The purpose of grounding is nothing more than safety and interference prevention. The purpose of grounding the PLC is mainly to suppress the interference attached to the power supply and input and output terminals. Therefore, the correct grounding system is one of the important measures for the PLC control system to resist electromagnetic interference. Grounding methods include floating grounding and direct grounding. For PLC control systems, direct grounding shall be used.
The specific grounding methods are as follows: Grounding of the shielding layer of the shielded wire: When the signal source is grounded, the shielding layer should be grounded on the signal side; when the signal source is not grounded, it should be grounded on the PLC side; when there is a connector in the middle of the signal wire, the shielding layer should be firm Connect and conduct insulation treatment, and be sure to avoid multi-point grounding; when the shielded twisted pair of multiple measuring point signals is connected to the multi-core pair-twisted general screen cable, the shielding layers should be connected to each other, and after insulation treatment, select the appropriate Single-point grounding at the grounding point.
Wiring of the input and output parts: PLC power lines, I/O power lines, input signal lines, output signal lines, AC lines, and DC lines should be wired separately as much as possible. The switch signal line and the analog signal line should also be wired separately, and the latter should be shielded, and the shielding layer should be grounded. Digital transmission lines should also use shielded wires, and the shielding layer should be grounded. Since the currents in the twisted-pair wires are in opposite directions and equal in magnitude, the noise caused by the induced current can be canceled each other, so the signal wires are mostly twisted-pair or shielded wires.
1. Grounding treatment of PLCIn the PLC control system. There are many forms of "land".
There are:
(1) Signal ground: It is the ground of the input signal component-the sensor. In order to suppress the interference attached to the power supply, input and output terminals, the PLC system should be well grounded. Under normal circumstances. The grounding method is related to the signal frequency. When the frequency is lower than 1 MHz, one-point grounding can be used; when the frequency is higher than 10 MHz, multiple-point grounding can be used;
Which ground to use between 1-10MHz depends on the actual situation.
Therefore, the control system composed of PLC is often grounded at one point. The cross-sectional area of ​​the ground wire cannot be less than 2 mm2. The grounding resistance cannot be greater than 100Q. The grounding wire is preferably a dedicated grounding wire. If this requirement is not met. Common grounding method can also be used. It is forbidden to use the grounding method in series with other equipment.
(2) Shielding ground: generally a shell or metal wire mesh set to prevent static electricity and magnetic field induction. Connect it to the earth's crust through a special copper wire.
(3) AC ground and protective ground: the N line of the AC power supply, usually it is the main place where noise is generated. The protective ground generally grounds the housing of the machine equipment or the housing of the independent device in the equipment. Used to protect personal safety and prevent leakage of equipment. When the AC power is transmitted, it is on the power wire at a considerable interval. There will be voltages of several mV or even several volts, and low-level signal transmission requires the circuit level to be zero. To prevent AC interference from low-level signals. Isolation and shielding should be added to the wires of the DC signal: the signal source and the AC power are not allowed to share the same ground wire: the grounding points are connected together by the grounding copper plate. Shield ground and protective ground cannot be twisted with power ground, signal ground and other grounds. They can only be connected to the grounding bronze plate independently of each other. In order to reduce the capacitive coupling noise of the signal, various shielding measures can be adopted. The distributed capacitance problem of electric field shielding can be solved by connecting the shielding ground to the ground. For purely anti-magnetic parts, such as magnetic field coupling of strong magnets, transformers, and large motors. High magnetic permeability materials can be used as the outer cover. Connect the cover to the ground to shield it.
2. Wiring of PLC input and outputPLC power cables, I/O power cables, input signal cables, output signal cables, AC cables, and DC cables should be wired separately as much as possible. The switch signal line and the analog signal line should also be wired separately. Both the switch signal line and the analog signal line should be shielded. And the shielding layer is reliably grounded. Because the current direction in the twisted pair is opposite. The same size can cancel the noise caused by the induced current. Therefore, the signal wire is mostly twisted pair or shielded wire.
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