Range of Anti-static Grounding Impedance for Assembly Lines
The grounding system of a computer room is a crucial means of preventing interference from parasitic capacitive coupling, protecting equipment and personnel, and ensuring the stable and reliable operation of the computer system.
The grounding system for electronic computers is the simplest, most economical, and most effective method in anti-interference design. Properly combining grounding with shielding can better solve noise problems.
Therefore, to ensure the safe, reliable, and stable operation of the computer system and the safety of equipment and personnel, appropriate grounding systems should be designed according to the different requirements of different types of computers.
According to the national standard "Computer Technical Requirements," specific requirements are specified for computer grounding systems. Computer stations generally have the following types of grounding:
DC ground for the computer system; resistance value not greater than 1Ω.
AC working ground; resistance value not greater than 4Ω.
Safety protection ground; resistance value not greater than 4Ω.
Anti-static grounding; resistance value not greater than 4Ω.
Lightning protection ground; resistance value not greater than 10Ω.
Grounding, in essence, connects a point in a circuit or a metal casing to the earth using a conductor. The goal is to facilitate the flow of grounding current; therefore, the lower the grounding resistance, the easier it is for the grounding current to flow. Furthermore, in computer systems, grounding should also minimize potential fluctuations that could cause noise. Therefore, lower grounding resistance is also better.
When handling computer grounding, the following two points should be noted:
Signal circuits and power supply circuits, as well as high-voltage and low-voltage circuits, should not share a common ground loop. Sensitive circuits should be isolated or shielded to prevent interference caused by ground return current and electrostatic induction.
The functions and implementation methods of several grounding wires are described below:
The role of AC working ground: In computer systems, many electrical devices use 380V/220V AC power, such as computer peripherals, transformers, fans in air conditioning cabinets, and maintenance equipment. According to national regulations, these must be grounded, i.e., the neutral point is grounded, also known as secondary grounding. Its function is to ensure personal safety and equipment safety.
In computer systems, there are many AC devices, but the secondary grounding of these devices is often overlooked, frequently causing unnecessary harm to people and equipment.
Specific Measures:
Connect the neutral point of the computer's external components in series with insulated wires to the neutral line of the distribution cabinet, and then ground it using a grounding busbar. Other AC devices, such as air conditioners, fresh air systems, and frequency and voltage stabilization equipment, should have their neutral points grounded independently according to electrical specifications.
Safety Protective Grounding: Properly grounding all equipment casings, including motors and air conditioners, within the computer room to the ground is called safety protective grounding. When insulation breaks down, the stray impedance between the casing and ground is very high, making the voltage on the casing essentially equal to the AC power supply voltage (220V). When a person touches the casing, and the insulation of the body to ground is poor, a considerable current will flow through the body to the ground, which is extremely dangerous. Grounding the casing completely changes this. When insulation breaks down, the grounding short-circuit current flows to the ground along both the grounding wire and the human body. Because the grounding resistance is very small, much smaller than the resistance of the human body... A large current flows into the earth through a grounding resistor, thus protecting personal safety.
Implementation Measures:
The safety ground in the computer room consists of connecting all the server racks in series with several insulated wires, then connecting them to the earth via a grounding busbar (multi-strand braided wire).
Other equipment in the computer room, such as air conditioners, is connected separately.
The DC ground of the computer system is also called the logic ground.
For the computer to function properly, all electronic circuits must operate at a stable base potential, i.e., a zero-potential reference point. When designing the DC ground, it is important to eliminate the noise voltage generated when current flows through a common ground impedance. We do not float the DC ground; instead, we connect it to the earth, meaning the digital circuits in the computer are connected to the earth at the same potential, with the resistance value determined according to the client's requirements.
In systems using a DC ground connection to the earth, a good safety ground system should also be present. Furthermore, in many computer systems, the DC ground and the server rack safety ground are separate, meaning they are insulated from each other within the computer room. This provides a low-resistance path for high-frequency interference to be released to ground, and also a low-resistance path for the discharge of static electricity from the chassis.
DC Grounding Connection and Selection:
Series Grounding: Multi-point grounding. Series grounding involves connecting the DC ground wires of each device in the computing system in series to the copper foil used as the DC ground wire. It should be noted that the direct conductor used in this case is multi-strand braided or copper strip and should be insulated from the chassis.
Parallel Grounding: Single-point grounding. In computer systems, multi-strand shielded flexible wire is used to connect to the copper ground wire, with insulating material placed under the copper block.
Lightning Protection Grounding. Lightning is a natural atmospheric discharge phenomenon. The discharge speed of lightning is very fast, and the change in lightning current is also very drastic. When a thundercloud begins to discharge, the lightning current increases sharply, reaching 200-300 kA during a lightning strike. The destructive effects of lightning can be basically divided into three categories. The first category is the effect of direct lightning strikes, where lightning strikes a building or equipment directly, causing damage. The second category is the secondary effects of lightning, commonly known as induced lightning. This refers to the effects produced by the magnetic and electrostatic effects of the lightning current. This manifests as the electromagnetic field generated by the lightning current changing drastically along with the current itself. Furthermore, electrostatic induction can induce very high voltages (up to hundreds of thousands of volts) on metal objects or electrical circuits, seriously endangering equipment and personnel. The third category involves the lightning current transmitting high voltage along electrical lines and pipes into buildings, creating a phenomenon known as potential introduction, which is, of course, extremely dangerous.