Selection Factors and Considerations for Molded Case Circuit Breakers
Selection Factors and Considerations for Molded Case Circuit Breakers
Molded case circuit breakers are a familiar sight, widely used in our daily lives. While there are many types of circuit breakers, why do so many people prefer molded case circuit breakers? What exactly is a molded case circuit breaker As the name suggests, a molded case circuit breaker is a circuit breaker primarily made of plastic. So how do we select the right type? And what considerations should we take into account when selecting one? Let's explore these questions together. Circuit breakers are classified by structure into universal type and molded case type. Molded case circuit breakers are suitable for power distribution systems with a rated voltage of 690V, a frequency of 50/60Hz, and a rated current of 16 to 1600A, or as protection equipment for transformers, motors, capacitors, etc. Their main functions are to distribute electrical energy, provide overload, short circuit, leakage, and undervoltage protection for various branches and equipment, and can also be used for infrequent switching of lines and equipment. Molded case circuit breakers (MCCBs) are widely used in industry, agriculture, transportation, mining, civil construction, and national defense, playing a significant role in power transmission and distribution, and circuit control and protection. They are a widely used product. However, due to users' limited or incomplete understanding of the characteristics and technical requirements of MCCBs, some concepts are easily confused, leading to frequent deviations and misunderstandings in practical applications. Applications and Uses of Molded Case Circuit Breakers MCCBs are widely applicable to power distribution systems with a rated voltage of 690V, frequency of 50/60Hz, and rated current of 16 to 1600A, or as protection equipment for transformers, motors, capacitors, etc. The primary function of MCCBs is to distribute electrical energy and provide overload, short-circuit, leakage, and undervoltage protection for various branches and equipment. They can also be used for infrequent switching of lines and equipment. Selection Methods for Molded Case Circuit Breakers Based on load type: Primarily divided into power distribution systems and motor protection. Based on load capacity: Select a MCCB with a rated current greater than the operating current of the load. Selection based on short-circuit current: The short-circuit breaking capacity of a molded case circuit breaker should be greater than the expected short-circuit current of the circuit. The short-circuit breaking capacity of a molded case circuit breaker determines its reliability; however, while ensuring circuit safety, it's unnecessary to blindly pursue high breaking capacity, which could lead to waste. 5 Important Parameters to Consider When Selecting a Molded Case Circuit Breaker 1. Circuit Breaker Frame Rating: The rated current of the circuit breaker frame rating refers to the rated current of the largest trip unit that can be installed in a frame and plastic housing of the same basic dimensions. The rated current of a circuit breaker refers to the current that the trip unit in the circuit breaker can continuously carry; it is also called the rated current of the circuit breaker trip unit. There are multiple frame ratings within the same series, and multiple rated currents within the same frame rating. For example, the DZ20 series includes frame ratings of 100, 225, 400, 630, 800, and 1250, while the 100 frame rating includes 16A, 20A, 25A, 32A, 40A, 50A, 63A, 80A, and 100A; the 225A frame rating includes 100A, 125A, 160A, 180A, 200A, and 225A. Both the DZ20-100 and DZ20-225 frame ratings have a 100A rated current, but the circuit breaker size, shape, and breaking capacity differ. Therefore, when selecting a circuit breaker, the model number must be filled in completely, specifying the rated current within the frame rating. Rated current classification is selected based on a priority coefficient of (1.25): on the one hand, to meet the needs of the maximum rated current of the line and electrical components; on the other hand, for standardization, to achieve optimal efficiency in conductor usage and processing. Therefore, the specified levels 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, thus affecting its protection performance to a certain extent. The trip unit current setting value refers to the operating current value adjusted by the trip unit. It refers to a multiple of the rated current In, which is the operating current value. For example, overcurrent settings to 1.2, 1.3, 5, or 10 times the current are written as Ir = 1.2In, 1.3In, 5In, 10In, etc. Currently, some electronic trip units have adjustable overload long-delay rated currents. The adjusted current is still the rated current, which is the maximum current that can be passed continuously. Rated operating current is the actual operating current of the circuit breaker contacts under a certain operating voltage when auxiliary contacts (accessories) are installed. This 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 designed for the circuit breaker. Electrical clearances and creepage distances should be determined with reference to this value. Some circuit breakers do not explicitly specify a rated insulation voltage; in such cases, the maximum value of the rated operating voltage should be considered the rated insulation voltage. Under no circumstances should the maximum rated operating voltage exceed the rated insulation voltage. The rated insulation voltage of the circuit breaker is related to the power frequency test voltage. Rated operating voltage refers to the voltage value related to breaking capacity and usage category. The rated operating voltage of molded case circuit breakers is mostly 50Hz, 380V, but some are 50Hz, 600V. Molded case circuit breakers with a rated operating voltage of 380V, 50Hz are absolutely not allowed to be used with 660V or 1140V power supply voltages. The rated control power supply voltage is the voltage of the molded case circuit breaker when the shunt trip unit and electric mechanism accessories are installed. Both AC and DC voltages are available; please specify AC or DC when selecting. 3. Rated Ultimate Short-Circuit Breaking Capacity The rated ultimate short-circuit breaking capacity refers to the breaking capacity under specified conditions. After operating according to the specified test procedure, the circuit breaker is not considered to continue carrying its rated current. The rated operating short-circuit breaking capacity refers to the breaking capacity under specified conditions. After operating according to the specified test procedure, the circuit breaker must be considered to continue carrying its rated current. To meet the needs of different users, many circuit breaker manufacturers in China now classify the short-circuit breaking capacity of the same frame class rated current into different levels. For example, Changshu Switchgear Factory's CM1-100 is divided into C basic type (25-35kA), L standard type (35-50kA), M higher breaking type (50-75kA), and H high breaking type (85-100kA). When selecting, users should ensure that the circuit breaker meets the ultimate short-circuit breaking capacity, and that the expected short-circuit current of the line meets the requirements. There is no need to artificially add a safety factor, which would be wasteful. The rated short-circuit breaking capacity can be 25%, 50%, 75%, or 100% of the rated ultimate short-circuit breaking capacity. Most circuit breakers are 50%–75%, with a very few reaching 100% (Ics=Icu), such as the NS products from Schneider Electric's Merlin Gerin. 4. Accessory Functions Accessories, as a derivative and supplement to the circuit breaker's functions, add control methods and expand protection capabilities. They are an integral and important part of the circuit breaker and mainly include auxiliary contacts, alarm contacts, shunt trip units, undervoltage trip units, electric operating mechanisms, and external rotary operating handles. (1) Auxiliary contacts are mainly used to display the open/closed status of the circuit breaker, but cannot indicate whether a fault trip has occurred. They are connected in the control circuit of the circuit breaker. For molded case circuit breakers with a frame rating of 100A, a single-break changeover contact is used; for 225A and above, a bridge contact structure is used, with a conventional heating current of 3A. For frame ratings of 400A and above, two normally open and two normally closed contacts can be installed, with a conventional heating current of 6A. (2) Alarm contacts are mainly used to trip the circuit breaker when the load experiences overload, short circuit, or undervoltage faults. The operating current of the alarm contacts is: AC380V, 0.3A; DC220V, 0.15A, generally not exceeding 1A, while the heating current can be in the range of 1-2.5A. (3) A shunt trip unit is an accessory for remotely controlling the tripping of the circuit breaker. Its voltage is independent of the main circuit voltage. The shunt trip unit operates on a short-time basis; the coil energization time generally cannot exceed 1 second, otherwise the coil will burn out. To prevent coil burnout, a microswitch is connected in series with the shunt trip coil of a molded case circuit breaker. When the shunt trip is energized, the armature is attracted, and the microswitch changes from normally closed to normally open. Since the control circuit of the shunt trip power supply is cut off, even if the button is manually pressed, the shunt coil will never be energized again. This avoids coil burnout. When the circuit breaker is closed again, the microswitch returns to the normally closed position. The shunt trip is available in various control voltages and different power frequencies to suit different occasions and power supplies. (4) Undervoltage release devices are used for overvoltage protection of lines and power equipment. When in use, the undervoltage release coil is connected to the power supply side of the circuit breaker. The circuit breaker can only be closed after the undervoltage release is energized; otherwise, the circuit breaker cannot be closed. Users should confirm that the line voltage and the working voltage of the undervoltage release are consistent. The working range of undervoltage is (70%～35%)Un. Undervoltage release devices also have various rated working voltages and different power frequencies to suit different occasions and power supplies. (5) Electric operating mechanisms are used for the automatic control and remote closing and opening of circuit breakers. There are two types: electric operating mechanisms and electromagnetic operating mechanisms. Electric operating mechanisms are driven by an electric motor and are generally suitable for circuit breakers with a frame-level rated current of 400A and above. Electromagnetic operating mechanisms are suitable for circuit breakers with a frame-level rated current of 225A and below. 5.Arc Distance When a circuit breaker interrupts a large short-circuit current, an arc is generated when its moving and stationary contacts separate. A portion of the arc or ionized gas is ejected from the arc outlet at the power supply end of the circuit breaker. The arc itself is a huge current, which can easily lead to phase-to-phase short circuits and grounding short circuits between exposed conductors and between exposed live conductors and "ground" (the metal casing of the complete set of equipment is grounded). To ensure safety, users should maintain a certain distance according to the data provided in the manufacturer's product catalog or instruction manual. If the height of the distribution box or cabinet is insufficient, products with a small or zero arc distance can be selected to ensure electrical safety. Selection Considerations for Molded Case Circuit Breakers 1. Differences in Breaking Capacity Molded case circuit breakers have two important breaking capacity indicators: rated operating short-circuit breaking capacity (Ics) (the breaking capacity under specified test conditions, including the circuit breaker's ability to continue carrying its rated current) and rated ultimate short-circuit breaking capacity (Icu) (the breaking capacity under specified test conditions, including the circuit breaker's ability to continue carrying its rated current). The difference lies in the rated ultimate short-circuit breaking capacity, which refers to the circuit breaker's ability to interrupt a three-phase short-circuit current at the outgoing terminals and then interrupt that same short-circuit current again. Whether it can subsequently make and break connections normally is not guaranteed. Rated operating short-circuit breaking capacity, on the other hand, requires the circuit breaker to be able to interrupt multiple times under the same conditions. 2. Coordination Between Circuit Breakers The selection of a single circuit breaker directly affects the selection of the overall wiring and conductor cross-sections. Circuit breakers must be selected based on the overall system composition to ensure that a fault at any point in the line can be cleared by the adjacent upstream circuit breaker.
