One of the most important reasons for calculating grounding and installation is that it protects people, appliances in the house from overvoltage. If suddenly lightning strikes a house or for some reason there is a power surge in the network, but at the same time the electrical system is grounded, all this excess electricity will go into the ground, otherwise there will be an explosion that can destroy everything in its path.
Electrical protection equipment
Growth in electricity consumption in all areas of life, at home and at work, requires clear safety rules for human life. Numerous national and international standards govern the requirements for the construction of electrical systems to ensure the safety of people, pets and property when using electrical appliances.
Electrical protection equipment installed during the construction of residential and public buildings must be checked regularly to ensure reliable operation for many years. Violations of safety rules in electrical systems can have negative consequences: threat to life of people, destruction of property orwiring destruction.
Safety regulations set the following upper limits for safe human contact with live surfaces: 36 VAC in dry buildings and 12 VAC in wet areas.
Grounding system
Earthing system is an absolutely essential technical equipment for every building, so it is the first electrical installation component to be installed in a new facility. The term grounding is used in electrical engineering to purposefully connect electrical components to earth.
Protective grounding protects people from electric shock when touching electrical equipment in the event of a malfunction. Masts, fences, utilities such as water pipes or gas pipelines must be connected with a protective cable by connecting to a terminal or grounding bar.
Problems of functional protection
Functional grounding does not provide safety as the name suggests, instead it creates uninterrupted operation of electrical systems and equipment. Functional grounding dissipates currents and noise sources to earth test adapters, antennas, and other devices that receive radio waves.
They determine the common reference potentials between electrical equipment and devices and thus prevent various malfunctions in private homes, such as TV or light flickering. Functional grounding can never perform protective tasks.
All requirements for protection against electric shock can be found in national standards. Establishing a protective earth is vital and therefore always takes precedence over functional.
Ultimate resistance of protective devices
In a system that is safe for people, protective devices must operate as soon as the fault voltage in the system reaches a value that can be dangerous to them. To calculate this parameter, you can use the above voltage limit data, choose the average value U=25 VAC.
Residual current circuit breakers installed in residential areas will not normally trip to earth until the short circuit current reaches 500 mA. Therefore, according to Ohm's law, with U=R1 R=25 V / 0.5 A=50 ohms. Therefore, in order to adequately protect the safety of people and property, the earth must have a resistance of less than 50 ohms, or R earth<50.
Electrode reliability factors
According to state standards, the following elements can be considered as electrodes:
- vertically inserted steel piles or pipes;
- horizontally laid steel strips or wires;
- recessed metal plates;
- metal rings placed around foundations or embedded in foundations.
Water pipes and other underground steel engineering networks (if there is agreement with the owners).
Reliable grounding with resistance less than 50 ohms depends on three factors:
- Land view.
- Type and soil resistance.
- Ground line resistance.
The calculation of the grounding device must begin with the determination of the resistivity of the soil. It depends on the shape of the electrodes. Earth resistivity r (Greek letter Rho) is expressed in ohm meters. This corresponds to the theoretical resistance of a 1 m grounding cylinder2, whose cross section and height are 1 m. gets higher). Soil resistivity examples in Ohm-m:
- marshy soil from 1 to 30;
- loess soil from 20 to 100;
- humus from 10 to 150;
- quartz sand from 200 to 3000;
- soft limestone from 1500 to 3000;
- grassy soil from 100 to 300;
- rocky land without vegetation - 5.
Installation of grounding device
The ground loop is mounted from a structure consisting of steel electrodes and connecting strips. After immersion in the ground, the device is connected to the house electrical panel with a wire or a similar metal strip. Soil moisture affects the placement level of the structure.
There is an inverse relationship between rebar length and groundwater level. The maximum distance from the construction site ranges from 1 m to 10 m. Electrodes for grounding calculation should enter the ground below the soil freezing line. For cottages, the circuit is mounted using metal products: pipes, smooth reinforcement, steel angle, I-beam.
Their shape must be adapted for deep entry into the ground, the cross-sectional area of the reinforcement is more than 1.5 cm2. The reinforcement is placed in a row or in the form of various shapes, which directly depend on the actual location of the site and the possibility of mounting a protective device. The scheme around the perimeter of the object is often used, however, the triangular grounding model is still the most common.
Despite the fact that the protective system can be made independently using the available material, many homebuilders purchase factory kits. Although they are not cheap, they are easy to install and durable in use. Typically, such a set consists of copper-plated electrodes 1 m long, equipped with a threaded connection for mounting.
Total streak calculation
There is no general rule for calculating the exact number of holes and dimensions of the ground strip, but the discharge of the leakage current is definitely dependent on the cross-sectional area of the material, so for any equipment, the size of the ground strip is calculated on the current that will be carried by this strip.
To calculate the ground loop, the leakage current is first calculated and the strip size is determined.
For most electrical equipment such as transformer,diesel generator, etc., the size of the neutral ground strip must be such that it can handle the neutral current of this equipment.
For example, for a 100kVA transformer, the total load current is about 140A.
The connected strip must be able to carry at least 70A (neutral current), which means a 25x3mm strip is sufficient to carry the current.
A smaller strip is used to ground the case, which can carry a current of 35 A, provided that 2 earth pits are used for each object as a backup protection. If one strip becomes unusable due to corrosion, which breaks the integrity of the circuit, the leakage current flows through the other system, providing protection.
Calculation of the number of protection pipes
The grounding resistance of a single electrode rod or tube is calculated according to:
R=ρ / 2 × 3, 14 × L (log (8xL / d) -1)
Where:
ρ=Ground resistance (ohmmeter), L=Electrode length (meter), D=Electrode diameter (meter).
Ground calculation (example):
Calculate the resistance of the ground insulating rod. It has a length of 4 meters and a diameter of 12.2 mm, a specific gravity of 500 ohms.
R=500 / (2 × 3, 14 × 4) x (Log (8 × 4 / 0, 0125) -1)=156, 19 Ω.
The grounding resistance of a single rod or tube electrode is calculated as follows:
R=100xρ / 2 × 3, 14 × L (log (4xL / d))
Where:
ρ=Ground resistance (ohmmeter), L=Electrode length (cm), D=Electrode diameter (cm).
Definitiongrounding structure
Calculation of the grounding of an electrical installation begins with determining the number of grounding pipes with a diameter of 100 mm, 3 meters long. The system has a fault current of 50 KA for 1 second and a ground resistivity of 72.44 ohms.
Current density at the surface of the earth electrode:
Poppy. allowable current density I=7.57 × 1000 / (√ρxt) A / m2
Poppy. allowable current density=7.57 × 1000 / (√72.44X1)=889.419 A / m2
The surface area of one diameter is 100 mm. 3m pipe=2 x 3, 14 L=2 x 3, 14 x 0.05 x 3=0.942 m2
Poppy. current dissipated by one ground pipe=Current density x Electrode surface area.
Max. current dissipated by one grounding pipe=889.419x 0.942=838A, Number of earth pipe required=Fault current / Max.
Number of ground pipe required=50000/838=60 pieces.
Earth pipe resistance (insulated) R=100xρ / 2 × 3, 14xLx (log (4XL / d))
Ground pipe resistance (insulated) R=100 × 72.44 / 2 × 3 × 14 × 300 × (log (4X300 / 10))=7.99 Ω / Pipe
Total resistance of 60 pieces of ground=7.99 / 60=0.133 Ohm.
Ground strip resistance
Ground strip resistance (R):
R=ρ / 2 × 3, 14xLx (log (2xLxL / wt))
An example of loop grounding calculation is given below.
Calculate a strip 12 mm wide, 2200 meters long,buried in the ground at a depth of 200 mm, the soil resistivity is 72.44 ohms.
Ground strip resistance (Re)=72, 44 / 2 × 3, 14x2200x (log (2x2200x2200 /.2x.012))=0, 050 Ω
From the above total resistance of 60 pieces of grounding pipes (Rp)=0.133 ohms. And this is due to the rough ground strip. Here net earth resistance=(RpxRe) / (Rp + Re)
Net resistance=(0.133 × 0.05) / (0.133 + 0.05)=0.036 Ohm
Ground impedance and number of electrodes per group (parallel connection). In cases where one electrode is insufficient to provide the required earth resistance, more than one electrode must be used. The separation of the electrodes should be about 4 m. The combined resistance of the parallel electrodes is a complex function of several factors such as the number and configuration of the electrode. Total resistance of a group of electrodes in various configurations according to:
Ra=R (1 + λa / n), where a=ρ / 2X3.14xRxS
Where: S=Distance between adjusting stem (meter).
λ=Factor shown in the table below.
n=Number of electrodes.
ρ=Ground resistance (Ohmmeter).
R=Resistance of a single rod in insulation (Ω).
| Factors for parallel electrodes in line | |
| Number of electrodes (n) | Factor (λ) |
| 2 | 1, 0 |
| 3 | 1, 66 |
| 4 | 2, 15 |
| 5 | 2, 54 |
| 6 | 2, 87 |
| 7 | 3.15 |
| 8 | 3, 39 |
| 9 | 3, 61 |
| 10 | 3, 8 |
To calculate the grounding of electrodes evenly spaced around a hollow square, such as the perimeter of a building, the above equations are used with a value of λ taken from the following table. For three rods located in an equilateral triangle or in an L-formation, the value λ=1, 66
| Factors for hollow square electrodes | |
| Number of electrodes (n) | Factor (λ) |
| 2 | 2, 71 |
| 3 | 4, 51 |
| 4 | 5, 48 |
| 5 | 6, 13 |
| 6 | 6, 63 |
| 7 | 7, 03 |
| 8 | 7, 36 |
| 9 | 7, 65 |
| 10 | 7, 9 |
| 12 | 8, 3 |
| 14 | 8, 6 |
| 16 | 8, 9 |
| 18 | 9, 2 |
| 20 | 9, 4 |
Calculation of loop protective grounding for hollow squares is carried out according to the formula of the total number of electrodes (N)=(4n-1). The rule of thumb is that parallel rods should be spaced at least twice as long to take full advantage of the additional electrodes.
If the separation of the electrodes is much greater than their length, and only a few electrodes are in parallel, then the resulting earth resistance can be calculated using the usual equation for resistance. In practice, the effective earth resistance will usually be higher than the calculated one.
Typically, a 4-electrode array can provide 2.5-3 times improvement.
An array of 8 electrodes usually gives an improvement of perhaps 5-6 times. The resistance of the original ground rod will be reduced by 40% for the second line, 60% for the third line, 66% for the fourth.
Electrode calculation example
Calculating the total resistance of a ground rod 200 units in parallel, at 4m intervals each, and if they are connected in a square. The ground rod is 4meters and a diameter of 12.2 mm, surface resistance 500 ohms. First, the resistance of a single ground rod is calculated: R=500 / (2 × 3, 14 × 4) x (Log (8 × 4 / 0, 0125) -1)=136, 23 ohms.
Next, the total resistance of the ground rod in the amount of 200 units in parallel: a=500 / (2 × 3, 14x136x4)=0.146 Ra (parallel line)=136.23x (1 + 10 × 0.146 / 200)=1.67 Ohm.
If the ground rod is connected to a hollow area 200=(4N-1), Ra (on an empty square)=136, 23x (1 + 9, 4 × 0, 146 / 200)=1, 61 Ohm.
Ground Calculator
As you can see, the calculation of grounding is a very complex process, it uses many factors and complex empirical formulas that are available only to trained engineers with complex software systems.
The user can only make a rough calculation using online services, for example, Allcalc. For more accurate calculations, you still need to contact the design organization.
Allcalc online calculator will help you quickly and accurately calculate the protective grounding in a two-layer soil consisting of a vertical ground.
Calculation of system parameters:
- The top layer of soil is highly moistened sand.
- Climatic coefficient- 1.
- The bottom layer of soil is highly moistened sand.
- Number of vertical groundings - 1.
- Top soil depth H (m) - 1.
- Vertical section length, L1 (m) - 5.
- Depth of the horizontal section h2 (m)- 0.7.
- Connection strip length, L3 (m) - 1.
- Diameter of the vertical section, D (m) - 0.025.
- Width of the horizontal section shelf, b (m) - 0.04.
- Electrical soil resistance (ohm/m) - 61.755.
- Resistance of one vertical section (Ohm) - 12.589.
- Length of the horizontal section (m) - 1.0000.
Horizontal grounding resistance (Ohm) - 202.07.
Calculation of the protective earth resistance is completed. The total resistance to the propagation of electric current (Ohm) - 11.850.
Ground provides a common reference point for many voltage sources in an electrical system. One of the reasons why grounding helps to keep a person safe is that the earth is the largest conductor in the world, and excess electricity always takes the path of least resistance. By grounding the electrical system at home, a person allows the current to go into the ground, which saves his life and the lives of others.
Without a properly grounded electrical system at home, the user risks not only household appliances, but their own lives. That is why in every house it is necessary not only to create a grounding network, but also to annually monitor its performance using special measuring instruments.
