Application of the hottest miniature circuit break

2022-08-15
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Application of miniature circuit breaker in small capacity terminal power distribution

(I) current situation of traditional terminal power distribution box

there are two typical connection schemes of traditional terminal power distribution box, as shown in Figure 1. One is that the power supply enters the bus of the distribution box after passing through (dz10 or DZ20) air circuit breaker (some also add fuses), and each output circuit passes through fuses, as shown in Figure 1 Scheme (a); The second is that the power supply enters the bus of the distribution box through the air circuit breaker, and each output circuit is controlled by the knife switch (it can also be dz10 or DZ20 air break Jinan assay tensile testing machine, which is a mechanical force adding testing machine circuit breaker and fuse used for static load, tension, contraction, zigzag, shear, stripping and other mechanical property experiments of materials), as shown in scheme (b) of Figure 1. The air circuit breaker of the incoming line of the power supply is mainly used for the on-off operation of the power supply and the subsequent short-circuit protection. In order to prevent the air circuit breaker from reliably and quickly acting when the circuit has a small current short-circuit, the fuses of each outgoing line are added, and the knife switches of each outgoing line are mainly used as disconnectors. The fuses of each outgoing line in scheme (a) can also give consideration to the function of disconnectors. DZ type air circuit breaker is also used as outgoing switch in power distribution scheme (b) of Figure 1. For small capacity terminal distribution, these combination and connection schemes have the following shortcomings:

1 It takes up a lot of space. The fuse is used as the short-circuit protection of the distribution box. The distribution box has a large volume, takes up a large space and is not beautiful. It is particularly prominent when the number of output branches of the distribution box is required to be large

2. There are many connection points of the circuit, and the installation process and reliability are poor. The fuse is used as the short-circuit protection of the distribution box. There are many connection points of the fuse and its terminal, and the contact reliability is poor. The maintenance workload is heavy, and it is easy to have a bad contact, causing the motor to be burned out due to lack of phase operation

3. The operation is unsafe and inconvenient. When only the fuse is used as the branch outlet protection, the distribution box does not have the isolation switch of each branch. When it is necessary for each branch to stop power transmission separately, it can only be completed by taking out or replacing the fuse core, which is not only troublesome, but also has the risk of electric shock

4. High cost. When using DZ type air circuit breaker as outgoing switch, it not only takes up large space, but also costs high

(II) characteristic analysis and verification of miniature circuit breaker

1 Characteristics of miniature circuit breaker

taking DZ miniature circuit breaker as an example, it is mainly divided into type D and type C according to the tripping form. It is mainly used for overload and short-circuit protection in distribution lines with AC 50Hz, rated voltage of 380V, and rated operating short-circuit current of not more than 6000a. It can also be used for infrequent on-off operation and conversion. Type C is used for lighting and civil power distribution, and type D is used for industrial power distribution. The main technical performance parameters of DZ (d) miniature circuit breaker are introduced as follows:

(1) protection function: overload long-time delay and instantaneous short-circuit protection function

(2) overcurrent tripping characteristics: as shown in Table 1

(3) rated breaking capacity: 4000A at 380V, 6000a at 220V

(4) mechanical and electrical life: 4000 times

dz type C-type and D-type miniature circuit breakers are different in that the former requires no tripping within 0.1 seconds when 5 times the rated current, while the latter requires no tripping within 0.1 seconds when 7 times the rated current. It has a stronger ability to withstand the instantaneous peak current and a higher reliability to avoid the starting current of the motor

2. According to the technical parameters provided by the manufacturer, type C of DZ miniature circuit breaker can be used for lighting circuit distribution and control, and type D can be used for low-voltage distribution and control of industrial power. Although the load of lighting circuit changes greatly, its impact load is relatively small. Therefore, miniature circuit breakers have been widely used in building lighting circuits, but they are rarely involved in the field of industrial power distribution with large impact load. The main reason is that the volume of miniature circuit breaker is too small, and its reliability of controlling impact load, short-circuit protection performance, overload protection performance and mechanical and electrical life are questioned

now only the performance analysis of the aforementioned miniature circuit breaker applied to low-voltage power distribution is as follows:

(1) the reliability of controlling impulse load. From table 1, it can be seen that the miniature circuit breaker in the form of D-type tripping can not trip within 0.1 seconds at 7 times the rated current

the starting current of small three-phase AC asynchronous motor is 4 ~ 7 times of the rated current, and the peak to peak value lasts for a short time, which can fully meet the requirements of non tripping when the motor is started

(2) short circuit 1. The performance of raw material protection can be seen from table 1. The miniature circuit breaker in the form of D-type tripping can trip within 0.1 seconds when 14 times the rated current, and the rated breaking capacity: 4000A at 380V, 6000a at 220V. It can meet the short-circuit protection requirements of small capacity power distribution

(3) overload protection performance it can be seen from table 1 that the miniature circuit breaker in the form of D-type tripping can be disengaged within 120 seconds at 2.55 times the rated current, which can meet the overload protection requirements of motors and distribution lines

(4) mechanical and electrical life. The mechanical and electrical life of DZ miniature circuit breaker: 4000 times. According to the survey and statistics, there are few mechanical and electrical failures of miniature circuit breaker applied to lighting distribution control, and it should also meet the requirements of industrial power distribution

3. Verification of characteristics of miniature circuit breaker

whether the performance of miniature circuit breaker is consistent with the technical parameters given by the manufacturer is the main factor affecting the actual use. We propose to carry out on-site test verification of miniature circuit breaker as follows: A. on load power on test to verify whether it can avoid the impact current of motor startup without tripping? B. Test on the ability to cut off short-circuit current and overload protection performance; C. Disassemble and check the contact burning and insulation damage after cutting off the short-circuit current for many times

(1) test of on load power on and off capacity

a. test object: three dz/20 (d) three-phase miniature circuit breakers (randomly selected). B. Test equipment: C620 lathe with spindle start (motor y132s-4 5.5kw380v, in =11.6a); Voltmeter (mf-47 multimeter level 2.5); Clamp on ammeter (t-302 level 2.5). C. Test method: after removing the original control circuit of C620 lathe, connect the dz/20 (d) three-phase miniature circuit breaker to control the 380V three-phase AC power supply. Through the power on and power off operation of the miniature circuit breaker, control the start and stop of the lathe motor with the spindle. Before each start, the lathe spindle must be in a static state. After repeated operation for 50 times, replace another miniature circuit breaker and repeat the above operations. D. Test results: the voltage is 380V, and the starting current (measured) is 62A. In the test of the on load power on and off capacity of the three miniature circuit breakers, everything worked normally, and the maximum surface temperature was about 40 ℃

(2) test of cutting off short circuit and overload current

a. test object: 1 dz/32 (d) three-phase miniature circuit breaker, 3 dz/20 (d) three-phase miniature circuit breakers. B. Test equipment: AC welding machine (BX type); Clamp on ammeter (t-302 level 2.5); Electronic stopwatch. C. Test method: according to the circuit in Figure 2, the short-circuit current at the secondary side of the AC welding transformer is used as the load of the simulated short-circuit and overcurrent, and the three-phase miniature circuit breaker is connected for the test. The simulated test current is divided into three grades: 2.55 times, 7 times and 14 times the rated current of the miniature circuit breaker, and the actual simulated test load current is detected with a clamp on ammeter. D. Test results: Table 2 only lists the test results of dz/32 (d) three-phase miniature circuit breaker, which are consistent with the characteristic parameters in the product manual

the test results of cut-off short circuit and overload current of the other three dz/20 (d) three-phase miniature circuit breakers are basically consistent with the characteristic parameters in the product manual

(3) destructive breaking and closing high current test and disassembly inspection

test method: add 12 times the rated current to four miniature circuit breakers respectively, and conduct on-load closing and breaking tests for 20 times. Each miniature circuit breaker only tests one phase, and then conduct disassembly inspection one by one. The specific conditions are as follows:

/20 ~ 32 (d) three-phase miniature circuit breakers are composed of three identical single-phase miniature circuit breakers with independent circuits, The internal circuits are not interconnected, only the mechanical connection between the operating mechanisms and handles of each phase and the connection between the four copper tube rivets between the shells. B. Each phase of the three-phase miniature circuit breaker is composed of static contact, moving contact, arc isolating plate, arc extinguishing grid, over-current tripping coil and its device. The static contact is a plane structure (3mm, 0.8 mm thick), made of special alloy materials, pressure welded on the static contact frame, and the moving contact is a spherical structure (about sr6mm), which is an integral structure. C. The static contact has severe burns, with a pit section of about 2.6 2.6mm and a maximum depth of about 0.4mm. The moving contact has moderate burns, with a depth of about 0.2mm. The arc isolating plate is covered with a relatively uniform copper metal vapor deposition layer. The arc extinguishing grid and plastic shell are relatively intact, and the appearance and insulation are almost free of damage

(III) application of miniature circuit breaker in terminal power distribution

1 Application in the distribution of electrical terminals of building lighting circuits since 1999, we have gradually used miniature circuit breakers as distribution and control switches in lighting circuits. With the passage of time and the increase of usage, the advantages of miniature circuit breakers have been recognized, and the doubts about their reliability have been gradually eliminated

in actual use, compared with the traditional terminal power distribution mode, the failure rate is significantly reduced, and there are few failures in normal use. In several years of use, the faults of miniature circuit breakers are mainly caused by the following conditions:

(1) the incoming and outgoing line connections of miniature circuit breakers are directly compressed with bolts, and in occasions with large load changes, they are not checked and tightened in time, resulting in poor local contact

(2) when used in occasions with large impulse current, such as computer (ordinary display, the degaussing coil current of the display is very large when it is started at the same time), the control switch in the classroom often has closing tripping and mechanism failure

(3) the contact of individual miniature circuit breakers is oxidized, resulting in contact failure

in the installation of household power distribution, in order to meet the requirements of large variable load, the rated capacity of the main power switch should choose a larger miniature circuit breaker, which should be equipped with leakage protection devices, and the miniature circuit breaker of the output branch should be selected according to the actual calculated load capacity

2. Application in small capacity power terminal distribution

we choose DZ series three-phase miniature circuit breaker as the load switch of each branch in the workshop distribution terminal box (taking into account short-circuit overcurrent protection). In order to improve the reliability of short-circuit overcurrent protection, DZ20 series air circuit breaker with good overcurrent protection performance is installed in the power incoming line of each distribution box as the secondary protection, and the capacity of each circuit breaker is determined according to the actual load capacity

about 200 dz/20 ~ 32 (d) three-phase miniature circuit breakers have been put into use since October 2003. Most of the loads are horizontal lathes such as C6140 lathes. In addition to the classic bromine flame retardant, C plastic materials rely on the performance advantages of mechanical processing equipment such as jk6140a CNC lathes and hk7140b CNC milling machines. The power of motors is between 4 ~ 9kw (the total capacity can reach 12KVA)

the fault rate has been relatively low for several years. The circuit has three short-circuit faults, and the miniature circuit breaker can reliably cut off the fault current without

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