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May 29,2021| LED Knowledge
The core components of the led display are composed of LED lamp beads and IC drivers. Since LEDs are sensitive to static electricity, excessive static electricity will cause the light-emitting diodes to break down. Therefore, grounding measures must be taken during the installation of large LED screens to avoid death. The risk of lights.
The working voltage of the large led screen is about 5V, and the general working current is below 20 mA. The working characteristics of the LED determine its resistance to static electricity and abnormal voltage or current impact. This requires us to recognize this in the production and use process, and pay enough attention to take effective measures to protect the large led screen. Power grounding is a commonly used protection method for large LED screens.
Why should the power supply be grounded? This is related to the working mode of the switching power supply. The LED large screen switching power supply is a series of methods such as filtering-rectification-pulse modulation-output rectification-filtering and other means to convert AC (AC) 220V city power into direct current (DC) 5V DC A device with stable power output.
To ensure the stability of the AC/DC conversion of the power supply, the power supply manufacturer connects an EMI filter circuit from the live wire to the ground wire in the circuit design of the AC220V input terminal according to the national 3C mandatory standard. To ensure the stability of the AC220V input, all power supplies will have filter leakage when working, and the leakage current of a single power supply is about 3.5mA. The leakage voltage is about 110V. In the case that the large led screen is not grounded, the leakage current may not only cause damage to the chip or burn out the lamp.
If more than 20 power supplies are used, the accumulated leakage current will reach 70mA or more. It is enough to cause the leakage protector to act and cut off the power supply. This is also the reason why the LED large screen cannot use the leakage protector. If the leakage protection is not connected and the large LED screen is not grounded, the superimposed current of the power supply will exceed the safety current of the human body, and the voltage of 110V is enough to cause death! After grounding, the voltage of the power supply shell is close to 0 to the human body. It shows that there is no potential difference between the power supply and the human body, and the leakage current is led to the ground. Therefore, the large led screen must be grounded.
But customers often use the wrong grounding method to ground the large led screen, the common ones are:
1. It is believed that the lower end of the column of the outdoor column structure is connected to the earth, so there is no need to make the LED large screen grounding;
2. It is considered that the power supply is locked on the box, and the boxes are connected to the structure by a lock, so the structure grounding means that the power supply is also grounded.
There is a misunderstanding between these two types of practices. The column is to maintain the connection with the foundation bolts, and the anchor bolts are embedded in the concrete. The resistance of the concrete is in the range of 100-500Ω. Too high grounding resistance will cause leakage current. It is not released in time or there are residues. The surface of the box body is sprayed with paint, and the paint is a poor conductor of electricity, which will lead to the poor grounding of the box body or increased grounding resistance, and electric sparks may interfere with the signal of the large LED screen.
As time goes by, the surface of the LED large screen box or structure will be oxidized and rusted, and the fixing parts such as screws will gradually loosen due to thermal expansion and contraction caused by the temperature difference. It will cause the effect of the grounding of the large led screen to be weakened or even completely invalid. Form a safety hazard. Lead to safety accidents such as leakage current, electric shock, chip interference and damage.
So, what should the standard grounding be like? There are 3 wiring terminals at the power input terminal, namely the live terminal, the neutral terminal and the ground terminal. The correct grounding method is to use the yellow and green two-color wire dedicated for grounding to connect all the power ground wire terminals in series and lock, and then lead out and connect to the grounding terminal.
If there is no ground terminal on-site, it can be connected to an iron water pipe or iron sewer pipe that is buried in and in good contact with the ground. To ensure good contact, the terminal should be welded on this natural grounding body, and then connect The ground wire is tightly locked on the terminal block and must not be bundled and connected. However, flammable and explosive pipelines such as gas shall not be used. Or bury the grounding body on site. The grounding body can be made of angle steel or steel pipe and buried horizontally or vertically in the ground as a simple grounding point. The grounding point should be selected in a remote place to prevent pedestrians or vehicles from destroying the grounding body.
When grounding, the grounding resistance must be less than 4 ohms to ensure timely discharge of leakage current. It should be noted that when the lightning protection ground terminal discharges the lightning current, it takes a certain time for the earth's current to spread, which will cause the earth's potential to rise in a short time.
If the LED large screen is grounded to the lightning protection ground terminal, then the earth The potential is higher than that of the large LED screen, and the lightning current will be transmitted to the LED screen along this ground wire, causing equipment damage. Therefore, the protective grounding of the large led screen must not be connected to the lightning protection grounding terminal, and the protection grounding terminal must be more than 20 meters away from the lightning protection grounding terminal. Prevent the counterattack of ground potential.