introduces the problems that are easy to occur when users choose and use molded case circuit breakers, so as to help users choose and use molded case circuit breakers reasonably

2. The rated current of the shell level of the circuit breaker refers to the rated current of the largest release that can be installed in the frame and plastic shell with the same basic size

rated current of circuit breaker refers to the current that the release in the circuit breaker can pass through for a long time, which is also called rated current of circuit breaker release

in the same series, there are multiple rated currents of the same shell level, and there are multiple rated currents of the same shell level. For example, DZ20 series has rated currents of 100, 225, 400, 630, 800, 1250 and other shell levels, while the rated currents of 100 shell levels include 16a, 20a, 25A, 32a, 40a, 50a, 63A, 80A, 100A; There are 100A, 125A, 160A, 180A, 200A and 225A rated currents in the rated current of 225A shell level. There is 100A rated current in both DZ and DZ shell levels, but the volume, shape and breaking capacity of the circuit breaker are different, so the model should be filled in completely when selecting, that is, the rated current of the circuit breaker within the rated current of the specific shell level. The rated current classification is selected according to the priority factor of (1.25): on the one hand, it meets and meets the needs of the maximum rated current of lines and electrical components

on the other hand, it is for standardization to obtain the best benefits of using wires and processing. Therefore, the levels specified are: 3 (6), 8, 10, 12.5, 16, 20, 25, 32, 40, 50, 63, 80100, 125, 160, 200, 250, 315, 400A, etc. Because of this regulation, when the calculated load of the line is 90a, only 100A specification can be selected, which affects its protection performance to a certain extent

the current setting value of the release refers to that the release is adjusted to the action current value. It refers to the multiple of the rated current in, which is the action current value. For example, if the overcurrent is set to 1.2, 1.3, 5, 10 times the current, it is written as IR = 1.2in, 1.3in, 5in, 10In, etc. At present, the overload long-time rated current of some electronic releases. The advanced automation system is not limited to one kind of test. It is adjustable. In fact, the adjusted current is still the rated current, which is the maximum current that can pass for a long time

the rated working current is the actual working current of the contact under a certain working voltage when the circuit breaker is equipped with auxiliary contacts (accessories). The current is 3A or 6a, which is used for control and protection circuits

3. Rated insulation voltage

rated insulation voltage is the voltage value of the design circuit breaker, and the electrical clearance and creepage distance should be determined with reference to this value. Some circuit breakers do not specify the rated insulation voltage. The maximum value of the rated working voltage should be regarded as the rated insulation voltage. In any case, the maximum rated working voltage does not exceed the rated insulation voltage. See Table 1 for the rated insulation voltage and power frequency test voltage of the circuit breaker

rated working voltage refers to the voltage value related to the making and breaking capacity and use category. The rated working voltage of molded case circuit breakers is mostly 50Hz and 380V, but there are also molded case circuit breakers with 50Hz and 600V, and the rated working voltage of 380V and 50Hz. It is absolutely not allowed to use 660V or 1140V power supply voltage

the rated control power supply voltage is the voltage when the molded case circuit breaker is equipped with shunt release and electric mechanism accessories. There are AC and DC voltages, and AC or DC must be indicated when selecting

4. The influence of spring rebound stress on experimental data is overcome. The rated limit short-circuit breaking capacity

the rated limit short-circuit breaking capacity refers to the breaking capacity under specified conditions. After acting according to the specified test procedure, the rated current of the circuit breaker will not be considered

rated operating short-circuit breaking capacity refers to the breaking capacity under specified conditions. After the specified test procedure, the rated current of the circuit breaker must be considered

in order to meet the needs of different users, many domestic circuit breaker manufacturers now have different levels of short-circuit breaking capacity for the rated current of the same shell level. For example, Changshu switchgear cm is divided into c basic type (25 ~ 35ka), l standard type (35 ~ 50kA), m higher breaking type (50 ~ 75kA), h high breaking type (85 ~ 100kA). When selecting, the user should meet the limit short-circuit breaking capacity of the circuit breaker, and the expected short-circuit current of the line can meet the requirements, without adding the insurance coefficient artificially, so as to avoid waste

the value of rated operating short-circuit breaking capacity can be 25%, 50%, 75% and 100% of the rated limit short-circuit breaking capacity. Most circuit breakers are 50% - 75%, and a few are 100% (ICS = ICU), such as ns products of Merlin Gerland company of Schneider, France

5. Accessory function

as the derivation and supplement of the function of the circuit breaker, accessories add control means and expand the protection function for the circuit breaker. They are an integral part of the circuit breaker, mainly including auxiliary contacts, alarm contacts, shunt release, undervoltage release, electric operating mechanism, external rotating operating handle and other accessories

1) the auxiliary contact is mainly used to display the opening and closing status of the circuit breaker, but it cannot display whether there is fault tripping. It is connected to the control circuit of the circuit breaker. The rated current of the shell frame of the molded case circuit breaker is 100, which is the single breakpoint switching contact. 225 and above are bridge contact structures, and the agreed thermal current is 3A; The rated current of the shell frame is 400 and above, and two normally open and two normally closed can be installed. The agreed heating current is 6A

2) the alarm contact is mainly used for free tripping in case of overload, short circuit, undervoltage and other faults of the load of the circuit breaker. The working current of the alarm contact is: AC380V, 0.3A, dc220v, 0.15A, which generally will not exceed 1a, while the heating current can be in the range of 1 ~ 2.5A

3) the shunt release is an accessory for remote control of opening. Its voltage can be independent of the voltage of the main circuit. The shunt release is a short-time working system, and the coil power on time generally cannot exceed 1s, otherwise the coil will be burned. In order to prevent the coil from burning out, a microswitch is connected in series in the coil of the shunt release of the molded case circuit breaker. When the shunt release is powered on, the armature is pulled, and the microswitch changes from normally closed to normally open. Because the control line of the shunt release power supply is cut off, even if the button is pressed artificially, the shunt coil is never powered on. The coil burning is avoided. When the circuit breaker is closed again, the microswitch is in the normally closed position again. The shunt release has a variety of control voltages and different power frequencies, which can be selected for different occasions and different power supplies

4) the undervoltage release is used as a long-term voltage protection for lines and power supply equipment. When in use, the coil of the undervoltage release is connected to the power side of the circuit breaker. The circuit breaker can be closed only after the undervoltage release is powered on, otherwise the circuit breaker cannot be closed. Users should confirm whether the operating voltage of the release is consistent with the "green difficulty" faced by the line and undervoltage industry. The working range of undervoltage is (70% - 35%) UN. The undervoltage release also has a variety of rated working voltages and different power supply frequencies, which can be selected for different occasions and different power supplies

5) electric operating mechanism is used for automatic control of circuit breaker and remote closing and opening. There are two kinds of electric operating mechanism and electromagnetic operating mechanism: the electric operating mechanism is driven by a motor, which is generally suitable for circuit breakers with a rated current of 400A and above, and the electromagnet operating mechanism is suitable for circuit breakers with a rated current of 225A and below

6. Arcing distance

when the circuit breaker breaks a large short-circuit current, its dynamic and static contacts will produce an arc when they are separated, and a part of the arc or ionized gas will be ejected from the arc nozzle at the power end of the circuit breaker. The arc itself is a huge current, which is easy to cause interphase short circuit and grounding short circuit accidents between exposed conductors and between exposed electrified bodies and "ground" (the metal shell of the complete set of equipment is grounded). To ensure safety, users should leave a certain distance according to the product samples of the manufacturer or the data provided in the instruction manual. If the height distance of distribution boxes and cabinets is not enough, products with small or zero arcing distance can be selected to ensure power safety

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