Discussion on several measures for safe operation

2022-08-23
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Talking about several measures for the safe operation of street lamps

at present, there are three common problems in the safe operation of street lamps:

1 The power supply of street lamps only has short-circuit and overload protection, but no leakage protection. The key reason for not using the leakage circuit breaker is that it goes into a misunderstanding. Some believe that the three-phase leakage circuit breaker has an action dead zone and cannot reliably protect people from electric shock, so it is better not to use it; Some believe that scrap steel with normal leakage current can be transported as raw material to xin3zhou special steel plant separated by 1 wall for smelting, which may cause malfunction of leakage circuit breaker and reduce the reliability of street lamp power supply; Some believe that there are irregularities in the installation of street lamps, such as only leading out the phase line of street lamps and borrowing the neutral line from elsewhere, resulting in the failure of the leakage circuit breaker to work normally. In fact, as a kind of safety appliance, the main function of leakage circuit breaker is to provide indirect contact protection. When a single-phase ground fault occurs, the contact voltage of the lamp pole can be reduced, which is beneficial to the protection of personal safety. If the rated leakage tripping current (I △ n) of the leakage circuit breaker is 200mA and the grounding resistance RD is 200, the contact voltage =rd*i n=200*0.2=40v, which is within the specified voltage range. Taking the author's area as an example, the average soil resistivity is 110, and the measured average value of single pole grounding resistance RD is 58. If there is no leakage protection and a single-phase grounding fault occurs, the contact voltage =un*rd/(rd+r0) = 220 58/(58+4) = 206v, close to the phase voltage, which is obviously very dangerous. When the coherence coefficient between the leakage breaking experimental curve and the theoretical fitting curve reaches more than 99%, it can also prevent the cable from overheating, shortening its service life and burning electrical equipment by leakage arc current due to the long-term existence of leakage current

2. The sensitivity check test was not conducted for the circuit breaker, and the rated tripping current was too large. When selecting circuit breakers, some only consider short-circuit protection, but ignore overload protection and single-phase end short-circuit sensitivity verification. The rated tripping current of circuit breakers is too large. Regardless of the nature of the load, 100A or even 150A is selected. This often leads to the circuit breaker still not tripping and losing its function when the grounding short-circuit current is large and the cable is heated

3. The process of using cable metal sheath as PE main line is complex and has low reliability. The construction process of using cable metal sheath as PE main line is complex. Each base street lamp needs to weld two welding points, and each welding point needs to be stripped, polished, tinned and welded. The operation is cumbersome, and the quality of welding points is difficult to guarantee. With so many series welding points, the reliability of PE main line connection can be imagined. In addition, it is almost impossible for the cable to be completely moisture-proof. As long as the moisture accumulates inside the cable, rust and corrosion of the metal sheath are inevitable. In general, after years of strengthening the production and quality control of food grade resin raw materials for street lamp low-voltage cables, the metal sheath will appear corrosion breakpoints and lose the protective effect of PE trunk lines. In this way, although the grounding protection of each street lamp is actually independent, once a single-phase grounding fault occurs, the dangerous voltage will exist for a long time

4. The quality of street lamp appliances, especially ballasts, is not up to standard, with serious heating, and the insulation is easy to be broken down, resulting in the electrification of lamp poles. Once touched by pedestrians, the consequences are unimaginable

for the above problems, it is recommended to adopt:

1 Leakage circuit breaker is used for the general protection of street lamp control box. The rated leakage tripping current of the leakage circuit breaker is generally 1.3 times greater than the maximum phase load current, and the sensitivity of the circuit breaker is verified, but it should not be too large; The rated leakage tripping current (I △ n) of the leakage circuit breaker is generally greater than 2 times the normal leakage current of the system. Considering that the leakage circuit breaker mainly prevents electrical fire, arc current and limiting contact voltage, and takes into account the power supply reliability of the street lamp system, generally (I △ n) can be 200mA, which can be appropriately increased in rainy seasons, but the maximum should not exceed 500mA. If the grounding resistance of 20 street lamps is measured in parallel, its value is not greater than 4 ohms. Even if the maximum leakage current value is 500mA, the safety margin is relatively large

2. Three phase five wire cables (vv5*25 or 5*16) will be used for all street lamp cables installed in the future, and one of them will be used as PE bus. The five core cable has no metal sheath, which is basically the same price as the four core cable (vv22) of the same specification. Although there is no metal sheath, there is another plastic pipe outside the cable, so the strength will not be affected. Since the PE trunk line has no breakpoint, it ensures that the grounding electrodes of all steel pole street lamps are connected reliably in parallel. In this way, even if the leakage protection fails to operate in case of grounding fault, the contact voltage can be controlled in a low range because the total grounding resistance of the street lamp system is less than the grounding resistance of the public transformer. For the original four core cable with metal sheath as PE trunk line, the spare one can be changed into PE bus. This small amount of transformation work can greatly improve the safety and reliability

3. TT system is still adopted for the grounding of street lamps, and TN-C-S system is not adopted. The reason is to prevent the fault voltage from spreading to the street lamp system after the low-voltage neutral point voltage of the public transformer rises, which reduces the safety of the protection. When installing electrical equipment, it is required that the N line and PE line are not wrongly connected and short circuited, and the N line is not allowed to be repeatedly grounded to maintain good insulation to the ground. In this way, even if the voltage of the N line of the public transformer increases, the street lamp pole can still maintain a low safety potential

4. Select high-quality street lamp appliances and lamps, do a good job in the type test of ballast, and check the acceptance. The compensation capacitor can improve the power factor of the system and play the role of energy saving and loss reduction. It should play its role as much as possible and cannot be deleted at will

5. Pay attention to some small details when installing the wiring. For example, the self-adhesive tape cannot be used for insulation protection alone, and three layers of plastic adhesive tape should be wrapped outside. The connecting line of the street lamp can be short or short at a glance. Only when all the small links are grasped, can the safe operation be guaranteed

in order to make it easy to judge the type of grounding fault, and focus on providing customers with a complete set of the most professional spring testing machine and parts testing configuration solutions with the best cost performance and convenient maintenance, it is recommended to choose to use the split leakage protector and zero sequence current transformer to detect the leakage signal, amplify the signal through the electronic amplification link, and output it to the coil after appropriate time delay to make the transfer contact act, Disconnect the protected circuit and send a light signal in cooperation with the equipped electrical appliance. Before the leakage protector is manually reset, the circuit cannot be connected and the leakage indicator light is always on. The street lamp maintenance personnel can judge whether it is a leakage fault or a sprint fault according to the state of the indicator light. When the leakage fault is determined, switch the conversion handle to the manual position, and the circuit breaker can operate with leakage current. Before finding the fault position, first measure the total leakage current value and the contact voltage of the lamp pole, and then use the clamp ammeter to detect whether there is zero current on the cable trunk (phase a, B, C and N lines). Using the basic principle of dichotomy, the fault position can be quickly found. Take 20 road light poles as an example, the fault point (section) can be found only four times at most

after troubleshooting, restore the conversion handle to the automatic position, and the system will enter the normal working state. When finding and dealing with faults, the low-voltage live working regulations shall be strictly observed to ensure the personal safety of maintenance and test personnel and other personnel

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