In the process of dust-free workshop consumption, the harm caused by static electricity is various. Firstly, due to electrostatic adsorption, dust particles will be adsorbed on the surface of devices and the whole machine, affecting the quality of production and forming defects; secondly, the energy of corona discharge will interfere with micro-electronic devices; moreover, when electrostatic is serious, it may cause spark discharge, and may become the ignition source or explosion of flammable and explosive gases, liquids or dust and their mixture.
In order to eliminate the harm caused by static electricity, some measures must be taken. There are many ways to eliminate static electricity. Grounding is the most common, economical, cumbersome and reliable way to eliminate static electricity. In a dust-free workshop, all equipment that generates static electricity should be firmly grounded. The central type of electrostatic connection mainly includes hard grounding and soft grounding. Hard grounding refers to the direct connection with the earth electrode as conductivity, while soft grounding is connected to the earth electrode through a current limiting resistance to limit the current flowing through the human body not to exceed 5mA safe value. There are three types of electrostatic grounding:
A. Direct grounding.
The metal conductor is directly connected with the earth to stop the electrical connection, so that the electrostatic potential of the metal conductor is close to or equal to the earth's potential.
B. Indirect grounding.
The electrostatic voltage conductor is electrically connected to the earth through all or part of the metal conductor in close contact with the surface.
C. Straddle grounding.
Two or more insulating metal conductors stop conductive electrical connection to establish a low impedance path for current supply activity, and then stop grounding. The anti-static grounding system sets grounding connection terminals according to different requests. Equipotential grounding grid or closed grounding copper ring should be set up in a room. The cross-sectional area of copper bars should not be less than 100 mm2. The anti-static grounding leads should be connected from the equipotential grounding grid or close to the grounding ring of copper bars. Multiple copper wires should be used for grounding lead, and the conductor cross section should not be less than 1.5 mm 2.
Equipotential anti-static grounding datum board should be set before the anti-static grounding system is connected to the earth. The main grounding line should be drawn from the datum board. The cross-sectional area of copper conductor should not be less than 95 mm, and insulated shielded cable should be used. The main grounding line shall be connected with the grounding grid or closed copper ring set in the anti-static area. The anti-static floor is used in the main consumer places. After the floor is laid, the metal wires in the anti-static floor are connected with the ground terminals reserved on the building pillars in the space where the anti-static floor is located. In addition, the grounding terminal must be connected with the grounding electrode through the main bar in the column, so that the static electricity flows to the grounding electrode along the main bar in the column through the grounding terminal.
The design of grounding system with different functions in dust-free workshop should follow the criterion of equipotential combination, in which DC grounding system can not be mixed with AC grounding system. Grounding design in electrostatic control should be based on lightning protection grounding of buildings. All kinds of functional grounding systems, including electrostatic shielding grounding, should be in the maintenance space of lightning protection grounding, that is to say, equipotential combination is necessary. All kinds of functional grounding systems should be self-contained and should not be mixed with each other. The low impedance electrical path of the electrostatic shielded grounding system should be less than 0.1_. The DC working grounding low impedance electrical circuit should be less than 0.02_.
Electrical Maintenance Grounding
Electrical maintenance grounding refers to the grounding of micro-electronics mounted metal housing to avoid contact voltage and step voltage endangering personal safety and equipment safety. When TN-S or TN-C-S system is used for grounding in electrical maintenance, the exposed metal parts of the electrical equipment without electricity are connected directly with the grounding points of the power grid. Once a single-phase grounding fault occurs due to insulation damage of one-phase winding, the grounding current (i.e. single-phase short-circuit current) of the single-phase grounding fault constituted by the outer shell of the equipment will be very large, so that the maintenance and installation of the line (such as fuses, low-voltage circuit breakers, etc.) can operate rapidly and cut off the circuit, thereby eliminating the risk of personal electric shock. In addition, if in the short time before the removal of the defective circuit, once someone touches the charged shell or frame, the resistance of the ground phase-to-ground circuit is much smaller than that of the human body. Therefore, the current basically passes through the phase-to-ground circuit to prevent the risk of electric shock.
In the dust-free workshop, the equipment on the consumption pipeline is intensive, and most of them are electrical equipment with metal shell. If the maintenance of grounding is not in place or does not meet the requirements, it is easy to cause staff electric shock risk when grounding fault occurs. Therefore, the maintenance of grounding problem can not be ignored, whether in the design process or in the construction process, we should really do a good job in the maintenance of grounding. Maintenance of grounding objects should stop mainly including metal frames or enclosures of transformers, high-voltage switchgear, distribution cabinets, control screens, fixed, portable and movable electrical appliances, metal maintenance tubes or bridges of power lines, junction box enclosures, armored cable sheaths, etc. Flat steel or copper conductors can be used to maintain grounding connection, requesting a reliable electrical access.
Information System Grounding
Dust-free workshop generally has integrated wiring system, automatic fire alarm system, IT information center, information points throughout the workshop and office for consumption monitoring and management, which touches on the problem of information system grounding.
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