1. Selectivity of low voltage circuit breaker
In order to ensure the reliability of the low-voltage distribution system, the selectivity of the low-voltage circuit breaker becomes an important part of the design of the terminal low-voltage distribution system.
In the power distribution system protected by the circuit breaker: when an electrical fault occurs, the circuit breaker closest to the fault point: the Qn action will cut the fault, while the other levels of the circuit breaker will not operate, thereby limiting the power failure caused by the fault to a minimum. In the range, other fault-free power supply circuits can still maintain normal power supply, which is the selectivity required for low-voltage circuit breakers. Non-selective low-voltage circuit breaker means that when an electrical fault occurs, the fault of the low-voltage circuit breaker QF3 closest to the fault point will be removed, while the other circuit breakers Qn, QF2, QF4 and QF5 are in the open state. Ensure that other fault-free circuits are properly powered. The selectivity of the low-voltage circuit breaker occupies a very important position in the design of the low-voltage power distribution system, which can bring convenience to the user and ensure the continuity of the power supply circuit. Therefore, in the absence of selective protection of household appliances, once electrical faults occur, the continuity of the distribution circuit cannot be guaranteed, so that household appliances such as refrigerators, range hoods, etc. are in a state of being shut down, which affects the daily life of users. .
Low-voltage circuit breakers used in low-voltage power distribution systems can be classified into two types, selective and non-selective. Selective low-voltage circuit breakers are available in two-stage protection and three-stage protection. The transient characteristics and short delay characteristics are suitable for short-circuit action, while the long delay characteristics are suitable for overload protection. Non-selective low-voltage circuit breakers are generally instantaneous and only used for short-circuit protection. There are also long delay actions, only for load protection.
In the low-voltage power distribution system, if the upper-level circuit breaker uses a selective circuit breaker, the next-level circuit breaker uses a non-selective circuit breaker or a selective circuit breaker, mainly using the delay action of the short-delay trip or Delay the action time to get the selectivity. When passing the delay action of the upper circuit breaker, please pay attention to the following points:
(1) Regardless of whether the next stage is a selective circuit breaker or a non-selective circuit breaker, the instantaneous overcurrent release setting current of the upper circuit breaker is generally not less than 1.1 of the maximum three-phase short-circuit current of the next-stage circuit breaker outlet end. Times
(2) If the next stage is a non-selective circuit breaker, in order to prevent short-circuit current from occurring in the protection circuit of the next-level circuit breaker, the short-stage overcurrent release device of the upper stage is not sufficient because of the insufficient sensitivity of the instantaneous action of this stage. The first action is to make it lose its selectivity. Generally, the setting current of the short-delay overcurrent release of the upper-level circuit breaker is not less than 1.2 times of that of the next-stage instantaneous overcurrent release;
(3) If the next stage is also a selective circuit breaker, in order to ensure selectivity, the short delay action time of the upper circuit breaker is at least 0.1S longer than the short delay action time of the next level circuit breaker.
In general, to ensure selective action between the upper and lower two-stage low-voltage circuit breakers, the upper-level circuit breaker should choose an over-current trip unit with a short delay, and its operating current is greater than that of the next-stage over-current release. Above the operating current level, at least the upper operating current IOP.1 is not less than 1.2 times the next-stage operating current IoP.2, ie IOP.1≥1.21OP.2.
2. Cascade protection of low voltage circuit breakers
In the design of low-voltage distribution systems, the selective coordination between the upper and lower stages of the low-voltage circuit breaker must be "selective, rapid and sensitive." The selectivity is related to the cooperation between the upper and lower two-stage low-voltage circuit breakers, and the rapidity and sensitivity are related to the characteristics of the protection appliances themselves and the operation mode of the lines. When the upper and lower two-stage circuit breakers are properly matched, the faulty circuit can be selectively removed to ensure that other fault-free circuits of the power distribution system continue to work normally. On the contrary, it affects the reliability of the power distribution system. Cascade protection is the specific application of the current limiting characteristics of the circuit breaker. The main principle is to use the current limiting function of the upper circuit breaker. When selecting the lower circuit breaker, the circuit breaker with lower breaking capacity can be selected to achieve cost reduction and cost saving. purpose.
The upper limit current-limiting circuit breaker QF1 can cut off the maximum expected short-circuit current at its installation. Since the low-voltage circuit breakers of the lower and lower stages in the low-voltage distribution system are installed in series, when a short circuit occurs at the outlet of the lower-stage and low-voltage circuit breaker QF2, the short circuit Due to the current limiting effect of the upper-stage low-voltage circuit breaker QF1, the actual value is much smaller than the expected short-circuit current at that point. That is to say, the breaking capacity of the lower-level low-voltage circuit breaker QF2 is greatly enhanced with the help of the upper-level low-voltage circuit breaker QF1, exceeding Its rated breaking capacity. This cascade protection also has certain conditions. For example, the adjacent loop cannot have an important load (because the QF1 loop is also powered off once QF1 trips), and the instantaneous setting of QF1 and the instantaneous setting of QF2 must be matched properly. Cascading data can only be determined experimentally, and the choice of the lower and lower voltage circuit breakers can only be determined by the manufacturer of the low voltage circuit breaker.
3. Sensitivity of low voltage circuit breaker
In order to ensure that the instantaneous or short-delay overcurrent release of the low-voltage circuit breaker is in the minimum operating mode of the system, it can reliably operate when the slightest short-circuit fault occurs within its protection range. The sensitivity of the protection of the low-voltage circuit breaker must meet the requirements of the "Design Specification for Low-Voltage Power Distribution" (CB50054-95). The sensitivity should be not less than 1.3, that is, SP=lK.min/IOP≥1.3, where the lOP instantaneous or short-delay overcurrent The operating current of the buckle, the single-phase short-circuit current or the two-phase short-circuit current of the IK.Min-circuit-protected line end in the system minimum operating mode, Sp--the sensitivity of the low-voltage circuit breaker.
When selecting a low-voltage circuit breaker, attention should also be paid to the verification of its sensitivity. For a selective circuit breaker with both a short delay and a transient overcurrent release, only the action of the short-delay overcurrent release should be verified. Sensitivity, there is no need to verify the sensitivity of the transient overcurrent release action.
4, the ambient temperature of the low-voltage circuit breaker
The overload protection of the low-voltage circuit breaker is completed by a thermal tripper. Usually, the rated current of the thermal trip unit of the low-voltage circuit breaker is determined by the manufacturer according to the IEC898 standard at a reference temperature of 30 ° C (2 conditions. The thermal trip is Made of a set of bimetals, when the line is overloaded, the overload current flows through the heating resistor wire to cause the bimetal to heat and deform, and the hook is opened to open the low voltage circuit breaker contacts. The trip unit has a direct relationship with the ambient temperature. If the ambient temperature changes, the rated current value of the low-voltage circuit breaker will change. The low-voltage circuit breaker is generally fixedly arranged on the switchboard and then installed in the distribution box. The installation method of the distribution box is divided into two types: clear and concealed. The heat dissipation effect of the assembled electrical box is better than that of the dark assembled electric box. The air in the dark assembled electric box is not suitable for convection, and the heat dissipation effect is poor. As a result, the temperature rise of the low-voltage circuit breaker in the distribution box causes the ambient air temperature to rise. Therefore, the actual operating temperature of the low-voltage circuit breaker is about 10 ° C - 15 ° C higher than the ambient temperature. Therefore, when the ambient temperature At or below the calibration 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. Take C65N/H as an example, if the low-voltage circuit breaker C65N/H is installed separately. The surrounding environment is 30 °C, the rated current of C65N/H is 20A, the actual working current is 20A; if the low-voltage circuit breaker C65N/H is installed alone, the surrounding environment is 40°C, and the rated current of C65N/H is 20A. The actual working current is 18.97A; if multiple C65N/H are installed in the distribution box, the surrounding environment is 30°C, and the rated current of C65N/H is 20A, the actual working current is 16A; if multiple C65N/H Installed in the distribution box, the surrounding environment is 40 ° C, the rated current of C65N / H is 20 A, the actual working current is 15.18 A. From this we can see that different ambient temperatures and different installation methods for low voltage disconnection The actual operating current value of the device has a certain influence.
Busbar Built In Current Transformer
Busbar Built In Current Transformer,Mini Busbar Built Current Transformer,Electric Meter Mini Current Transformer,Current Transformer With Busbar Inside
Zibo Tongyue Electronics Co., Ltd , https://www.tongyueelectron.com