Measurement of resistance of grounding devices

What is grounding

Along with insulation, grounding is the most important means of protection against electric shock, which determines electrical safety. At first glance, it may seem strange in the literal sense of the word "bury money in the ground." But when it comes to human health and life, then any costs to prevent an accident or mitigate its consequences will be justified! For this, working grounding, lightning protection grounding and protective grounding are used.

Service ground is the deliberate connection to earth of specific points in an electrical circuit (for example, neutral points of windings of generators, power and instrument transformers, and when using earth as a return conductor). Working grounding is designed to ensure the proper operation of electrical installations in normal and emergency conditions and is carried out directly or through special devices (breakdown fuses, arresters, resistors).

Lightning protection grounding is a deliberate connection to the ground of arresters and lightning rods in order to divert lightning currents from them to the ground.

Protective grounding is grounding performed for the purpose of electrical safety (according to, hereinafter - PUE), i.e. deliberate connection to the ground of non-conductive metal parts that may be energized and designed to protect people from electric shock if accidentally touched. In addition, grounding devices perform other safety-related functions: they remove static electricity at explosive and fire hazardous facilities (for example, at a gas station). Dangerous voltage on any current conducting surface can be due to various reasons: static electricity charges, potential carry-over, lightning discharge, induced voltage, etc.

In practice, the most common occurrence is an accidental short-circuit of a phase to the case due to mechanical damage to current-carrying conductors or cable insulation failure. Touching the body of such a faulty installation is actually a single-phase touch mode, although in this case a person does not violate safety rules. The voltage under which a person will find himself touching the body in Figure 1 at low values ​​of the line capacitance is determined by the formula U pr = I h ∙ R h... With equal insulation resistances of the phase wires, flowing through the body R h= 1kOhm current, will be determined by the state of insulation relative to earth I h = 3U ph / (3R h + R iso).

Rice. 1. Shock by current when a phase is closed on a case isolated from earth

measurement of resistance of conductors of connection to earth and potential equalization (metal connection) (2p); measurement of resistance of grounding devices according to a three-pole circuit (3p); measurement of resistance of grounding devices according to the four-pole scheme (4p); measuring the resistance of multiple grounding devices without breaking the grounding circuit (using a current clamp); measurement of resistance of grounding devices by the method of two clamps; measurement of resistance of lightning protection (lightning rods) according to the four-pole circuit by the impulse method; measurement of alternating current (leakage current); measurement of soil resistivity by the Wenner method with the possibility of choosing the distance between the measuring electrodes; high noise immunity;

In such a situation, protective grounding in Figure 2 will reduce the touch voltage to a safe one by reducing the potential of the electrical installation housing and equalizing the potential of the base on which the person stands, to a value close to the potential of the grounded installation U corp = U z = I z ∙ r z. The grounding resistance r z is about 100 times less than the resistance of the human body, so the touch voltage will be low.

Grounding provides safety in situations where the earth fault current is not sufficient to trip the circuit breaker, and therefore is the main form of protection against electric shock in power supply systems with isolated neutral of a transformer or generator. In a network with a solidly grounded neutral in Figure 3, the earth fault current I z = U f / (r 0 + r z) is determined only by the ratio of the grounding resistances r 0 and r z and does not depend on the state of the insulation. If r 0 and r z are equal, the voltage on the grounded housing will be dangerous for a person U corpus = U z = 0.5 ∙ U f, which proves the ineffectiveness of grounding, in this case, grounding or an RCD is used to protect against electric shock.

The protective effect of grounding is based on several principles:

• reduction to a safe value of the potential difference between the grounded device and other conductors that have a natural ground.
• Leakage current drainage when voltage appears in the circuit of the grounded device. In a properly designed system, the appearance of a leakage current leads to the immediate operation of the residual current device (RCD) and de-energizes the network section. Maximum permissible shutdown time by GOST R IEC 60755-2012 is 0.3 s (0.5 s for selective), but in reality modern high-quality RCDs have a speed of about 20-30 ms.
• in systems with a solidly grounded neutral - initiation of the circuit breaker operation when the phase hits the grounded surface. The maximum permissible time of protective automatic shutdown in such a system according to clause PUE is, respectively, 0.4 / 0.2 s for voltages of 220/380 V.

In electrical engineering, the concepts of natural and artificial grounding are distinguished.

TO natural grounding includes conductive structures permanently buried in the ground, such as water pipes. Since their resistance is not standardized, such natural grounding structures cannot be used as the grounding of an electrical installation. When a dangerous potential appears on a water pipe, the life of an unlimited number of people is threatened. Therefore, the PUE clause prohibits the use of ordinary communications or engineering systems as PE conductors. To ensure guaranteed safety conditions in buildings and structures, a potential equalization system is used, which provides for the electrical connection of all metal structures and a zero protective conductor.

Artificial grounding Is the deliberate electrical connection of any point in the electrical network, electrical installation or equipment with a grounding device. The grounding device consists of a grounding conductor (a conductive part or a set of interconnected conductive parts that are in electrical contact with the ground directly or through an intermediate conductive medium) and a grounding conductor connecting the part to be grounded to the grounding conductor. The construction of grounding can be very diverse: from a simple metal rod to a complex set of elements of a special shape (Fig. 4).

Rice. 4. Grounding design: a) pin, b) loop, c) multi-element

The quality of grounding is determined by the value of the resistance to current spreading through the ground (the lower the better), which can be reduced by increasing the area of ​​the grounding electrodes and reducing the electrical resistivity of the soil, for example, by increasing the number of grounding electrodes or their depth.

The grounding system should be subject to periodic checks during operation so that corrosion or changes in soil resistivity cannot significantly affect its parameters. The grounding device may not show its malfunction for a long time until a dangerous situation arises.

In the Russian Federation, the requirements for grounding and its structure are described in chapter 1.7 of the PUE. The highest permissible values ​​of the resistance of grounding devices for various conditions are indicated in the EIC table and in table 36 of the Rules for the technical operation of consumer electrical installations (hereinafter - PTEEP), and the frequency of measurements is given in table 26 of Appendix 3 of PTEEP. The resistance of the ground electrode must not exceed the specified value at any time of the year.

According to clause 1.17.118 of the PUE, the identification mark is placed at the points of entry of grounding conductors into buildings. The dimensions and type of the "Grounding" sign are set in GOST 21130-75 "Grounding clamps and grounding signs. Design and dimensions ".

Rice. 5. Sign "Grounding"

Grounding systems

For electrical installations with voltage up to 1 kV, in accordance with the following types of grounding of AC and DC systems are used:

The first letter indicates the state of the power supply neutral to ground:

• T - grounded neutral (lat. Terra);
• I - isolated neutral (English Isolation).

The second letter indicates the state of exposed conductive parts relative to earth:

• T - open conductive parts are grounded, regardless of the relation to the ground of the neutral of the power source or any point of the supply network;
• N - open conductive parts are connected to the dead-grounded neutral of the power supply.

Subsequent letters after N denote the combination in one conductor or separation of functions for the zero working and zero protective conductors:

• S - zero working N and protective PE conductors are separated (English Separated);
• С - functions of zero protective and zero working conductors are combined in one PEN-conductor (Combined);
• N - zero working (neutral) conductor (eng. Neutral);
• PE - protective conductor (zero protective or grounding conductor, protective conductor of the equipotential bonding system) (English Protective Earth);
• PEN - combined zero protective and zero working conductors (English Protective Earth and Neutral).

Theory of measuring grounding and soil resistivity

The resistance of a single-element ground electrode system is influenced by several factors:

• the resistance of the metal of the ground electrode and the resistance of the contact of the conductor with the pin. An artificial ground electrode is made of copper, black or galvanized steel (PUE item) and a connecting conductor of the appropriate size and section is used (Table 1.7.4 PUE), therefore, if there is reliable contact with the grounding conductor, the value of these resistances can be neglected;
• the resistance of the contact of the pin with the ground. If the pin is firmly driven into the ground to a sufficient depth and does not have traces of paint, oil and significant corrosion on its underground surface, then the resistance of contact with the ground can also be ignored;
• resistance of the earth (soil). Imagine the earthing rod in Fig. 11 in the form of an electrode surrounded by concentric layers of soil of the same thickness.

The layer adjacent to the electrode has the smallest surface, but the greatest resistance. As the distance from the electrode increases, the layer surface increases, and its resistance decreases. The contribution of the resistance of the removed layers to the total soil resistance quickly becomes negligible. The area beyond which the resistance of the earth layers can be neglected is called the effective resistance area. Its size depends on the depth of immersion of the electrode in the ground. When calculating the resistance of the earth, the resistivity of the soil is considered unchanged. The grounding resistance for the case of one electrode is determined by the Dwight formula:

R = ρ / 2πL ∙ ((ln4L) -1) / r

where R is the grounding resistance, Ohm.
L is the depth of immersion of the electrode under the ground, m.
r is the radius of the electrode, m.
ρ is the average soil resistivity in Ohm.m.

An analysis of Dwight's formula shows that an increase in the pin diameter decreases the grounding resistance insignificantly, in particular, doubling the diameter reduces the resistance by less than 10%. The depth of the electrode has a much stronger effect. In theory, doubling the depth reduces the grounding resistance by 40%. The main factor that ultimately determines the grounding resistance and the depth of grounding of the pin required to provide a given resistance is soil resistivity. To a large extent, it depends on the content of electrically conductive minerals and electrolytes in the soil, i.e. water with salts dissolved in it. Soil resistivity varies greatly depending on the region of the world and the season. Dry desert soil or permafrost has high resistance.

Due to the dependence of soil resistivity on temperature and moisture content, the resistance of the grounding device also changes throughout the year. Since the temperature stability and moisture content in the soil increases with distance from the surface, the grounding system will be effective all year round if the ground electrode is placed at a considerable depth exceeding the maximum freezing depth.

The need to measure soil resistivity and grounding device resistance arises already at the design and installation stage. To measure the grounding resistance, special devices are used that use the principle of a potential drop created by an alternating current flowing between the auxiliary and the tested electrode.

The three-pole or three-wire (3p) resistance measurement circuit in Fig. 12 is the main one and consists in installing two measuring electrodes (current electrode H and voltage (potential) electrode S) in the ground near the grounding device (E) according to a single-beam circuit. The voltage electrode (S) is placed on the same line between the tested grounding device (E) and the current electrode (H) in the region of zero potential. For accurate measurement, it is necessary that the potential at the auxiliary voltage electrode is measured outside the effective resistance zones of both the grounding device and the auxiliary current electrode. The zero potential region also expands with increasing distance between the measured ground and the auxiliary current electrode. In practice, the 62% method is used, which provides the highest accuracy, provided the soil is homogeneous. Using this method, you can easily find the place of installation of the auxiliary voltage electrode (point of zero potential), when the electrodes are located along a straight line.

The device measures the amount of current flowing in the created circuit and the voltage between the investigated ground electrode and the voltage electrode. The result of the measurement is the value of the resistance of the grounding device calculated according to Ohm's law. In urban environments, it is difficult to find a place to install the two auxiliary electrodes at the required distance. But with a well-developed infrastructure, next to the measured ground electrode (N) there may be another ground (M) with a known resistance, Fig. 14. In this case, a two-point measurement method (2p) is used, which shows the resistance of two earthing devices in series. Therefore, the second ground must be so good that its resistance can be neglected. In addition, it is necessary to additionally determine the resistance of the test leads and subtract it from the obtained result. This simplified method is used as an alternative method and is not as accurate as the standard 3-wire (62% method) as it is highly dependent on the distance between the measured and the auxiliary ground.

In the case when extremely high measurement accuracy is required, a four-pole or four-wire (4p) circuit is used, which excludes the influence of the resistance of the test leads.

All of the above methods for the time of measurement require disconnecting the investigated ground electrode system from the general grounding system (untwisting the threaded connection / dismantling the welded joint). For multi-element grounding, such a process is very laborious, therefore, in Sonel devices, it is possible to carry out measurements without disconnecting the investigated ground electrode system. With this method (3p + clamps), the current electrode (H) and the voltage electrode (S) are placed in the ground in the same way as in the classical three-pole method, but the current is measured using the clamps installed on the investigated ground electrode system. The device determines the resistance of the earthing switch, on which the current clamp is installed (calculates the resistance by the magnitude of the current through the investigated earthing switch and ignores the current flowing through the adjacent earthing switches).

After measuring the resistance values ​​of individual grounding elements R E1, R E2, R E3 ... R EN, the total resistance value R E in Figure 16 is calculated by the formula:

Measuring the resistance of grounding devices on the territory of megalopolises presents enormous difficulties. Especially in the city center, where the buildings are especially dense, it is impossible to install auxiliary electrodes due to the road surface or paving slabs. In the case of a complex grounding system, the elements of which are not connected underground, the two-clamp method is used. If the earths are connected underground, this method can only establish the continuity of the circuit. Transmitting clamps by means of electromagnetic induction excite current in the measured circuit, and additional clamps measure it. It does not matter which one is at the top, it is important to maintain a minimum distance between them (> 3cm) in order to exclude the influence of the transmitting clamp on the clamp meter.

After measurement, the device will show the value of resistance R E, which for a four-element grounding in Figure 17 can also be calculated using the formula:

As follows from the above relation, the value of R E will be the sum of the measured resistance of the ground electrode system and the result of the parallel connection of the remaining ground electrodes. Therefore, the obtained value of the grounding resistance will be slightly overestimated (additional measurement error). This is a fatal method error. Since the resulting value of the parallel connection of the remaining grounding elements will be the less, the more such grounding devices are, it is recommended to perform measurements by this method only in multi-element systems.

As follows from Dwight's formula, soil resistivity directly affects the design of grounding devices (the depth of the ground electrode at a given resistance and the number of elements). When designing large grounding systems, it is important to find areas of least ground resistance in order to design the most economical option with the minimum number of elements.

To measure soil resistivity according to the Wenner method, implemented in Sonel devices, four electrodes are used, placed linearly at equal distances, Figure 18. The soil resistivity value is automatically calculated during the measurement by the formula: ρ = 2πd ∙ U / I [Ohm ∙ m].

A characteristic feature of the Wenner method is the directly proportional dependence of the distance between the electrodes and the depth at which the current flows. The limiting value for the depth of penetration of the current into the ground is 0.7 ∙ d. By performing a series of resistivity measurements, while changing the distance between the electrodes, you can roughly estimate at what depth its smallest value is. Then the electrodes should be turned at right angles to the line on which the measurements were taken and the entire series should be repeated. If the device shows a significant scatter of results, which makes it difficult to perform measurements, then it is likely that there are underground communications in this place (water pipes, metal structures, etc.). In this case, it is necessary to rearrange the electrodes several meters away from the place where non-uniform readings were observed, and repeat the measurement of the soil resistivity. Close results indicate the homogeneity of the soil and the correctness of the measurements.

The obtained data are used for geophysical study of the underlying rocks in order to determine the zones and depth of occurrence. In addition, the rate of corrosion of underground pipelines can be estimated from the value of soil resistance. A significant reduction in soil resistance leads to an intensification of the corrosion process and requires a special protective treatment of underground metal surfaces.

Conclusions:

1. Measurement of the resistance of the grounding device is carried out in the dry period of the year.
2. Salts and minerals dissolved in water give the soil the properties of an electrolyte, therefore it is necessary to use alternating current to measure the grounding resistance.
3. To avoid the influence of power-frequency currents and their higher harmonics, use a frequency of the measuring voltage that is not a multiple of 50 Hz (60 Hz).
4. The best grounding accuracy is provided by the 4p circuit using the 62% method.
5. Measurement of resistance using two clamps has a methodical error, therefore it is recommended to use it only in multi-element grounding systems.
6. Wenner's method allows you to quickly and easily measure soil resistivity.

Lightning protection

In the earthing systems discussed above, which are primarily intended for protection against electric shock, the behavior of low frequency currents is important.

The task of lightning protection grounding is to divert a lightning strike into the ground. The impulsive nature of this discharge determines the significant effect of the inductive component of the grounding, therefore, only the part of the grounding located in the immediate vicinity of the discharge site is effectively used to drain the lightning current. Grounding with low static resistance, which guarantees good basic protection, will not provide sufficient lightning protection parameters - especially in the case of large grounding systems, which, having low static resistance, can have many times the dynamic impedance. In the Russian Federation, at present, in addition to regulatory documents establishing requirements for lightning protection of buildings: "Instructions for lightning protection of buildings and structures" RD 34.21.122-87 and "Instructions for arranging lightning protection of buildings, structures and industrial communications" CO 153-343.21.122- 2003, in 2011 the first two parts of GOST R IEC 62305-2-2010 “Risk Management. Lightning protection ”, which are translations of the IEC 62305 standard, which consists of four parts. Unfortunately, none of these instructions covers the issue of practical application of protection devices against lightning and switching overvoltages.

Bibliography:

Electrical Code, Edition 7.
Rules for the technical operation of electrical installations of consumers, introduced since 2003.
GOST R IEC 61557-5-2008 “Electrical safety. Apparatus for testing, measuring or controlling protective equipment. Part 5. Resistance of the ground electrode to the ground "
GOST R 50571.1-2009 Low-voltage electrical installations, part 1 "Basic provisions, assessment of general characteristics, terms and definitions".
GOST R IEC 60755-2012 "General requirements for protective devices controlled by differential (residual) current".
GOST R IEC 62305-2-2010 “Risk management. Lightning protection ", part 1 and part 2
"Instructions for lightning protection of buildings and structures" RD 34.21.122-87.
"Instructions for the device of lightning protection of buildings, structures and industrial communications" CO 153-343.21.122-2003.
A.V. Sakara. "Organizational and methodological recommendations for testing electrical equipment and devices for electrical installations of consumers" Moscow, JSC "Energoservice", 2004.