What are the specifications and requirements for electrical grounding?
The protection methods for electrical system configuration include: protective grounding, protective neutral connection, repeated grounding,
working grounding, etc. A good electrical connection between a part of electrical equipment and the earth is called grounding. The metal
conductor or metal conductor group directly contacting with the earth soil is called the grounding body: the metal conductor connecting the
grounding part of the electrical equipment to the grounding body is called the grounding wire; The grounding body and grounding wire are
collectively referred to as grounding devices.
Grounding concept and type
(1) Lightning protection grounding: grounding for the purpose of quickly introducing lightning into the earth and preventing lightning damage.
If the lightning protection device shares a general grounding grid with the working grounding of the telegraph equipment, the grounding resistance
shall meet the minimum requirements.
(2) AC working grounding: metal connection between a point in the power system and the earth directly or through special equipment. Working
grounding mainly refers to the grounding of transformer neutral point or neutral line (N line). N wire must be copper core insulated wire. There
are auxiliary equipotential terminals in the power distribution, and the equipotential terminals are generally in the cabinet. It must be noted that
the terminal block cannot be exposed; It shall not be mixed with other grounding systems, such as DC grounding, shielding grounding, anti-static
grounding, etc; It cannot be connected with PE line.
(3) Safety protection grounding: safety protection grounding is to make a good metal connection between the uncharged metal part of electrical
equipment and the grounding body. The electrical equipment in the building and some metal components near the equipment are connected with
PE lines, but it is strictly prohibited to connect the PE lines with N lines.
(4) DC grounding: In order to ensure the accuracy and stability of each electronic equipment, a stable reference potential must be provided in addition
to a stable power supply. The insulated copper core wire with large section area can be used as the lead, one end of which is directly connected with the
reference potential, and the other end is used for DC grounding of electronic equipment.
(5) Anti static grounding: the grounding to prevent the interference of static electricity generated in the dry environment of the computer room in the
intelligent building to electronic equipment is called anti-static grounding.
(6) Shielding grounding: in order to prevent external electromagnetic interference, the shielding wire or metal pipe inside and outside the electronic
equipment enclosure and the equipment are grounded, which is called shielding grounding.
(7) Power grounding system: in electronic equipment, to prevent interference voltage of various frequencies from invading through AC and DC power
lines and affecting the operation of low-level signals, AC and DC filters are installed. The grounding of filters is called power grounding.
The functions of grounding are divided into protective grounding, working grounding and anti-static grounding
(1) The metal shells, concrete, poles, etc. of electrical equipment may be electrified due to insulation damage. In order to prevent this situation from
endangering personal safety and avoiding electric shock accidents, the metal shells of electrical equipment are connected with the grounding device
to protect the grounding. When the human body touches the electrical equipment with the shell electrified, the contact resistance of the grounding
body is far less than the human body resistance, Most of the current enters the earth through the grounding body, and only a small part flows through
the human body, which will not endanger human life.
(2) The grounding conducted to ensure the reliable operation of electrical equipment under normal and accident conditions is called working
grounding. For example, the direct grounding and indirect grounding of neutral point as well as the repeated grounding of zero line and lightning
protection grounding are all working grounding. In order to introduce lightning into the ground, connect the grounding terminal of lightning
protection equipment (lightning rod, etc.) to the ground to eliminate the harm of lightning overvoltage to electrical equipment, personal property,
also known as overvoltage protection grounding.
(3) The grounding of fuel oil, natural gas storage tanks, pipelines, electronic equipment, etc. is called anti-static grounding to prevent the impact
of electrostatic hazards.
Requirements for installing grounding device
(1) The grounding wire is generally 40mm × 4mm galvanized flat steel.
(2) The grounding body shall be galvanized steel pipe or angle steel. The diameter of the steel pipe is 50mm, the pipe wall thickness is not less
than 3.5mm, and the length is 2-3 m. 50mm for angle steel × 50mm × 5 mm.
(3) The top of the grounding body is 0.5~0.8m away from the ground to avoid thawing soil. The number of steel pipes or angle steels depends
on the soil resistivity around the grounding body, generally not less than two, and the spacing between each is 3~5m
(4) The distance between the grounding body and the building shall be more than 1.5m, and the distance between the grounding body and the
independent lightning rod grounding body shall be more than 3m.
(5) Lap welding shall be used for the connection of grounding wire and grounding body.
Methods for reducing soil resistivity
(1) Before the installation of the grounding device, the resistivity of the soil around the grounding body shall be understood. If it is too high,
necessary measures shall be taken to ensure that the grounding resistance value is qualified.
(2) Change the soil structure around the grounding body within 2~3m of the soil around the grounding body, and add substances that are
impermeable to water and have good water absorption, such as charcoal, coke cinder or slag. This method can reduce the soil resistivity to
the original 15~110.
(3) Use salt and charcoal to reduce soil resistivity. Use salt and charcoal to tamp in layers. The charcoal and fine are mixed into a layer, about
10~15cm thick, and then 2~3cm of salt is paved, a total of 5~8 layers. After paving, drive into the grounding body. This method can reduce the
resistivity to the original 13~15. However, salt will be lost with running water over time, and it is generally necessary to replenish it once more
than two years.
(4) The soil resistivity can be reduced to 40% by using long-acting chemical resistance reducer. The grounding resistance of electrical equipment
shall be tested once each year in spring and autumn when there is less rain to ensure that the grounding is qualified. Generally, special
instruments (such as ZC-8 grounding resistance tester) are used for testing, and ammeter voltmeter method can also be used for testing.
The contents of grounding inspection include
(1) Whether the connecting bolts are loose or rusted.
(2) Whether the corrosion of grounding wire and grounding body below the ground is desoldered.
(3) Whether the grounding wire on the ground is damaged, broken, corroded, etc. The power line of overhead incoming line, including the neutral
line, shall have a section of no less than 16 mm2 for aluminum wire and no less than 10 mm2 for copper wire.
(4) In order to identify the different uses of various conductors, the phase line, working zero line and protective line shall be distinguished in
different colors to prevent the phase line from being mixed with the zero line or the working zero line from being mixed with the protective zero
line. In order to ensure the correct connection of various sockets, the three-phase five wire power distribution mode shall be used.
(5) For the automatic air switch or fuse of the power supply at the user end, a single-phase leakage protector shall be installed in it. The user lines
that have been out of repair for a long time, aging insulation or increased load, and the section is not small, should be replaced as soon as possible
to eliminate electrical fire hazards and provide conditions for the normal operation of the leakage protector.
(6) In any case, the protective grounding wire and neutral wire of the three item five wire system equipment in the power electrical system shall not
be less than 1/2 of the phase line, and the grounding wire and neutral wire of the lighting system, whether three item five wire or single item three
wire system, must be the same as the item line.
(7) The main line of working grounding and protective grounding is allowed to be shared, but its section shall not be less than half of the section
of phase line.
(8) 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 to be grounded in series in one grounding wire.
(9) The section of bare copper grounding wire of 380V distribution box, maintenance power box and lighting power box shall be>4 mm2, the section
of bare aluminum wire shall be>6 mm2, the section of insulated copper wire shall be>2.5 mm2, and the section of insulated aluminum wire shall be>4 mm2.
(10) The distance between the grounding wire and the ground should be 250-300mm.
(11) Working grounding shall be painted on the surface with yellow and green stripes, protective grounding shall be painted on the surface with black,
and neutral line of equipment shall be painted with light blue mark.
(12) It is not allowed to use the metal sheath or metal mesh of snakeskin pipe, pipe insulation layer and cable metal sheath as the grounding wire.
(13) When the ground wire is welded, the lap welding shall be used for welding the ground wire. The lap length must meet the requirements that the flat
steel is 2 times its width (and at least 3 edges are welded), and the round steel is 6 times its diameter (and double-sided welding is required). When the
round steel is connected with the flat iron, the lap welding length is 6 times of the round steel (and double-sided welding is required).
(14) Copper and aluminum wires must be crimped with fixing screws to connect with the grounding bar, and shall not be twisted. When flat copper
flexible wires are used as grounding wires, the length shall be appropriate, and the crimping lug shall be connected with the grounding screw.
(15) During the operation of the equipment, the operator shall check that the grounding wire of the electrical equipment is well connected with the
grounding grid and electrical equipment, and there is no breakage that reduces the section of the grounding wire, otherwise it will be treated as a defect.
(16) During the acceptance of equipment maintenance, it is necessary to check that the grounding wire of electrical equipment is in good condition.
(17) The Equipment Department shall regularly check the grounding of electrical equipment, and timely notify the rectification in case of any problem.
(18) The grounding resistance of electrical equipment shall be monitored according to the provisions of the cycle or during major and minor maintenance
of the equipment. If problems are found, the causes shall be analyzed and handled in a timely manner.
(19) The grounding of high-voltage electrical equipment and the grounding resistance of the grounding grid shall be conducted by the Equipment
Department in accordance with the Code for Handover and Preventive Test of Electric Equipment, and the grounding of low-voltage electrical equipment
shall be conducted by the department under the jurisdiction of the equipment.
(20) The incoming short circuit current of the grounding device adopts the maximum symmetrical component of the maximum short circuit current
flowing into the ground through the grounding device in case of internal and external short circuit of the grounding device. The current shall be determined
according to the maximum operation mode of the system after 5 to 10 years of development, and the short circuit current distribution between the
grounding neutral points in the system and the separated grounding short circuit current in the lightning conductor shall be considered.
The following equipment must be grounded
(1) Secondary coil of current transformer.
(2) Enclosures of distribution boards and control panels.
(3) The enclosure of the motor.
(4) The shell of the cable joint box and the metal sheath of the cable.
(5) The metal base or housing of the switch and its transmission device.
(6) Metal base of high-voltage insulator and bushing.
(7) Metal pipes for indoor and outdoor wiring.
(8) Metering meter grounding terminal.
(9) Enclosures for electrical and lighting equipment.
(10) Metal frame of indoor and outdoor power distribution equipment and metal barrier of live parts.
Relevant requirements for motor grounding
(1) The motor grounding wire should be connected with the grounding grid of the whole plant by flat iron. If it is far from the grounding main
line or the flat iron grounding wire is arranged to affect the beauty of the environment, the natural grounding body should be used as far as
possible, or flat copper wire should be used as the grounding wire.
(2) For motors with grounding screws on the shell, the grounding wire must be connected with the grounding screw.
(3) For motors without grounding screws on the shell, it is required to install grounding screws at appropriate positions on the motor shell to
connect with the grounding wire.
(4) The motor shell with reliable electrical contact with the grounded base may not be grounded, and the grounding wire shall be arranged
neatly and beautifully.
Relevant requirements for switchboard grounding
(1) The grounding wire of the distribution board should be connected with the grounding grid of the whole plant by flat iron. If it is far from
the grounding main line or the flat iron grounding wire layout affects the beauty of the environment, the natural grounding body should be
used as far as possible, or soft copper wire should be used as the grounding wire.
(2) When bare copper conductor is used as grounding wire of low-voltage switchboard, the section shall not be less than 6mm2, and when
insulated copper wire is used, the section shall not be less than 4mm2.
(3) For the distribution board with grounding screw on the shell, the grounding wire must be connected with the grounding screw.
(4) For the distribution board without grounding screw on the shell, it is required to install grounding screw at the proper position of the
distribution board shell to connect with the grounding phase line.
(5) The shell of the distribution board with reliable electrical contact with the grounding body can be ungrounded.
Inspection and measurement method of grounding wire
(1) Before the test, a sufficient safety distance shall be kept from the tested equipment to prevent accidental contact with live and rotating parts,
and the test shall be carried out by two people.
(2) Before the test, select the resistance gear of the multimeter, short the two probes of the multimeter, and the resistance gear of the calibration
meter indicates 0.
(3) Connect one end of the probe to the ground wire and the other end to the special terminal for equipment grounding.
(4) When the tested equipment does not have a special grounding terminal, the other end of the probe shall be measured on the enclosure or
metal component of the electrical equipment.
(5) The main grounding grid or the reliable connection with the main grounding grid must be selected as the grounding terminal, and the
surface oxide must be removed to ensure good contact.
(6) The value shall be read after the meter indication is stable, and the grounding resistance value shall comply with the regulations.
Post time: Oct-10-2022