Under normal or abnormal conditions, in order to ensure normal and reliable operation, a reliable metal connection must be made between a certain point in the power supply system and the ground, which is called working grounding. The main functions are:
â‘ Reduce the contact voltage of the human body. In a system where the neutral point is insulated from the ground, when one phase is grounded and the human body touches the other phase, the human body will be subjected to line voltage, but for the neutral point grounding system, the human body will be affected by the line voltage. Is the phase voltage.
â‘¡ Cut off the faulty equipment quickly. In a neutral-point insulated system, when one phase is grounded, the ground current is only capacitive current and leakage current, and the value is very small, not enough to make the protection device act to cut off the faulty equipment. In the neutral grounding system, a single-phase grounding short circuit will be caused when the ground is touched, which can make the protection device act quickly to cut off the fault.
â‘¢Reduce the danger of high pressure and low pressure.
The role of the grounding device-protective groundingUnder normal working conditions, the shells of various electrical appliances are not charged. However, due to some reasons, the enclosure may be electrified after the equipment insulation is damaged. Once a person or animal comes into contact with such an apparatus with an electrified enclosure, there is a danger of electric shock. In order to prevent this phenomenon from endangering personal safety when it occurs, the metal shell of the electrical equipment that is not charged when the electrical equipment is normal, and the metal part of the power distribution device are well electrically connected to the earth, which is called protective grounding.
The function of the protective grounding is to set up the grounding for personal safety to prevent the shell from being charged and endangering the personal safety.
When the neutral point of electrical equipment is grounded, this point is called the zero point. The wire drawn from the zero point is called zero. When the shell of electrical equipment is connected to the neutral line, it is called protective zero connection. When the protective grounding is adopted in the low-voltage power network where the neutral point is directly grounded, the grounding resistance value of the equipment shell of the neutral point grounding resistance is large, and the system voltage is low (such as 220v), so that the shell-touching fault current is small and the protection cannot be started. The device cuts off the power supply, and the equipment shell is live for a long time, which endangers personal safety. Because it is not economical to lay grounding grids in low-voltage power grids to reduce grounding resistance, the existing neutral wire is usually used as a protective grounding wire to form a protective way . The resistance of the neutral line is small, and the current is large in the event of a fault, so that the protection device can reliably act to cut off the power supply, ensuring the safety of personnel and equipment. This method is simple and economical, so it is often used in low-voltage power grids.
The role of the grounding device-lightning protection groundingThe grounding is set up for the needs of lightning protection, the purpose is to make the lightning current smoothly lead into the earth, so as to help reduce the lightning overvoltage.
The unit of measurement in the lightning protection and grounding list pricing is an item, and the comprehensive unit price in the list pricing table is the price of each item. When submitting inventory, if lightning protection and grounding are each a system, then one lightning protection device and one grounding device can be calculated separately. If a group of grounding poles are used for lightning protection and grounding, they can be combined into one code, and the list quantity is one item.
The grounding lead-out device can effectively prevent groundwater seepage when the subway platform power equipment grounding, signal equipment grounding and lightning protection grounding are connected to the integrated grounding grid, and can well meet the stray current protection requirements.
In the subway grounding system, due to fault current (stray current) protection requirements, the grounding grid is located under the structural floor. Therefore, when the electrical equipment of the subway platform is grounded, it must be connected to the grounding grid through the grounding down conductor to achieve reliable grounding. In the past, the grounding down conductor was usually drawn directly from the structural floor or through a PVC pipe. With this kind of measure, it is easy to fail to meet the protection requirements of fault current (stray current), groundwater is easy to seep out from the grounding lead, and it fails to meet the design requirements of subway grounding.
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