Grounding resistance is of course the smaller the better, according to the different requirements of the equipment, the standard is 4 - 10 ohms, the maximum can not be greater than 10 ohms, 4 ohms below the better, but generally difficult to do.
Standard grounding resistance specification requirements:
1, independent lightning protection grounding resistance should be less than or equal to 10 ohms;?
2, independent safety protection ground resistance should be less than or equal to 4 ohms;?
3. Independent AC working grounding resistance shall be less than or equal to 4 ohms;?
4, independent DC working ground resistance should be less than or equal to 4 ohms;?
5, *** with the grounding body (joint grounding) should be no greater than the grounding resistance of 1 ohm.
Extended information:
There are many factors affecting the grounding resistance: the size of the grounding electrode (length, thickness), shape, number, depth of burial, the surrounding geographic environment (such as flat, ditch, sloping land is different), soil moisture, texture and so on. In order to ensure good grounding of the equipment, the use of meters to grounding resistance measurement is essential.
The measurement of grounding resistance can be divided into: voltage and current meter method, ratio meter method and bridge method. According to the specific measuring instrument and cloth pole number can be divided into: hand-operated ground resistance meter method, clamp-type ground resistance meter method, voltage and current meter method, three-pole method and four-pole method.
In the measurement of grounding resistance, some factors cause grounding resistance inaccuracy:
(1) the ground network around the soil composition is inconsistent, geological inconsistency, close, dry and wet degree is not the same, with the dispersion of the ground surface stray currents, in particular, the overhead ground, underground water pipes, cable sheaths and so on, the impact of the test is particularly large. Solution: take different points for measurement, take the average value.
(2) The test line direction is not right, the distance is not long enough. Solution: Find the right test direction and distance.
(3) Auxiliary grounding electrode resistance is too large. Solution: Splash water at the ground stake or use resistor to reduce the grounding resistance of the current pole.
(4) test clip and grounding measurement point contact resistance is too large. Solution: Grind the contact point with a file or sandpaper, and use the test line clamp to adequately clip the grounding contact.
(5) interference effects. Solution: adjust the release direction, try to avoid interference in the direction of the meter readings to reduce the jump.
(6) meter use problems. Battery power is insufficient, the solution: replace the battery.
(7) meter accuracy decline. Solution: Recalibrate to zero.
The accuracy of the test value of the grounding resistance is one of the important factors to determine whether the grounding is good. Once the test value is inaccurate, either a waste of labor and material resources (measured value is large), or will bring safety hazards to the grounding equipment (measured value is small).
Measuring Instruments
(1) grounding resistance measurements are sometimes carried out in the field, therefore, the measurement instrument should be strong and reliable, with power supply in the machine, light weight, small size, and have a strong adaptability to harsh environments.
(2) Greater than 20dB anti-interference ability, can prevent the soil stray current or electromagnetic induction interference.
(3) The meter should have an input impedance greater than 500kW in order to reduce the measurement error caused by the contact resistance between the auxiliary pole probe and the soil.
(4) the frequency of the measurement signal within the instrument should be between 25Hz ~ 1kHz, the measurement signal frequency is too low and too high is prone to polarization effects, or the increase in inductance between the leads of the test pole rod leads, so that the role of inductance or capacitance between the leads, resulting in a larger measurement error, i.e., cloth pole error.
(5) in the case of power consumption allows, should try to improve the test current, a larger test current is conducive to improve the anti-interference performance of the instrument.
(6) The meter should be easy to operate, the readings are preferably digital display, in order to reduce the reading error.
Reference:
Baidu Encyclopedia-Grounding Resistance