Soaring knowledge, why sometimes the hand touched the industrial control chassis will have the feeling of electric shock

  2019-08-09  Macase  

Compared with commercial machines, IPCs are characterized by strong anti-electromagnetic interference. Industrial PCs are often used when integrating DCS systems or PLC systems. “Why do people sometimes feel the electric shock when they touch the industrial control cabinet?” This is because the industrial control cabinets are made of high-quality metal materials, so that the industrial control cabinet has a strong shielding effect on the electromagnetic signals, and the electromagnetic induction around the industrial control cabinet. Inductive power will be formed on the chassis. If the chassis is well grounded, the induction will lead the earth through the ground wire, so that the person touches the chassis and will not be charged. However, if the chassis is not grounded, the static electricity will accumulate more and the voltage can be as high as 100V. If the grounding fault is induced, the voltage will reach 20-40V.

The industrial computer adopts the 220V power supply, which inevitably has the phenomenon of unbalanced power supply. This will cause the grounding voltage and grounding current of the AC220V zero line to exist to different extents. In serious cases, it will also damage the components in the industrial computer. . The potential of the human body standing on the ground is the local ground potential, but the chassis of the industrial computer is not necessarily the ground potential. The reason is: the zero line potential in the AC 220V power supply is basically similar to the local ground potential, and the difference between the live line and the ground potential, that is, the potential of the human body is about 220V. After the power supply enters the manual control, it generally passes. Electronic components such as power switches, fuses, coils, etc., electronic components and circuit boards are fixed in the chassis, and there are distributed capacitance and leakage resistance. For these reasons, it can be understood as two impedances, one connected to the neutral line in the power line and the other connected to the power line.
Industrial control chassis ground impedance equivalent diagram
As shown in the figure above, the potential of the chassis to ground is:
The potential of the human body is basically the ground potential, so the alternating voltage between the human body and the chassis is also the voltage represented by the above formula. When a person touches the industrial control cabinet, this voltage is added to the human body, which makes people feel the contact inductance. Since the impedances of Zt and Zz are large, the contact inductance is not serious. If the power cord is fired, the neutral pair is Change, the AC voltage on the chassis will be:
The voltages expressed by the above two equations may vary greatly, so sometimes the power plug is swapped, and the originally charged ones become uncharged, or vice versa. This contact inductance is also related to the insulation performance of the shoes worn by people. If the insulation performance of the shoes is good, when the chassis is touched, the voltage is mainly dropped between the shoes and the earth, so there is no inductance.
In order to protect the safety of the operator, grounding should be done carefully, and the chassis of the industrial computer should be connected to the ground wire. The grounding resistance should be checked and tested regularly to ensure that the grounding resistance of the ground wire meets the requirements of lightning protection grounding specifications.