良好的接地是保证PLC可靠工作的重要条件，可以避免偶然发生的电压冲击危害。接地的目的通常有两个，其一为了安全，其二是为了抑制干扰。完善的接地系统是PLC控制系统抗电磁干扰的重要措施之一。PLC控制系统的地线包括系统地、屏蔽地、交流地和保护地等。接地系统混乱对PLC系统的干扰主要是各个接地点电位分布不均，不同接地点间存在地电位差，引起地环路电流，影响系统正常工作。例如电缆屏蔽层必须一点接地，如果电缆屏蔽层两端A、B都接地，就存在地电位差，有电流流过屏蔽层，当发生异常状态如雷击时，地线电流将更大。
ELAU-MC-41110400
SCHNEIDER ELAU-MC-41110400 若系统地与其它接地处理混乱，所产生的地环流就可能在地线上产生不等电位分布，影响PLC内逻据电路和模拟电路的正常工作。PLC工作的逻辑电压干扰容限较低，逻娟地电位的分布干扰容易影响PLC的逻辑运算和数据存贮，造成数据混乱、程序跑飞或死机。模拟地电位的分布将导致测量精度下降，引起对信号测控的严重失真和误动作。此外，屏蔽层、接地线和大地有可能构成闭合环路，在变化磁场的作用下，屏蔽层内又会出现感应电流，通过屏蔽层与芯线之间的耦合，干扰信号回路。
ELAU-MC-41110400
A good grounding is an important condition to ensure the reliable operation of the PLC, which can avoid accidental voltage surge hazards. There are usually two purposes for grounding, one is for safety, and the other is to suppress interference. A complete grounding system is one of the important measures for PLC control systems to resist electromagnetic interference. The grounding wire of the PLC control system includes system grounding, shielding grounding, AC grounding, and protection grounding. The interference of the chaotic grounding system on the PLC system is mainly due to the uneven distribution of potential at various grounding points, and the existence of ground potential differences between different grounding points, causing ground loop current and affecting the normal operation of the system. For example, the cable shielding layer must be grounded at one point. If both ends A and B of the cable shielding layer are grounded, there is a ground potential difference, and there is current flowing through the shielding layer. When abnormal conditions such as lightning strikes occur, the ground wire current will be greater.
ELAU-MC-41110400
SCHNEIDER ELAU-MC-41110400 If the system grounding and other grounding processes are chaotic, the generated ground current may generate unequal potential distribution on the ground wire, affecting the normal operation of the logic circuit and analog circuit in the PLC. The logic voltage interference tolerance of PLC operation is low, and the distribution interference of logic ground potential can easily affect the logic operation and data storage of PLC, causing data confusion, program runaway or crash. The distribution of simulated ground potential will lead to a decrease in measurement accuracy, causing serious distortion and misoperation of signal measurement and control. In addition, the shielding layer, grounding wire, and ground may form a closed loop. Under the action of a changing magnetic field, induced current will appear inside the shielding layer, which interferes with the signal circuit through the coupling between the shielding layer and the core wire.
ELAU-MC-41110400
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