Problems needing attention in using low voltage ci

Problems that should be paid attention to when using low-voltage circuit breakers in power distribution systems

low voltage is widely used in building low-voltage power distribution systems, which is a kind of protective electrical components. When designing low-voltage distribution system, attention should be paid to the selectivity and cascade protection of low-voltage circuit breakers; Select and set the rated current of the overcurrent release of the low-voltage circuit breaker to ensure the action sensitivity of the overcurrent release; When the ambient temperature is greater than or less than the calibrated temperature value, the rated current value of the low-voltage circuit breaker shall be adjusted according to the temperature and current carrying capacity correction factor provided by the manufacturer

the low-voltage circuit breaker has the functions of open circuit protection, overload protection, control and isolation. It is suitable for the low-voltage distribution system at the terminal of industrial and civil buildings, and is widely used in the low-voltage distribution system. If the low-voltage circuit breaker is not designed properly in the distribution system, it will affect the normal operation of the power supply circuit. The author takes this opportunity to talk about the low-voltage circuit breaker and some problems that should be paid attention to in the design of low-voltage distribution system

1. Selectivity of low-voltage circuit breakers

in order to ensure the reliability of low-voltage distribution system, the selectivity of low-voltage circuit breakers has become an important part of the design of terminal low-voltage distribution system. In the distribution system protected by the circuit breaker, when an electrical fault occurs, the circuit breaker QF3 closest to the fault point acts to remove the fault, while the circuit breakers at other levels do not act, so as to limit the power cut caused by the fault to the minimum range, so that other fault free power supply circuits can still maintain normal power supply, which is the selectivity required for low-voltage circuit breakers. The non selective low-voltage circuit breaker refers to that when an electrical fault occurs, the low-voltage circuit breaker QF3 closest to the fault point acts to remove the fault, while the circuit breakers QF1, QF2, QF4 and qf5 at other levels act, which are in the open state, which cannot ensure the normal power supply of other fault free circuits. The selectivity of low-voltage circuit breaker plays a very important role in the design of low-voltage distribution system. It can bring convenience to users and ensure the continuity of power supply circuit. Therefore, under the non selective protection of household appliances, once there is an electrical fault, the continuity of the distribution circuit cannot be guaranteed, making household appliances such as refrigerators, platoons, etc. in the state of shutdown and waiting to start, affecting the daily life of users

low voltage circuit breakers used in low-voltage distribution systems can be divided into selective and non selective categories according to their protection performance. There are two types of selective low-voltage circuit breakers: two-stage protection and three-stage protection. The instantaneous characteristic and short time delay characteristic are suitable for short-circuit action, while the long time delay characteristic is suitable for overload protection. Non selective low-voltage circuit breakers are generally instantaneous and only used for short-circuit protection. Some are long time-delay actions, which are only used for overload protection. Three protection characteristic curves of the low-voltage circuit breaker are shown

in low-voltage distribution system, if the upper circuit breaker adopts selective circuit breaker and the lower circuit breaker adopts non selective circuit breaker or selective circuit breaker, it is mainly to use the difference of delay action or delay action time of short time-delay release to obtain selectivity. Brief introduction of experts: please pay attention to the following problems when the delay action of the upper circuit breaker:

⑴ whether the next stage is a selective circuit breaker or a non selective circuit breaker, the setting current of the instantaneous overcurrent release of the upper circuit breaker shall generally not be less than 1.1 times the maximum three-phase short-circuit current at the outlet end of the lower circuit breaker

⑵ if the next stage is a non selective circuit breaker, in order to prevent the short-circuit current in the circuit protected by the next stage circuit breaker, the short-time delay overcurrent release of the upper stage will act first and lose its selectivity due to the insufficient instantaneous action sensitivity of this stage. Generally, the setting current of the short time-delay overcurrent release of the upper circuit breaker is not less than 1.2 times that of the next instantaneous overcurrent release

⑶ if the next level is also a selective circuit breaker, in order to ensure selectivity, the short delay action time of the upper level circuit breaker is at least 0.1s longer than that of the next level circuit breaker

generally speaking, to ensure the selective action between the upper and lower low-voltage circuit breakers, the upper circuit breaker should choose the overcurrent release with short time delay, and its action current should be more than one level higher than the action current of the lower overcurrent release, at least the action current IOP of the upper level 1 not less than the action current IOP of the next level 2, i.e. iop 1≥1.2iop. 2。

2. Cascade protection of low-voltage circuit breakers

in the design of low-voltage distribution system, the selective cooperation between the upper and lower levels of low-voltage circuit breakers must have "selectivity, rapidity and sensitivity". The selectivity is related to the cooperation between the upper and lower low-voltage circuit breakers, while the rapidity and sensitivity are related to the characteristics of the protective apparatus and the line operation mode respectively. If the upper and lower circuit breakers are properly coordinated, the fault circuit can be selectively cut off to ensure that other fault free circuits of the distribution system continue to work normally. Otherwise, it will affect the reliability of distribution system. Cascade protection is a specific application of the current limiting characteristics of circuit breakers. Its main principle is to use the current limiting effect of the upper circuit breaker. When selecting the lower circuit breaker, the circuit breaker with lower breaking capacity can be selected to achieve the purpose of reducing costs and saving expenses

the upper current limiting circuit breaker QF1 can disconnect the maximum expected short-circuit current at its installation location. Because the upper and lower low-voltage circuit breakers in the low-voltage distribution system are installed in series, when a short circuit occurs at the outlet of the lower low-voltage circuit breaker QF2, the actual value of the short-circuit current is much smaller than the expected short-circuit current at that location due to the current limiting effect of the upper low-voltage circuit breaker QF1, that is, The breaking capacity of the lower low-voltage circuit breaker QF2 is greatly enhanced with the help of the upper low-voltage circuit breaker QF1, exceeding its rated breaking capacity. This cascade protection also has certain conditions, such as the adjacent circuit can not have important loads? Because once QF1 trips, QF3 circuit will also be powered off, and the instantaneous setting value of QF1 and QF2 should also be properly matched. The cascade data can only be measured by experiments, and the coordination selection of upper and lower low-voltage circuit breakers can only be provided and determined by the manufacturer of low-voltage circuit breakers. As shown in Table 1, for example, if the upper level low-voltage circuit breaker c65h has a breaking capacity of 10karms, the lower level can choose c65n low-voltage circuit breaker with a breaking capacity of 10karms. (* indicates stepless connection selectivity between upper and lower low-voltage circuit breakers)

3 Sensitivity of low-voltage circuit breaker

in order to ensure that the instantaneous or short-time-delay overcurrent release of low-voltage circuit breaker can act reliably in case of the slightest short-circuit fault within its protection range under the minimum operation mode of the system. The sensitivity of low-voltage circuit breaker protection must meet the provisions of code for design of low voltage power distribution (gb50054-95), and its sensitivity should not be less than 1.3, that is, sp=n/iop ≥ 1.3, where IOP - the action current of instantaneous or short-time-delay overcurrent release, N - the single-phase short-circuit current or two-phase short-circuit current at the end of the line protected by the circuit breaker by Xue Jian, the chief representative of KANGLONG Far East Beijing Representative Office, under the minimum operation mode of the system, Sp - sensitivity of low voltage circuit breaker

when selecting low-voltage circuit breakers, attention should also be paid to the verification of their sensitivity. For selective circuit breakers with both short-time-delay and instantaneous overcurrent release, only the action sensitivity of the short-time-delay overcurrent release needs to be verified, and the action sensitivity of the instantaneous overcurrent release does not need to be verified

4. The ambient temperature of the low-voltage circuit breaker

the overload protection of the low-voltage circuit breaker depends on the thermal release. Generally, the rated current of the thermal release of the low-voltage circuit breaker is set by the manufacturer under the reference temperature of 30 ℃ according to iec898 standard. The thermal release is made of a group of bimetallic sheets. When the circuit is overloaded, the overload current flows through the heating resistance wire to make the bimetallic sheet heat, deform and bend, and push the hook open to disconnect the contact of the low-voltage circuit breaker. The thermal release of low-voltage circuit breaker is directly related to the ambient temperature. If the ambient temperature changes, the rated current value of low-voltage circuit breaker will change, as shown in Table 2. Low voltage circuit breakers are generally arranged and fixed on the distribution board in an orderly manner, and then installed in the distribution box. The installation methods of the distribution box are divided into open installation and concealed installation. The heat dissipation effect of the open installation distribution box is better than that of the concealed installation distribution box. The air in the concealed installation distribution box should not be convective, and its heat dissipation effect is poor, resulting in the temperature rise of the surrounding air temperature due to the temperature rise of the low-voltage circuit breaker in the distribution box. Therefore, the actual working temperature of the low-voltage circuit breaker is about 10 ℃ ~ 15 ℃ higher than the temperature of the surrounding environment. Therefore, when the ambient temperature is greater than or less than the calibrated temperature value, we must correct the rated current value of the low-voltage circuit breaker according to the temperature and current carrying capacity correction coefficient table provided by the manufacturer

taking c65n/h as an example, if the low-voltage circuit breaker c65n/h is installed individually, its surrounding environment is 30 ℃, and the rated current of c65n/h is 20a, its actual working current is 20A; If the low-voltage circuit breaker c65n/h is installed individually, its surrounding environment is 40 ℃, and the rated current of c65n/h is 20a, its actual working current is 18.97a; If multiple c65n/h are installed in the distribution box, the surrounding environment is 30 ℃, and the rated current of c65n/h is 20a, the actual working current is 16A; If multiple c65n/h are installed in the distribution box, the surrounding environment is 40 ℃, and the rated current of c65n/h is 20a, the actual working current is 15.18a. From this, we can see that different ambient temperatures and different installation methods have a certain impact on the actual working current value of the low-voltage circuit breaker

5. Selection and setting of low-voltage circuit breaker release

⑴ selection of rated current of low-voltage circuit breaker overcurrent release

the rated current of low-voltage circuit breaker overcurrent release is not less than the calculated current i30 of the line, i.e. ≥ i30

⑵ setting of action current of overcurrent release of low-voltage circuit breaker

① setting of action current of instantaneous overcurrent release. Among the objects protected by the low-voltage circuit breaker, there are some electrical equipment. These electrical equipment will generate peak current several times its rated current in a short time during the startup process, so that the low-voltage circuit breaker can withstand large peak current in a short time. Action current IOP of instantaneous overcurrent release? o? You must avoid the peak current ipk of the line, that is, IOP? o? ≥ krel · ipk, where krel is the reliability coefficient. When selecting the circuit breaker, attention should be paid to make the setting current of the instantaneous overcurrent release of the low-voltage circuit breaker avoid the peak current, so as to avoid causing the maloperation of the low-voltage circuit breaker

② setting of action current and action time of short time-delay overcurrent release. Action current IOP of short time-delay overcurrent release? s?， It should also avoid the peak current ipk of the line, i.e. IOP? s? ≥ krel · ipk, where krel is the reliability coefficient. The action time of the short time-delay overcurrent release is generally divided into three types: 0.2S, 0.4s and 0.6s, which are determined according to the protection selectivity of the front and rear protection devices. The action time of the previous level of protection should be one time difference longer than that of the next level of protection

③ setting of action current and action time of long-time overcurrent release. The long-time overcurrent release is mainly used to protect overload, so its action current IOP? l? Only need to avoid the maximum load current of the line, i.e. calculate the current i30, i.e. IOP? l? ≥krel. I30, where krel is the reliability coefficient. The action time of the long-time overcurrent release should avoid the duration of the allowable short-time overload, so as to avoid causing the maloperation of the low-voltage circuit breaker

④ coordination requirements between the action current of the overcurrent release and the protected line