Rated limit and rated operating short-circuit brea

Rated limit and rated operating short-circuit breaking capacity of the circuit breaker

when designing and selecting the short-circuit breaking capacity of the low-voltage circuit breaker, the basic principle that users should follow is: the rated short-circuit breaking capacity of the circuit breaker the line may have (expected) short-circuit current

there are two types of short-circuit breaking capacity specified in iec947 - 2 of the International Electrotechnical Commission and gb14048.2 low voltage circuit breakers for low voltage switchgear and control equipment of China; Rated limit short-circuit breaking capacity ICU and rated operating short-circuit breaking capacity ICs

1. The definitions of ICU and ICs

are respectively defined as follows: ICU refers to the conditions specified according to the specified test procedures, excluding the breaking capacity of the circuit breaker to continue to carry its rated current capacity; ICs refers to the conditions specified according to the specified test procedure, including the breaking capacity of the circuit breaker to continue to carry its rated current capacity

icu is o t Co; The test procedure for ICs is o t co t Co. ICs is one more time than ICU. After the program test, it can completely break and extinguish the arc without exceeding the specified damage, which is considered as passing the ICU test. In addition to the same qualification standard as ICU, the IC tensile testing machine s also needs to increase the tests of temperature rise and 5% life times of switching on and off the rated voltage and current. The ICs test conditions are more stringent

2. Relationship between ICU and ICs

icu and ICs are both important technical performance indicators of circuit breakers. From the actual situation, according to the needs of line protection and different structures of circuit breakers, the ICs determined by iec947 - 2 and gb14048.2 have 4 or 3 values, which are 25%, 50%, 75% and 100%icu (for class a circuit breakers, i.e. plastic shell type), or 50%, 75% and 100%icu (for class B circuit breakers, i.e. universal or frame type). The circuit breaker manufacturer shall determine the ICs value of its products. All ICU percentages conforming to the above standards are valid and qualified products

most of the universal circuit breakers (not all specifications) have three-stage protection functions of overload long time delay, short-circuit short time delay and short-circuit instantaneous action, which can realize selective protection. Therefore, most trunk lines (including the outgoing terminal of the transformer) use it as the main (protection) switch, while the molded case circuit breaker generally does not have the short-circuit short-time delay function (only the overload long-time delay and short-circuit instantaneous two-stage protection), and cannot be used for selective protection. They can only be used for branches

due to different use (application) conditions, iec92 marine electrical standard suggests that the universal circuit breaker with three-stage protection should focus on its ICs, while the molded case circuit breaker widely used in branch lines should ensure that it has enough ICU. The author's understanding of this is: the replacement of a new circuit breaker after the fault current of the main line is cut off should be careful. The domestic diaphragm market will develop towards high-end diaphragm products with high porosity and thin film. The long power failure of the main line will affect the power supply of a large number of users. Therefore, in case of short-circuit fault, it is required to have two CO and continue to carry the rated current for a period of time; The circuit breaker with only two-stage protection used in the branch has completed its mission after breaking the limit short-circuit current and making and breaking again. It no longer carries the rated current and can be replaced with a new one (the power outage area is limited to the branch during replacement, so the impact is small), and its ICs can be less than the limit short-circuit current. The specified values of ICs on the two types of circuit breakers are also different. The minimum allowable ICs for the molded case type is 25%icu, while the minimum allowable ICs for the universal type is 50%icu. There are few circuit breakers with ICs = ICU. Even if they are universal, there are few ICs = 100%icu [there is a molded case circuit breaker with rotating double breaking (point) technology, which has excellent current limiting performance and a large margin of short-circuit breaking capacity. ICs = ICU can be achieved, but the price is very high]. In the field of materials, the DW15 universal circuit breaker ICs = (60% ~ 75%) ICU in China, and the DW45 intelligent universal circuit breaker ICs = (62.5% ~ 65%) ICU. Internationally, ABB's f is the asymptote stress column of the corresponding point, and Schneider's M-series ICs is only about 70% ICU. For molded case circuit breakers, most of the ICs of various new types in China are between (50% ~ 75%) ICU. Some manufacturers' advertisements or samples claim that its circuit breaker ICs = ICU. If it is not of current limiting type, it has moisture. The most reliable and serious way to select it is to ask them for the test certificate or type test report of ICs = ICU

3. ICU and ICs select a transformer with a capacity of 1600KVA. The rated current of its secondary side is 2312a, the percentage of impedance voltage UK is taken as 6%, and the maximum expected short-circuit current should be 38.5ka. The rated short-circuit breaking capacity of the circuit breaker used for protection should be 40ka. If 2500A of DW15 - 2500y or 2500A of DW45 - 3200 is selected as the main switch, it is competent. Because the transformer in the modern power center is very close to the distribution cabinet, or even installed together, even if the branch has a rated current of 100A, its expected short-circuit current is very large. Therefore, it is also required that the short-circuit breaking capacity of the molded case circuit breaker in the line should reach 380V and 40ka

some articles concluded that a new type of molded case circuit breaker (shell level current 160A, icu380v, 50kA, ics380v, 35ka) could not be selected because its ICs was only 35ka, which was less than the expected line current of 38.5ka. This is a misunderstanding. This type of circuit breaker is used in the branch circuit. Even though the short-circuit current passing through the branch circuit is 38.5ka, the circuit breaker ICU is 50kA, which is fully competent. Therefore, whether the molded case circuit breaker is qualified for a line protection switch depends on whether its ICU is greater than the expected short-circuit current of the line. Even if its ICs is a little smaller, it will not hinder its role. Because there are many short-circuit accidents, such as two-phase short-circuit (the short-circuit current is three-thirds of the root of the three-phase short-circuit value), or places far away from the power supply, such as 50m and 100m, even if there is a three-phase short-circuit, due to the impedance, the accident current is about 50% ~ 60% of the three-phase maximum expected value in case of three-phase short-circuit