Electrostatic grounding means that the equipment container and pipeline are connected to the earth through the metal wire and the grounding body to form an equipotential, and have a minimum resistance value. 'The bridging means that the metal equipment and the pipelines are connected by metal wires to cause an equipotential body. Obviously, the purpose of grounding and bridging is to artificially create an equipotential rest with the earth, causing no disaster caused by the spark of the electrostatic potential difference. However, another purpose of the pipeline bridging is when there is stray current. Give it a good path to avoid sparks at the breaks and cause accidents. The bridge for this purpose can be used normally, and it needs to be connected beforehand.
It takes a certain time for the electric charge accumulated inside the insulating oil to be introduced into the earth through the grounding body. The time required in the previous section is t"3t. Therefore, even a well-grounded metal container cannot eliminate the generation of electric power in the oil. Accumulation. Some people try to set up a metal mesh in the insulating liquid and clean it to eliminate static electricity. This is exactly the opposite. Because the metal mesh can not increase the electrical conductivity of the insulating liquid, it increases the solid-liquid contact area and gives a new static charge. The creation has created a good opportunity.
A liquid having a resistivity greater than 1011 ohms has a condition for generating static electricity. For this, a medium having a resistivity of less than 10/8 ohm centimeters is a good conductor of static electricity. From this point of view, the total grounding resistance of the boundary can satisfy 10/6. Ohm, the static charge can run smoothly.
Since the electrostatic current is micro-ampere (10-8 amps), if the potential difference caused by the grounding body is not more than 10 volts, the grounding resistance can be up to 10-6 ohms. If the current is taken to 10-4 amps, the voltage is Taking 0.1 volts, and considering the ease of use, the total resistance of the metal conductor portion of the electrostatic grounding device can be less than 100 ohms. Therefore, the grounding resistance of the static electricity is suitable in the range of 10-2-10-6 ohms. Although the value of the grounding resistance for the purpose of antistatic alone can be higher, it is necessary to pay attention to the connection must be secure, otherwise it is in the virtual connection or In the case of looseness, there will be a high potential and there is a danger of discharge.
In order to protect the equipment and buildings and structures in the refinery from lightning, a lightning protection device shall be provided. The grounding resistance values ​​in the device have different requirements according to the lightning protection level:
Lightning resistance resistance value
I lightning protection buildings, structures and storage tanks 5-10 ohms
Class II lightning protection buildings, structures and storage tanks 10 ohms
Class III lightning protection buildings, structures and storage tanks 20-30 ohms
Obviously, when using the joint grounding grid, no matter which level of grounding resistance value is more than static, when the grounding body is an artificial grounding body, generally use 50X50x5 mm angle steel or steel pipe with wall thickness greater than 3.5 mm. , cut into a length of 2.5 meters, into the ground, the top of which is 700 mm from the floor. Each grounding body is connected with 40×4 mm flat steel and connected with the equipment. The resistance value is based on the measured value.
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