Grounding device refers to the grounding device buried undergroundelectrodeFrom the grounding electrode toequipmentThe general name of connecting wires between.
Grounding device is also called grounding integration device:electricalElectrical connection between equipment or other objects and groundequipment。（Building electrical construction technology）。The grounding device consists of grounding electrode (plate)Ground bus(indoor, outdoor), groundingDownlead(Ground jumper）The framework is composed of grounding.It is used to connect the electrical system with the earth.The metal objects that are in direct contact with the earth to realize electrical connection are grounding electrodes.It can be an artificial grounding electrode or a natural grounding electrode.This grounding electrode can be assigned with some electrical function, such as system grounding, protective grounding or signal grounding.The grounding busbar isbuildingThe reference potential point of the electrical device, through which the part to be grounded in the electrical device is connected with the grounding electrode.It also plays another role, that is, it connects all equipotential bonding wires in the electrical device, so as to realize theTotal equipotential bonding。The connecting line between the grounding electrode and the grounding busbar is called the grounding electrode lead.
Safety isolation transformersafety isolating transformer。Supply tools, other equipment and distribution circuit safety ultra-low voltage transformers.Its input winding and output winding are electrically isolated by at least equivalent to double insulation or reinforced insulation[1]。
classification
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The grounding device is composed of grounding body (round steelAngle steel, flat steelSteel pipeAnd connectionground wireComposition.
Working grounding: to ensurePower systemGrounding required for normal operation.for exampleNeutral point directly grounded systemThe neutral point of the transformer in is grounded, which is used to stabilize the potential of the power grid to the ground, thus reducing the insulation to the ground.
Protective grounding: also known as safety grounding, is the grounding set for personal safety, that is, electrical equipment enclosure (includingCableLeather) must be grounded to prevent the electrification of the shell from endangering personal safety.
I&C grounding: thermal control system of power plantdata acquisition system , computer monitoring system, transistor or microcomputerRelay protection systemAnd telecontrol communication system to stabilize potential and prevent interference.also known asElectronic systemGrounding.
The grounding resistance refers to the resistance encountered when the current enters the ground through the grounding body and spreads around.The earth has a certain resistivity. If there is a current flowing through the earth, there will be different potentials everywhere.After the current is injected into the ground through the grounding body, it spreads around in the form of current field. The farther away from the grounding point, the larger the hemispherical dispersion area, the smaller the current density in the ground. Therefore, it can be considered that the resistance per unit diffusion distance and the current density in the ground are close to zero at a distance (15~20m away), where the potential is zero.ascurveU=f (r) is the potential distribution on the ground surface (r is the distance from the lightning current injection point).
The potential Um at the grounding point andGrounding currentThe ratio of I is defined as the grounding resistance R of the point, R=Um/I. When the grounding current is a fixed value, the smaller the grounding resistance, the lower the potential Um, and vice versa.The grounding resistance mainly depends on the structure, size, buried depth and localSoil resistivity。Since the resistivity of the metal grounding body is far less than the soil resistivity, the resistance of the grounding body itself can be ignored in the grounding resistance[2]。
Installation
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(1) General requirements
First, make full use of the natural grounding body to save investment. If the resistance of the natural grounding body measured on the spot has met the requirements of the grounding resistance value and the thermal stability conditions, it is not necessary to install a manual grounding device, otherwise, it should be installed as a supplement.
The artificial grounding device shall be arranged so that the potential distribution near the grounding device is as uniform as possible to reduce the touch voltage and step voltage and ensure personal safety.
(2) Utilization of natural grounding body
The steel structure and reinforcement of the building, the rail of the crane, the buried metal pipeline (except the pipeline of combustible liquid and combustible explosive gas), and the metal sheath of the cable laid underground with the quantity of no less than two can be used as the natural grounding body.Buildings available for transformer and distribution substationreinforced concreteThe foundation is used as a natural grounding body.When using the natural grounding body, make sure that the electrical connection is good.
(3) Installation of artificial grounding body
The artificial grounding body has two basic structural types, namely, vertical burial and horizontal burial.
The commonly used vertical grounding body is a steel pipe with a diameter of 50 mm and a length of 2.5 m or an L50 × 5 angle steel. In order to reduce the impact of external temperature changes on the stray resistance, the upper end of the buried vertical grounding body should not be less than 0.7 m from the ground.
For grounding devices laid in places with strong corrosivity, hot tin plating, hot galvanizing and other anti-corrosion measures shall be taken according to the nature of corrosion, or the section shall be appropriately enlarged.
When multiple grounding bodies are close to each other, the dispersion of incoming current will crowd out each other, which is called shielding effect.This reduces the utilization rate of grounding devices, so the spacing between vertical grounding bodies should not be less than 5m, and the spacing between horizontal grounding bodies should not be less than 5m.
The grounding grid shall be arranged so that the ground potential is evenly distributed as far as possible to reduce the touch voltage and step voltage.The outer edge of the artificial grounding grid shall be closed, and each corner of the outer edge shall be made into an arc.The grounding grid of 35~110kV/6~10kV substation shall be laid with horizontal voltage equalizing tape.In order to reduce the contact voltage of the building, the horizontal distance between the grounding body and the foundation of the building shall be no less than 1.5m, generally 2~3m.
Equipotential bonding
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Equipotential bondingIt is a measure to equalize the potential and reduce the contact voltage and eliminate the risk of electric shock caused by potential difference by adding some connecting wires without adding protective appliances.It is economical and can effectively prevent electric shock.
Equipotential bonding usually includesTotal equipotential connectionandAuxiliary equipotential bondingTwo.The so-called total equipotential connection is to connect the PE line or PEN line of the electrical device with all nearby metal pipe components (such as grounding trunk lines, water pipes, gas pipes, heating and air conditioning pipes, and if possible, the reinforcement and metal components of the building) at the entrance to the building and the equipotential connection terminal board (namely the grounding terminal board).The total equipotential connection reduces the touch voltage by equalizing the potential, and eliminates the dangerous voltage introduced into the building from the power line.
The main purpose of the total equipotential connection is not to shorten the action time of the protective appliance, but to make the potential between the exposed conductive part and the external conductive part that can be touched at the same time approximately equal, that is, to reduce the contact voltage below the safe value.The safe voltage value is 50V under normal conditions and 25V in humid environment.When automatic power cut is adopted as a measure to prevent indirect electric shock, the total equipotential connection is indispensable.
Auxiliary equipotential connection is also calledLocal equipotential bondingIt is to connect PE line or PEN line with all accessible exposed conductive parts and external conductive parts in a local range, so that they are at the same potential in a local range, as a supplement to the total equipotential connection.The main purpose of local equipotential bonding is to reduce the contact voltage below the safe voltage.
WithLightning protection deviceIf the lightning protection device cannot be isolated from other facilities and personnel in the building, equipotential bonding shall also be adopted.
When some electrical devices are located outside the action area of the total equipotential bonding, they shall be installedResidual current circuit breakerAnd the PE line of this part should be isolated from the PE line of the power incoming line and connected to a separate grounding electrode to eliminate the dangerous voltage from outside.
quality standard
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GB50169-92 Code for Construction and Acceptance of Grounding Devices
1. The buried depth of the top surface of the grounding body shall meet the design requirements.When there is no regulation, it should not be less than 0.6m.Angle steel and steel pipe grounding body shall be configured vertically.In addition to the grounding body, the vertical part of the outgoing line of the grounding body and the welding part of the grounding device shall be subject to anti-corrosion treatment;Before anti-corrosion treatment, the surface must be derusted and the residual welding flux at the welding position must be removed.
2. The spacing of vertical grounding body shall not be less than 2 times of its length.The spacing of horizontal grounding body shall meet the design requirements.When there is no design specification, it should not be less than 5m.
3. The grounding wire shall be protected from mechanical damage and chemical corrosion;Steel pipes or other solid protective pipes shall be installed where the grounding wire passes through the walls, floors and floors, and anti-corrosion measures shall be taken for the parts with chemical corrosion.
4. The grounding main line shall be connected to the grounding grid at two or more different points.
5. The grounding of each electrical device shall be connected to the grounding main line with a separate grounding wire. It is not allowed to connect several electrical devices that need grounding in series in one grounding wire.
6. After the grounding body is laid, the backfill soil of the trench shall not be mixed with stones and construction waste;The soil taken from outside shall not have strong corrosivity;The backfill shall be compacted layer by layer.
7. The grounding body (wire) shall be connected by welding, and the welding must be firm without faulty welding.The grounding wire connected to the electrical equipment shall be connected with galvanized bolts.
8. The grounding body (wire) shall be welded by lap welding, and the lap length must meet the following requirements:
(1) The flat steel shall be twice its width (and at least 3 edges shall be welded).
