How to ensure the reliability of grounding and zero connection when installing the distribution cabinet?

The methods to ensure the reliability of grounding and zero connection of the distribution cabinet are as follows:

First, the grounding aspect

Select the appropriate grounding material and connection mode.

Grounding electrode material: grounding electrode is the basic part of grounding system. In terms of material selection, angle steel, steel pipe and flat steel are commonly used. For example, angle steel grounding electrode generally adopts hot dip galvanized angle steel, the specification can be ∠50×50×5mm, etc., because hot dip galvanizing treatment can improve the corrosion resistance of angle steel and prolong its service life. Galvanized steel pipe with a diameter of about 50mm, such as SC50, is usually selected as the steel pipe grounding electrode. The flat steel grounding electrode is usually-40×4mm hot-dip galvanized flat steel, which has the advantage of being easy to connect with other grounding devices.

Connection mode: The connection between grounding electrode and grounding trunk line must be firm and reliable. Generally, the welding method is adopted. When welding, it is necessary to ensure that the weld is full and there is no virtual welding. For example, when welding the flat steel with the angle steel grounding electrode, the flat steel should be close to the outside of the angle steel and welded on three sides, and the weld length should be not less than 2 times the width of the flat steel. If bolt connection is adopted, check washer, such as spring washer, should be used, and the specifications of bolts should meet the connection requirements to ensure the tightness of connection and prevent looseness.

Correct installation of grounding electrode

Installation depth and spacing: the installation depth of grounding electrode is the key factor to ensure grounding effect. Generally, the depth from the top of grounding electrode to the ground should not be less than 0.6m. For example, in ordinary soil environment, the effective length of grounding electrode (calculated from the part below the ground) is generally not less than 2.5m. The spacing between multiple grounding electrodes is also very important. Usually, it is required that the spacing between grounding electrodes is not less than twice the length of grounding electrodes, so as to avoid the mutual shielding effect between grounding electrodes and ensure the effective leakage of grounding current.

Improvement of soil conditions: the resistivity of soil has a great influence on grounding effect. If the soil resistivity is high, some measures can be taken to reduce the resistivity. For example, the resistance reducer is filled around the grounding electrode, and the common resistance reducer includes chemical resistance reducer and physical resistance reducer. Chemical drag reducer reduces soil resistivity by ion exchange, but it should pay attention to its potential pollution to soil and environment. Physical drag reducer is mainly some materials with good electrical conductivity, such as graphite, which is relatively friendly to the environment. In addition, the soil replacement method can also be used to replace the high resistivity soil around the grounding electrode with low resistivity soil, such as clay and black soil.

Ensure a good connection between the grounding trunk and the cabinet.

Connection point treatment: the connection point between the grounding trunk line and the cabinet body of the distribution cabinet is the key part of the grounding system. The connection mode can be welding or using special grounding terminals. If it is welding, it is necessary to clean up the paint, rust and other impurities at the joint to ensure the welding quality. When the grounding terminal is used for connection, it is necessary to ensure that the specifications of the terminal match the connecting bolts of the grounding trunk and the cabinet, and tighten the bolts to make the connection points contact well.

Clear grounding sign: at the connection point between the grounding trunk line and the cabinet, an obvious grounding sign should be set, and the yellow-green grounding sign is generally used. The signboard should be marked with the word "grounding" and should be firmly fixed to facilitate inspection and maintenance.

Second, zero connection

Distinguish between system types

In TN-C system, the working zero line (PEN line) also serves as the protection zero line. In this system, the PEN line should ensure the continuity of connection from the power supply end to the distribution cabinet and the subsequent electrical equipment. In TN-S system, the working zero line (N line) and the protection zero line (PE line) are separated. When installing the distribution cabinet, these two lines should be strictly distinguished and cannot be confused. For example, the color of PE line is yellow and green, and N line is generally light blue, so it can be preliminarily judged whether the lines are connected correctly by color.

Connection mode and reliability check

Connection mode: the connection of zero protection should also be firm and reliable. For TN-S system, PE line should be reliably connected with the metal shell and frame of distribution cabinet. Generally, the connection is made through the PE line terminal block, and the connecting bolts should be tightened, and the fastening of the bolts should be checked regularly. In the process of connection, it is necessary to avoid mechanical damage to the PE line, such as avoiding being squeezed or scratched at the door and other positions.

Reliability check: The grounding resistance tester can be used to check the reliability of the zero-connection system. Under normal circumstances, the resistance value of PE line should be very small, and the repeated grounding resistance of PE line is generally required to be no more than 10Ω (the specific value may vary according to different regions and standards). At the same time, it is necessary to check whether there is electrical connection between PE line and N line, so as to prevent the connection between PEN line and PE line and N line from being wrong, which may lead to safety accidents.

Regular inspection and maintenance

Grounding and neutral system need to be inspected and maintained regularly. The inspection period can be determined according to the actual situation, and it is generally recommended to check once every quarter or half a year. The inspection contents include the corrosion of grounding electrode, the looseness of connection point, the change of grounding resistance and zero resistance, etc. If the grounding electrode is found to be seriously corroded, it should be replaced in time; If the connection point is loose, tighten it in time; If the grounding resistance or neutral resistance does not meet the requirements, corresponding measures should be taken for rectification, such as laying grounding electrodes again or improving soil conditions.