Various grounding analyses in electrical control systems

(1) Signal ground: It is the ground of the input signal element-sensor.

(2) AC ground: The N line of the AC power supply, which is usually the main place where noise is generated.

(3) Shielded ground: A shell or metal mesh generally set up to prevent static electricity and magnetic field induction, connected to the earth's crust through a special copper wire.

(4) Protective ground: Generally, the outer casing of the machine equipment or the outer casing of each independent device in the device is grounded to protect personal safety and prevent leakage of the device.

In order to suppress the interference superimposed on the power supply and input and output terminals, the electrical control system should be well grounded . In general, the grounding method is related to the signal frequency. When the frequency is lower than 1MHZ, one-point grounding can be used; when it is higher than 10MHZ, multi-point grounding is used; between 1MHZ and 10MHZ, the type of grounding used depends on the actual situation. Therefore, the control system composed of various controllers often adopts one-point grounding, the cross-sectional area of ​​the grounding wire is not less than 2mm, the grounding resistance is not greater than 100Ω, and the grounding wire is preferably a dedicated ground wire. If this requirement cannot be met, a public grounding method can also be used. It is forbidden to use the method of series grounding with other devices.

When AC power is transmitted, there will be several millivolts or even several volts of voltage on the power wire at a considerable distance, while low-level signal transmission requires the voltage along the way to be zero. In order to prevent AC from interfering with low-level signals, isolation shielding should be added to the DC signal wire; the signal source and AC are not allowed to share a ground wire; each grounding point can be connected together through a grounding copper sheet.

The shielding ground and protective ground cannot be twisted together with the power ground, signal ground and other grounds, and can only be connected to the grounding copper sheet separately. In order to reduce the capacitive coupling noise of the signal, a variety of shielding measures can be adopted. The distributed capacitance problem of electric field shielding can be solved by connecting the shielding ground to the earth. For purely anti-magnetic parts, such as strong magnets, transformers, and magnetic field coupling of large motors, high magnetic permeability materials can be used as outer covers, and the outer covers can be connected to the earth for shielding.