Five important parameters to pay attention to when selecting molded case circuit breakers

Molded case circuit breakers (plastic case air insulated circuit breakers) are widely used in the low-voltage power distribution industry and are used to cut off or isolate fault currents within normal and rated ranges to ensure the safety of lines and equipment.

Common tripping modes of plastic case circuit breakers are single magnetic tripping, thermal magnetic tripping (complex tripping), and electronic tripping.

Single magnetic tripping means that the circuit breaker will only trip when a short-circuit fault occurs in the line. We usually use this type of switch in heater circuits or motor circuits with overload protection functions.

Thermal-magnetic tripping trips when a short-circuit fault occurs on the line or the line current exceeds the rated current of the circuit breaker for a long time. Therefore, it is also called compound tripping and is often used in ordinary power distribution situations.

Electronic tripping is a mature technology that has emerged in recent years. Circuit breakers with electronic tripping have adjustable magnetic tripping current, thermal tripping current, and tripping time. They are applicable to a wider range of situations, but the cost of circuit breakers is high.

In addition to the above three types of trippers, there is also a circuit breaker specially used for motor circuit protection. Its magnetic tripping current is generally more than 10 times the rated current to avoid the peak current when the motor starts and ensure the smooth start of the motor. Do not move the circuit breaker.

Because users do not have a deep or comprehensive understanding of the characteristics and technical requirements of molded case circuit breakers, some concepts are easily confused with each other, and some deviations and misunderstandings often occur in practical applications. The editor of Tongyida below introduces in detail the important parameters that users need to pay attention to when selecting and using molded case circuit breakers to help users choose and use molded case circuit breakers reasonably.

Five important parameters to pay attention to when selecting molded case circuit breakers

1. Circuit breaker frame grade

　　The rated current of the circuit breaker frame class refers to the rated current of the largest trip unit that can be installed in a frame and plastic enclosure of the same basic size.

　　The rated current of the circuit breaker refers to the current that the tripper in the circuit breaker can pass for a long time, also known as the rated current of the circuit breaker tripper.

　　There are multiple rated currents of multiple frame levels in the same series, and there are multiple rated currents within the same frame level rated current. For example, the DZ20 series has 100, 225, 400, 630, 800, and 1250 frame rated currents, while the 100 frame rated currents include 16A, 20A, 25A, 32A, 40A, 50A, 63A, 80A, and 100A. Current; the 225A frame grade rated current includes 100A, 125A, 160A, 180A, 200A, and 225A rated current. Both DZ20-100 and DZ20-225 have a rated current of 100A in both frame grades, but the volume, shape and breaking capacity of the circuit breaker are different. Therefore, when selecting, the model must be filled in completely, that is, the circuit breaking within the rated current of the specific frame grade. rated current of the device. The rated current classification is selected according to the (1.25) priority coefficient: on the one hand, it conforms to and meets the needs of the maximum rated current of circuits and electrical components; on the other hand, it is for standardization to obtain the best benefits of using wires and processing. Therefore, the levels it stipulates are: 3(6), 8, 10, 12.5, 16, 20, 25, 32, 40, 50, 63, 80, 100, 125, 160, 200, 250, 315, 400A, etc. Due to this regulation, when the calculated load of the line is 90A, only the 100A specification can be selected, which affects its protection performance to a certain extent.

　　The current setting value of the release device refers to the operating current value that the release device is adjusted to. It refers to the multiple of the rated current In and is the operating current value. For example, if the overcurrent is adjusted to 1.2, 1.3, 5, or 10 times the current, it is written as Ir=1.2In, 1.3In, 5In, 10In, etc. Nowadays, the overload long-delay rated current of some electronic trippers is adjustable. The adjusted current is actually still the rated current, which is the maximum current that can pass for a long time.

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

2. Rated insulation voltage

　　The rated insulation voltage is the voltage value for which the circuit breaker is designed. The electrical clearance and creepage distance should be determined with reference to this value. Some circuit breakers do not clearly 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. The rated insulation voltage of the circuit breaker is the same as the power frequency test voltage. See Table 1.

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

　　The rated control power supply voltage is the voltage when the molded case circuit breaker is equipped with a shunt release and electric mechanism accessories. There are two voltages: AC and DC. Be sure to indicate AC or DC when selecting.

3. Rated ultimate short circuit breaking capacity

　　Rated ultimate short-circuit breaking capacity refers to the breaking capacity under specified conditions. After operating according to the prescribed test procedures, the circuit breaker continues to carry its rated current regardless of the condition.

　　Rated operating short-circuit breaking capacity refers to the breaking capacity under specified conditions. After following the specified test procedures, it must be considered that the circuit breaker continues to carry its rated current.

　　In order to meet the needs of different users, many domestic circuit breaker manufacturers now divide the short-circuit breaking capacity of the same frame level rated current into different levels. For example, Changshu Switch Factory CM1-100 is divided into C basic type (25~35kA), L standard Type (35～50kA), M high breaking type (50～75kA), H high breaking type (85～100kA). When users choose, they must meet the ultimate short-circuit breaking capacity of the circuit breaker. The expected short-circuit current of the line can meet the requirements. There is no need to artificially add an insurance factor to avoid waste.

　　The value of the rated operating short-circuit breaking capacity can be 25%, 50%, 75%, or 100% of the rated limit short-circuit breaking capacity. Most circuit breakers are 50% to 75%, and a few have 100% (Ics=Icu). Such as the NS products of Merlin Gerin Company of Schneider of France.

4. Attachment function

　　As a derivation and supplement of the function of the circuit breaker, accessories add control means and expand protection functions to the circuit breaker. They are an integral and important part of the circuit breaker. They mainly include auxiliary contacts, alarm contacts, shunt releases, and undervoltage. Releases, electric operating mechanisms, external rotating operating handles and other accessories.

　　1) The auxiliary contact is mainly used to display the opening and closing status of the circuit breaker but cannot display whether it has tripped due to a fault. It is connected to the control circuit of the circuit breaker. The rated current of the molded case circuit breaker frame is 100, which is a single break point transfer contact. 225 and above are bridge contact structures, with an agreed heating current of 3A; frames with a rated current of 400 and above can be equipped with two normally open and two normally closed contacts, with an agreed heating current of 6A.

　　2) The alarm contact is mainly used to trip freely when the load of the circuit breaker encounters overload, short circuit or under-voltage faults. The working current of the alarm contact is: AC380V, 0.3A, DC220V, 0.15A, which generally does not exceed 1A, while the heating current can range from 1 to 2.5A.

　　3) The shunt release is an accessory for remote control of opening. Its voltage can be independent of the main circuit voltage. The shunt release is a short-time working system. The coil energization time generally cannot exceed 1s, otherwise the coil will burn. In order to prevent the coil from burning out, a microswitch is connected in series with the coil of the shunt release of the molded case circuit breaker. When the shunt release is energized and the armature is closed, the microswitch changes from the normally closed state to the normally open state. Due to the shunt tripping The control circuit of the machine power supply is cut off, and even if the button is pressed artificially, the shunt coil will no longer be energized. This avoids coil burnout and the micro switch returns to the normally closed position when the circuit breaker is closed again. The shunt tripper has a variety of control voltages and different power frequencies, which can be used in different occasions and different power supplies.

　　4) The undervoltage release is used for long-term voltage protection of lines and power equipment. When in use, the undervoltage release coil is connected to the power side of the circuit breaker. The circuit breaker cannot be closed until the undervoltage release is energized, otherwise the circuit breaker cannot be closed. Go to the gate. The user should confirm whether the working voltage of the line and the undervoltage release are consistent. The working range of undervoltage is (70%~35%)Un. Undervoltage releases also have a variety of rated operating voltages and different power frequencies, which can be selected for different occasions and different power supplies.

　　5) The electric operating mechanism is used for automatic control of circuit breakers and remote closing and opening. There are two types of electric operating mechanisms and electromagnetic operating mechanisms: the electric operating mechanism is driven by an electric motor and is generally suitable for circuit breakers with a frame rating of 400A and above. The electromagnet operating mechanism is suitable for circuit breakers with a frame rating of 225A and below.

5.Flashover distance

　　When the circuit breaker breaks a large short-circuit current, an arc is generated when the moving and static contacts are separated. A part of the arc or ionized gas is ejected from the arc outlet at the power end of the circuit breaker. The arc itself is a huge current, which can easily cause Phase-to-phase short circuit and ground short-circuit accidents between exposed conductors and exposed live bodies and "ground" (the metal shell of the complete equipment is grounded). To ensure safety, users should keep a certain distance based on the data provided by the manufacturer's product sample or instruction manual. If the height distance between distribution boxes and cabinets is not enough, you can choose products with small or zero flashover distance to ensure the safety of electricity.