Earth resistance measurement: a review of practical measurement methods
Grounding is used in the implementation of various projects of electrical systems. The very concept of "grounding" is schematically considered by connecting a portion of an electrical circuit to the earth's potential.
The ground loop contains a conductor and an electrode embedded deep in the ground. The traditional action in electrical practice is to measure the grounding resistance of networks that are still being launched and are already in operation. We will describe how and how this important action is performed.
The content of the article:
- What are measurements necessary for?
- Overview of measuring methods
- General guidelines for measuring USG
- Conclusions and useful video on the topic
What are measurements necessary for?
A brilliant solution to the following tasks is achieved by perfect zero resistance in the grounding circuit:
- Prevent voltage on the case of technological machines.
- To achieve an effective reference potential of electrical equipment.
- Completely eliminate static currents.
True electrical engineering experience shows: it is impossible to get a result at a perfect zero.
In any case, a grounded electrode produces some kind of resistance.
The specific value of resistance is determined by:
- electrode resistance at the point of contact with the conductive bus;
- the contact area between the earth electrode and the ground;
- soil structure giving different resistance.
The practice of measuring the resistance of the ground loop indicates that the first two factors can be neglected, but subject to logical conditions:
- The ground electrode is made of highly conductive metal.
- The body of the electrode pin is carefully cleaned and firmly planted in the ground.
The third factor remains - the resistive surface of the soil. It is seen as the main design part for measuring the resistance of the ground loop.
It is calculated thanks to the formula:
R = pL / A,
where: p is the soil resistivity, L is the conditional deepening, A is the working area.
To protect the owners of the house / apartment, all types of powerful home electrical equipment must be equipped with grounding:
When testing resistance, each of the grounding lines is checked separately. The resistance between the grounding element and each non-conductive part of the electrical equipment, which may be under voltage, should be less than 0.1 ohms.
Overview of measuring methods
There are several options for measuring resistance ground loop, each of which quite accurately allows you to determine the desired value.
3-point detection system
So, for example, a 3-point circuit technique is often applied based on the effect of a potential drop.
Measurements are carried out in three main steps:
- Measurement of voltage at the electrode E1 and probe E2.
- Measurement of current strength on the electrode E1 and probe E3.
- Calculation (by the formula R = E / I) of the resistance of the grounding electrode.
For this technique, the accuracy of measurements is logically dependent on the installation location of the E3 probe. It is recommended to introduce it into the soil at a distance - optimally outside the so-called area of ESE (effective resistance of electrodes) E1 and E2.
Measurements on the technology "62%"
If the soil structure for the placement of the grounding electrode differs in homogeneous content, the “62%” method for determining the resistance of ground loops promises good performance.
The method is applicable to circuits with a single grounding electrode. The accuracy of the testimony here is due to the possibility of the location of the working probes in a straight section, relative to the grounding electrode.
Control probe installation points
|Electrode deepening, m||Distance to probe E1, m||Distance to probe E2, m|
Simplified point-to-point method
The use of this method of measurement requires the presence of another good grounding in addition to the one that will be examined. The technique is relevant for densely populated areas, where it is often not possible to widely operate auxiliary working electrodes.
The point-to-point measurement method is different in that it simultaneously shows the result for two grounding devices connected in series. This explains the requirements for high quality performance of the second grounding, so as not to take into account its resistance.
To perform the calculations, the resistance of the ground bus is also measured. The result obtained is subtracted from the results of general measurements.
The accuracy of this method leaves much to be desired in comparison with the two above. Here, the distance between the ground electrode plays an important role, the resistance of which is measured by the second ground. By default, this technique is not applied. This is a kind of alternative when you cannot use other measurement methods.
Accurate four-point measurement
For most resistance measurement options, the 4-point technology is considered to be the most optimal way, in addition to 2-and 3-point ones. Such measurement technology is endowed with instruments similar to the tester of the 4500 series. Judging by the name of the method, four working electrodes are placed in one line and at equal distances on the working platform.
The current generator of the device is connected to the extreme electrodes, as a result of which a current flows between them, the value of which is known. At the other terminals of the device, two internal working electrodes are connected.
A voltage drop value is present on these terminals. The final measurement result is the ground resistance (in Ohms), the value of which the device displays on the display.
4500 Series instruments are often used to measure touch voltage. Using a special module, the device generates a small voltage in the ground - an imitation of cable damage.
At the same time, the current flowing through the ground circuit is indicated on the scale of the device. The readings on the screen are taken as a basis and multiplied by the estimated value of the current in the ground. In this way, the touch voltage is calculated.
For example, the maximum value of the expected current in the fault area is 4000A. A value of 0.100 is indicated on the screen of the device. Then the magnitude of the touch voltage will be 400V (4000 * 0.100).
Measurement with the instrument S.A6415 (6410, 6412, 6415)
The uniqueness of this method is the ability to take measurements without disconnecting the grounding circuit. It should also be emphasized that it is permissible to measure the total resistance of the grounding device by including the resistive component of all the connections in the grounding circuit.
The principle of operation is approximately as follows:
- A special transformer in the circuit creates current.
- Current flows in an educated circuit.
- Using a synchronous detector, the measured signal is recorded.
- The received signal is converted by the ADC.
- The result is displayed on the LCD.
The device is equipped with a module (selective amplifier), thanks to which the useful signal is effectively cleaned of various kinds of interference - n.ch. and h. noise. The paws of ticks in their articulated state form an excited circuit, covering the ground conductor.
Instructions for measuring with the instrument S.A6415
The sequence of actions when working with the device of the C.A6415 series is intelligibly described in the instructions attached to this unique device.
For example, there is a need to measure the grounding resistance of an electrical module (transformer, electric meter, etc.).
- Open the grounding bus by removing the protective cover.
- Grab the ground conductor (bus or directly electrode) with the pliers.
- Select measuring mode “A” (current measurement).
The maximum current value of the device is 30A, therefore, if this figure is exceeded, measurement cannot be performed. Remove the instrument and try again at another point.
When the current value obtained on the scale falls within the permissible range, you can continue to work by switching the device to the resistance measurement "?".
The result displayed on the display will show the total resistance value, including:
- electrode and ground bus;
- neutral contact with the ground electrode;
- contact of the connections on the line between the neutral and the ground electrode.
When working with pincers, it should be borne in mind: the overestimated readings of the device regarding grounding resistance are, as a rule, due to poor contact of the grounding electrode with the ground.
Also, a torn current-carrying bus may be the cause of high resistance. High resistance figures at the connection points (splices) of the conductors can also affect the readings of the device.
General guidelines for measuring USG
Before ground circuit, for example, for a gas boiler, it is necessary to obtain accurate information about in which area the ground electrode will be laid. Often, it is proposed to refer to the existing tables to determine the “p” values of the soil.
However, this option with tables provides purely indicative data. Therefore, you should not rely on them. The true values of soil resistance may vary significantly.
Option # 1: single layer primer
If the soil has a homogeneous component, its resistivity is measured by the method of "test electrode".
The method involves performing a certain procedure in two stages:
- Take a rod control probe with a length slightly greater than the depth of the design tab.
- Immerse the probe in the ground strictly vertically to the depth of the project bookmark.
- The end remaining above the surface of the earth is used to measure the spreading resistance (Rr).
- USG is determined by the formula p = Rr * Ψ.
It is advisable to perform the procedure several times at various points on the job site. Alternative measurements help to achieve accurate soil resistance measurements.
Option # 2: multi-layer soil
For such a situation, the USG is measured by the method of stepwise sensing. That is, the control probe is immersed to the working depth in steps and in the position of each step, resistivity measurements are performed. Calculations of average USG are made using formulas for each individual measurement.
Then, based on the climatic characteristics of the area, they find the values for seasonal changes. In this way (quite complicated), the calculated values of the UGS of the upper layers are obtained.The underlying layers are considered as not subject to seasonal changes and therefore the calculation for them is limited to a somewhat simplified measurement and calculation.
Work of this kind, of course, is carried out by qualified personnel representing specialized organizations. So, utilities are usually responsible for the operation of power panels in residential buildings. It is permitted to take any measurements at these points only through access to these services.
Electrical circuits are dangerous systems. Despite the fact that household communications are designed for voltages of less than 1000V, this voltage is fatal to humans. All necessary safety precautions must be observed when handling electrical equipment. The layman often such measures are simply unknown.
With the features of the grounding construction for the bath in a city apartment will introduce next articlecontaining the rules and guidelines for the work.
Conclusions and useful video on the topic
Taking measurements in practice using the instrument:
The execution of work related to the verification of grounding resistance is required, regardless of the complexity of the electrical circuit and the category of the facility where electrical equipment is installed or installed and operated. Many specialized organizations are ready to provide such services.
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