The purpose of grounding is simply for safety and to prevent interference. For PLCs, the main purpose of grounding is to suppress interference attached to the power supply and input/output terminals. Therefore, a proper grounding system is one of the important measures for PLC control systems to resist electromagnetic interference. Grounding methods include floating ground and direct grounding. Direct grounding should be used for PLC control systems.
The specific grounding methods are as follows: Grounding of the shielding layer of the shielded cable: 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 joint in the middle of the signal line, the shielding layer should be firmly connected and insulated, and multiple grounding points must be avoided; when the shielded twisted pair of multiple measurement point signals is connected to the multi-core twisted pair shielded cable, each shielding layer should be connected to each other and insulated, and a suitable grounding point should be selected for single-point grounding.
Input/output wiring: PLC power lines, I/O power lines, input signal lines, output signal lines, AC lines, and DC lines should all be routed separately as much as possible. Digital signal lines and analog signal lines should also be routed separately, with the latter using shielded cable and the shield grounded. Digital transmission lines should also use shielded cable, and the shield should be grounded. Because the current in twisted-pair cables is opposite in direction and equal in magnitude, it can cancel out noise caused by induced current; therefore, signal lines often use twisted-pair or shielded cable.
1. PLC grounding treatment
In a PLC control system, there are various forms of "ground".
Mainly:
(1) Signal ground: This is the ground of the input signal element—the sensor. In order to suppress interference attached to the power supply and input/output terminals, the PLC system should be properly grounded. Generally, the grounding method is related to the signal frequency. When the frequency is below 1MHz, a single-point grounding can be used; when it is above 10MHz, multiple-point grounding should be used.
The choice of grounding method for the 1-10MHz range depends on the specific circumstances.
Therefore, control systems composed of PLCs commonly use single-point grounding. The cross-sectional area of the grounding wire should not be less than 2 mm², and the grounding resistance should not exceed 100 Ω. Ideally, the grounding wire should be a dedicated grounding wire. If these requirements cannot be met, a common grounding method can be used. Connecting the grounding wire in series with other equipment is prohibited.
(2) Shielding ground: Generally, it is an outer shell or metal mesh installed to prevent static electricity and magnetic field induction. It is connected to the earth's crust through a special copper wire.
(3) AC ground and protective ground: The N line of the AC power supply is usually the main source of noise. The protective ground is generally used to ground the casing of the machine or the casing of the independent components inside the equipment to protect personal safety and prevent equipment leakage. When AC power is transmitted, there will be a voltage of several mV or even several V on the power conductors over a considerable distance, while the transmission of low-level signals requires the circuit level to be zero. To prevent AC power from interfering with low-level signals, isolation shielding should be added to the conductors of DC signals: the signal source and AC power are not allowed to share a ground wire; all grounding points are connected together through grounding copper plates. The shielding ground and protective ground cannot be twisted together with the power ground, signal ground and other grounds. They can only be connected to the grounding copper plates independently. To reduce the capacitive coupling noise of the signal, various shielding measures can be adopted. For the distributed capacitance problem of electric field shielding, it can be solved by connecting the shielding ground to the earth. For purely antimagnetic parts, such as the magnetic field coupling of strong magnets, transformers and large motors, a high magnetic permeability material can be used as an outer cover, and the outer cover can be connected to the earth for shielding.
2. PLC input/output wiring
PLC power lines, I/O power lines, input signal lines, output signal lines, AC lines, and DC lines should be routed separately as much as possible. Digital signal lines and analog signal lines should also be routed separately. Both digital and analog signal lines should use shielded cables, and the shielding layer should be reliably grounded. Because the current in twisted-pair cables flows in opposite directions and is of equal magnitude, noise caused by induced current can be canceled out. Therefore, signal lines often use twisted-pair or shielded cables.
