LED display is the main component of the LED and driver chip, is a collection of microelectronics products, LED operating voltage is about 5V, the general operating current is 20 mA or less. Its working characteristics determine its resistance to static electricity and abnormal voltage or current shock is very fragile. This requires us to recognize this point in the production and use process and give enough attention to take measures to protect the led display. Power grounding is the most common method of protection for LED displays.
Why should the power supply be grounded? This is related to the operating mode of the switching power supply. Our LED display switching power supply is a series of means for converting AC (AC) 220V to DC (DC) 5V DC through a series of filtering, rectification, pulse modulation, output rectification, and filtering. A device with stable power output. In order to ensure the stability of the power AC/DC conversion, the power supply manufacturer bridges an EMI filter circuit from the live line to the ground line in the circuit design of the AC220V input terminal according to the national 3C mandatory standard. In order to ensure the stability of AC220V input, all the power supply will have filter leakage at work. The leakage current of a single power supply is about 3.5mA. The leakage voltage is about 110V. Leakage current may not only damage the chip but also cause the lamp to burn out if the display is not grounded. If more than 20 power supplies are used, the accumulated leakage current reaches more than 70 mA. Sufficient to cause the leakage protector to operate, cut off the power supply. This is also the reason why our display cannot use the leakage protector. If you do not miss the warranty and the display is not grounded, the power supply superimposed leakage current will exceed the human safety current, 110V voltage is enough to cause death! After grounding, the voltage of the power supply enclosure is close to zero for the human body. It shows that there is no potential difference between the power supply and the human body, and the leakage current is introduced into the earth. Therefore, the LED display must be grounded.
However, customers often use the wrong grounding method to ground the display. Commonly there are:
1. It is considered that the lower end of the pillar of the outdoor pillar structure is connected with the earth, so it is not necessary to do the display grounding;
2. It is considered that the power supply is locked on the box body, and the box body is connected with each other with a lock and a structure. Therefore, the structure grounding represents that the power supply is also grounded.
There are misunderstandings in these two types of practices. Our pillars are connected to the anchor bolts of the foundation. The anchor bolts are embedded in the concrete. The resistance of the concrete is in the range of 100-500 Ω. High grounding resistance will cause leakage. Current drain is not timely or there is residue. We have paint on the surface of the cabinet, and the paint is a poor conductor of electricity, which will lead to bad contact or grounding resistance of the box connected to the ground, and there may be spark interference with the screen signal. As time passes, oxidation and corrosion will occur on the surface of the cabinet or structure, and the fasteners such as screws will gradually loosen due to thermal expansion and contraction caused by temperature changes. It will lead to weakening or even complete failure of structural grounding. Form a security risk. Caused a leakage current, chip damage caused by interference and other security incidents.
So, what should be the standard grounding? As shown in the following figure: There are three terminal blocks on the input end of the power supply. They are the live terminal, the neutral terminal, and the grounding terminal. The correct grounding method is to connect and lock all the power ground terminals in a ground-specific yellow-green two-color cable, and then lead out to connect to the ground.
If the site does not have a grounding terminal, it can be connected to a pipe that is buried in the ground and maintains good contact with the earth, such as iron water pipe or iron sewer pipe. To ensure good contact, the connection terminal should be welded to this type of natural grounding body, and then The ground wire is locked on the terminal block and must not be tied. However, gas and other inflammable and explosive pipes must not be used. Or bury the grounding body on site. The grounding body can adopt angle steel or steel pipe and be buried horizontally or vertically in the ground as a simple grounding point. The grounding point should be chosen in a remote place so as to prevent pedestrians or vehicles from damaging the grounding body.
When we ground, the grounding resistance must be less than 4 ohms to ensure timely leakage of the leakage current. It should be noted that the grounding terminal for lightning protection needs a certain period of time for the diffusion of the lightning current due to the diffusion of the lightning current, which will cause the ground potential to rise in a short time. If the LED display screen is grounded and connected to the lightning protection ground, the earth potential at this time Higher than the display, lightning current will be transmitted along the ground to the screen, causing equipment damage. Therefore, the protective grounding of the LED display screen must not be connected to the lightning protection ground. The protection ground must be more than 20 meters away from the lightning protection ground. Prevent ground potential counterattack.
Summary of LED grounding considerations:
1, each power supply must be grounded from the ground terminal, and locked;
2, grounding resistance must not be greater than 4Ω;
3, the ground wire should be the exclusive wire, it is forbidden to connect with the zero wire;
4. No open air or fuses shall be installed on the ground;
5, ground and ground terminals should be away from lightning protection ground 20 or more;
It is strictly forbidden for some devices to use protective grounding instead of protective zero, resulting in a wrong connection between the protective grounding and the protection zero connection. When the insulation of a protective earthing device is damaged and the phase line collides with the shell, the neutral voltage appears on the neutral line, thus causing a dangerous voltage on the outer shell of the protective zero-connect device.
Therefore, in the line powered by the same bus, the protection earth and the protection zero cannot be mixed, that is, a part of the electrical equipment cannot be connected to zero, and another part of the electrical equipment is grounded. Normally city power adopts zero protection. Therefore, if you use electrical power equipment, you should take zero protection.
4, protection zero attention
(1) In a protective zero-connected electrical system. The zero line plays a vital role. Once the neutral line is disconnected, the electrical equipment connected to the line behind the disconnected line is equivalent to no protective zero and protective grounding. If there is a leakage of the electrical equipment enclosure behind the zero line break, it will not constitute a short circuit, but will melt. Body fuse. Not only the enclosure of this equipment has a voltage, but also the enclosure of all devices behind the disconnected line has a voltage close to the phase voltage of the power supply, and the danger of electric shock will be increased.
All electrical equipment connected to the protection of the zero line should be connected in parallel to the power supply on the zero line, not allowed in series, and pressed with bolts, solid and reliable, good contact. On the zero line, it is forbidden to install fuses and separate switches. In a special environment, zero line anti-corrosion treatment is self-evident.
(2) In the three-phase four-wire distribution system where the neutral point is not grounded, it is not allowed to use protective zero, and only the protective grounding can be used. Grounding occurs in any phase of the system. The entire system is still operating normally. However, if the earth and the grounding wire are at equal potential, the voltage drop of the shell of the consumer equipment connected to the zero line is equal to the phase of the grounding wire from the grounding point to the neutral point. The voltage value is very dangerous.
(3) In the protective zero-crossing system, the work ground and the zero line must be separated at a neutral point of the power supply and the terminal should be grounded repeatedly. In the neutral grounding system, except that the neutral point of the transformer is grounded, the neutral line is again grounded at one or more places along the zero line.
Its role is to reduce the voltage to ground when the electrical equipment enclosure is leaking. When the neutral line breaks, it also reduces the risk of electric shock. When the casing of the electrical equipment is leaking, a short-circuit loop is formed by the phase line and the zero line, and the short-circuit current will be melted and insured. The enclosure of the device is powered down and there is no risk of electric shock. However, the equipment is charged before the fuse is blown, which is still dangerous for the human body. If a grounding device is added near the device, repeated grounding is performed, and the voltage of the conductor portion of the charged device decreases to ground.
(4) The protective zero connection must be equipped with a reliable and sensitive short-circuit protection device. Therefore, it is forbidden to use a copper wire or other metal material instead of a metal fuse wire that meets the requirements. Otherwise, the zero protection will lose its protective effect.