Instructions for the use of the shaker and test standards for grounding resistance
Shaking meter is also called ground resistance meter, ground resistance meter, and ground resistance tester. According to the power supply mode, the shaker is divided into traditional hand-cranked and battery-driven; according to the display mode, the shaker is divided into pointer type and digital; according to the measurement method, the shaker is divided into pile type and clamp type. At present, the traditional hand-cranked tables are rarely used. The most popular ones are the analog or digital grounding tables, and the more popular ones in power systems and telecommunication systems are the clamp-on earthing tables.
For buildings, structures, power distribution rooms, high-voltage transmission lines, etc. with lightning protection grounding devices on the construction drawings, the grounding resistance value of the grounding body shall be measured in time after the completion of the underground part of the lightning protection grounding body; when the unit project is completed A retest is also required as one of the materials for the completion of the project.
The test method with ZC29B-2 type shaking table is as follows:
(1) When measuring the grounding resistance at the two terminals E-E, short-circuit it with a chrome-plated copper plate and connect it to the 5m-long pure copper wire that comes with the meter. The other end of the wire is connected to the test point of the grounding body to be tested. on. When measuring the resistance of the shield, loosen the chrome-plated copper plate, connect one E terminal to the ground, and the other E terminal to the shield.
(2) The P-pillar is connected to the 20m pure copper wire that comes with the instrument, and the other end of the wire is connected to the pin.
(3) The C-pillar is connected to the 40m pure copper wire that comes with the instrument, and the other end of the wire is connected to pin 2.
2 Technical requirements for grounding resistance tester settings
(1) The grounding resistance tester should be placed 1～3m away from the test point, and the placement should be stable and easy to operate.
(2) The terminal of each terminal must be in good contact and the connection must be firm.
(3) The two grounding electrode pins should be set at 20m and 40m to the left and right of the grounding body to be tested; if the two pins are connected by a straight line, the grounding body to be tested should be basically on this straight line.
(4) Do not use other wires to replace the 5m, 20m, 40m pure copper wires that come with the instrument configuration.
(5) If the grounding resistance tester is the center of the circle, the angle between the two pins and the tester should not be less than 120°, and they should not be set in the same direction.
(6) The soil quality of the two-pin installation must be solid and cannot be installed in the mud, backfill, beside tree roots, grass, etc.
(7) The ground resistance test can only be carried out after 7 consecutive sunny days after rain.
(8) The grounding body to be tested should be derusted first to ensure reliable electrical connection.
3 The operating essentials of the ground resistance tester
(1) The measurement of the grounding resistance starts only after the tester is set up to meet the specifications.
(2) Before measurement, the grounding resistance gear knob should be turned at the maximum gear position (x10), and the grounding resistance value adjustment knob should be placed at the 6-7Ω position.
(3) Rotate the handle slowly. If the pointer of the galvanometer quickly deflects to the right from the 0 balance point in the middle, it means that the original range gear is selected too large. You can select the gear to x1. If the deflection direction is the same as before, you can The gear selection turns to x0 1 gear.
(4) After selecting step (3), slowly turn the handle, and the pointer of the galvanometer will shift from the 0 balance point to the right, indicating that the ground resistance value is still too large. While turning the handle slowly, the ground resistance knob should be clockwise slowly Turn, when the pointer of the galvanometer returns to 0, gradually increase the speed of the handle so that the speed of the handle reaches 120 rpm. At this time, the ground resistance
The indicated resistance value is multiplied by the gear multiple to measure the grounding resistance value of the grounding body. If the pointer of the galvanometer is slowly deflected to the left, it means that the resistance value of the ground resistance knob is lower than the actual ground resistance value. You can turn it counterclockwise slowly to increase the resistance value indicated by the meter.
(5) If the pointer of the galvanometer is erratic when turning the handle slowly, it means that the ground of the two grounding pins is not dense or there is a poor contact of a connector. At this time, you should recheck the ground or the ground of the two pins. Each joint.
(6) When measuring the grounding resistance of a static pressure pile with a grounding resistance measuring instrument, it is normal for the pointer of the galvanometer to swing slightly from side to side at 0 o'clock.
(7) When the pointer of the galvanometer slowly moves to the 0 balance point, the handle of the instrument generator can be accelerated, and the rated speed of the handle is 120 rpm. It is strictly prohibited to increase the rotation speed of the handle when the pointer of the galvanometer still has a large deflection.
(8) After the measuring instrument is used, the resistance value gear should be placed at the maximum position, that is, the x10 gear. Arrange the three test leads that come with the instrument configuration, clean up the dirt on the two pins, and pack them for storage.
Grounding requirements and grounding resistance standards:
The grounding of AC electrical installations should meet the following requirements:
1 When the high-voltage side of the distribution transformer works in a low-resistance grounding system, the grounding resistance of the protective grounding grid should meet the requirements of the following formula:
R≤2000/I (12.4. 1-1)
In the formula, R――The maximum grounding resistance (Ω) considering seasonal changes;
I-the short-circuit current (A) flowing through the grounding grid for calculation.
2 When the high-voltage side of the distribution transformer works in an ungrounded system, the grounding resistance of the electrical device should meet the following requirements:
1) The grounding resistance of the grounding grid shared by high-voltage and low-voltage electrical installations should meet the requirements of the following formula, and should not exceed 4Ω:
R≤120/I (12.4.1-2)
2) The grounding resistance of the grounding grid only used for high-voltage electrical installations should comply with the following
Type requirements, and should not exceed 100,:
Ruler≤250/I (12.4.1-3)
In the formula, R――The maximum grounding resistance (Ω) considering seasonal changes;
I-ground fault current (A) for calculation.
3 In the power grid where the neutral point is grounded by the arc suppression coil, when the grounding
When the resistance is calculated according to the formulas of this code <12.4.1-2) and (12.4.1-3), the ground fault current shall be determined as follows:
1) For the grounding grid of a substation or electrical device equipped with arc suppression coils, the calculated current should be 1.25 times the sum of the rated currents of the arc suppression coils in the same power grid connected to the same grounding network;
2) For substations or electrical installations without arc suppression coils, the calculated current should be the maximum possible residual current when the largest arc suppression coil is disconnected from the power network, and it should not be less than 30A.
4 In areas with high soil resistivity, when the grounding resistance of the grounding grid reaches the above specified value and the technology and economy are unreasonable, the grounding resistance of the electrical device can be increased to 30Ω, and the grounding resistance of the substation grounding grid can be increased to 15Ω. It meets the requirements of Article 12.6.1 of this code.
In the low-voltage system, the grounding resistance of the neutral point of the distribution transformer should not exceed 4Ω. In areas with high soil resistivity, when it is difficult to reach the above-mentioned grounding resistance value, a grid-type grounding grid can be used, but it should meet the requirements of Article 12.6.1 of this code.
The grounding resistance of the power distribution device should meet the following requirements:
1 When the distribution transformer that supplies power to a building is installed outside the building, the following requirements should be met:
1) For the high-voltage side of the distribution transformer working in ungrounded, arc suppression coil grounded and high resistance grounded systems, when the grounding resistance of the protective grounding grid of the transformer meets the requirements of the formula (12.4.3) and does not exceed 4Ω, The power grounding point of the low-voltage system can share the grounding grid with the protective grounding of the transformer. The grounding resistance of electrical installations should meet the following requirements:
R≤50/I (12.4.3)
In the formula, R――The maximum grounding resistance (Ω) of the grounding grid in consideration of seasonal changes;
I――Single-phase ground fault current; the arc suppression coil grounding system is the residual current at the fault point.
2) Low-voltage cables and overhead lines are introduced into buildings. For TN-S or TN-CS systems, the protective conductor (PE) or protective earth neutral conductor (PEN) should be grounded repeatedly, and the grounding resistance should not exceed 10Ω; for TT systems , The protective conductor (PE) is grounded separately, and the grounding resistance should not exceed 4Ω;
3) When the high-voltage side of the distribution transformer that supplies power to the low-voltage system is working in a low-resistance grounding system, the low-voltage system must not share the grounding grid with the protective grounding of the power distribution transformer, and the power grounding point of the low-voltage system should be at an appropriate distance from the distribution transformer. Set up a dedicated grounding grid at the site, and its grounding resistance should not exceed 4Ω.
2 When the distribution transformer that supplies power to a building is installed in the building, it shall meet the following requirements:
1) For the high-voltage side of the distribution transformer working in an ungrounded, arc-suppression coil grounded, and high-resistance grounded system, when the grounding resistance of the grounding grid of the transformer is not greater than 4Ω, the power grounding point of the low-voltage system can be connected to the transformer for protective grounding Shared grounding grid;
2) The high-voltage side of the distribution transformer works in a low-resistance grounding system. When the grounding resistance of the protective grounding grid of the transformer meets the requirements of the formula (12.4.1-1) of this code and the building adopts total equipotential bonding, The power grounding point of the low-voltage system can share the grounding grid with the protective grounding of the transformer.
The lightning arrester to protect the distribution transformer shall share the grounding grid with the protective grounding of the transformer.
The grounding conductor of the lightning arrester to protect the circuit breaker, load switch and capacitor bank on the distribution pole should be connected to the equipment shell, and the grounding resistance should not be greater than 10Ω.
In the TT system, when the system grounding point and the exposed conductive part of the electrical device have been connected with total equipotentiality, the exposed conductive part of the electrical device need not be equipped with a grounding grid; when the total equipotential connection is not performed, the exposed conductive part of the electrical device is exposed A grounding grid for protective grounding shall be provided, and its grounding resistance shall meet the requirements of the following formula.
R≤50/Ia (12.4.6-1)
In the formula, R――The maximum grounding resistance (Ω) of the grounding grid in consideration of seasonal changes;
Ia——Ensure that the protective electrical appliance cuts off the action current of the fault circuit (A).
When the residual action current protector is used, the grounding resistance should meet the requirements of the following formula:
R≤25I△n such as (12.4. 6-2)
In the formula I△n-residual operating current protector operating current (mA).
The protective grounding of the exposed conductive parts of the electrical equipment of the IT system can share the grounding grid, or use a separate grounding grid for single grounding or group grounding. Each ground
The grounding resistance of the network should meet the following requirements:
R≤50/Id (12.4.7)
In the formula, R――The maximum grounding resistance (Ω) of the grounding grid in consideration of seasonal changes;
Id-the fault current (A) of the first short-circuit fault between the phase conductor and the exposed conductive part.
The grounding of each electrical system of the building should use the same grounding grid. The grounding resistance of the grounding grid should meet the minimum requirement.
The grounding of overhead lines and cable lines should meet the following requirements:
1 In the low-voltage TN system, the PEN conductors or PE conductors at the terminals of the trunk and branch lines of the overhead line should be grounded repeatedly. Cable lines and overhead lines should be grounded repeatedly in accordance with 12.2.2 of this code at the entry point of each building. The PEN conductor after the residual current operated protector is not allowed to be repeatedly grounded. Except for the neutral point of the power supply, the neutral conductor (N) should not be grounded repeatedly.
The grounding resistance of each repeated grounding grid of the low-voltage line should not be greater than 10Ω. In the power network where the grounding resistance of electrical equipment is allowed to reach l0Ω, the grounding resistance value of each repeated grounding shall not exceed 30Ω, and the repeated grounding shall not be less than 3.
2 In residential areas with non-asphalt ground, the reinforced concrete poles of 10(6)kV high-voltage overhead distribution lines should be grounded, and the metal poles and towers should be grounded, and the grounding resistance should not exceed 30Ω. For low-voltage overhead lines and high- and low-voltage common-pole lines of the power supply neutral point directly grounded system, except for those with residual current-operated protectors at the outlet end, the iron cross arm or iron rod of the reinforced concrete pole should be connected with the PEN conductor, and the steel bar The reinforcement of the concrete pole should be connected with the PEN conductor.
3 The metal sheaths at both ends of the power cables laid through metal conduits should be grounded, and the metal sheaths of the power cables in the substation can be grounded by the main grounding grid. When using all-plastic cables, 1-2 grounding conductors grounded at both ends should be laid along the cable trench.
The following are the standard requirements for grounding resistance:
1. The grounding resistance of independent lightning protection should be less than or equal to 10 ohms;
2. The grounding resistance of independent safety protection should be less than or equal to 4 ohms;
3. The grounding resistance of independent AC work should be less than or equal to 4 ohms;
4. The independent DC working grounding resistance should be less than or equal to 4 ohms;
5. Anti-static grounding resistance is generally required to be less than or equal to 100 ohms.
6 The common grounding body (joint grounding) should not be greater than 1 ohm grounding resistance