Grounding systems: types, description, installation

The main reason for the need for grounding in electrical networks is safety. When all metal parts of the electrical equipment are grounded, then, even in the case of broken insulation, dangerous voltages will not be created on its case, they will be prevented by reliable grounding systems.

Tasks for grounding systems

The main tasks of security systems operating on the grounding principle:

1. Safety for human life, in order to protect against electric shock. Provides an alternative path for the passage of emergency current so that it does not cause damage to the user.
2. Protection of buildings, machinery and equipment in the event of a power outage so that exposed conductive parts of the equipment do not reach deadly potential.
3. Overvoltage protection due to lightning strike, which can lead to dangerous high voltages in the electrical distribution system or from inadvertent human contact with high voltage lines.
4. Voltage stabilization. There are many sources of electricity. Each transformer can be considered as a separate source. They should have a common accessible point of negative energy discharge. Earth is the only such conductive surface for all energy sources, so it was adopted as the universal standard for current and voltage discharge. If there were no such common point, it would be extremely difficult to ensure security in the power system as a whole.

Grounding system requirements:

• It must have an alternative path for dangerous current to flow.
• Lack of hazardous potential on open conductive parts of equipment.
• It must have a low impedance sufficient to provide the necessary current through the safety device so that it turns off the power (<0.4 sec).
• Must have good corrosion resistance.
• Must be able to dissipate a large short circuit current.

Description of grounding systems

The process of connecting the metal parts of electrical apparatus and equipment with a mass of earth with a metal device that has little resistance is called grounding. When grounding, live parts of the devices are directly connected to the ground. Grounding provides the return path for leakage current and therefore protects the power system equipment from damage.

When a malfunction occurs in the equipment, a current imbalance forms in all three of its phases. Grounding discharges the fault current to earth and, therefore, restores the operating balance of the system. These protective systems have several advantages, such as eliminating overvoltage by discharging it to the ground. Grounding ensures equipment safety and increases service reliability.

Zeroing Method

Zeroing means connecting the supporting part of the equipment to the ground. When a malfunction occurs in the system, a dangerous potential is created on the external surface of the equipment, and any person or animal, accidentally touching the surface, can receive an electric shock. Zeroing dumps dangerous currents to the ground and, therefore, neutralizes the current shock.

It also protects equipment from lightning strikes and provides a discharge path from arresters and other extinguishing devices. This is achieved by connecting the parts of the installation to the ground with a grounding conductor or electrode in close contact with soil located at a certain distance below the ground.

The difference between grounding and grounding

One of the main differences between grounding and grounding is that when grounding, the current-carrying part is connected to ground, while when grounding, the surface of the devices are connected to ground. Other differences between them are explained below in the form of a comparative table.

comparison table

TN protective wires

These types of grounding systems have one or more directly grounded points from an energy source. Open conductive parts of the unit are connected to these points using protective wires.

In world practice, a two-letter code is used.

Used letters:

• T (the French word Terre means "earth") is a direct connection of a point with the earth.
• I - no point is connected to the ground due to the high impedance.
• N - direct connection to the neutral of the source, which, in turn, is connected to the ground.

Based on the combination of these three letters, there are types of grounding systems: TN, TN-S, TN-C, TN-CS. What does this mean?

In a TN type grounding system, one of the source points (generator or transformer) is connected to ground. This point is usually a star point in a three-phase system. The case of the connected electrical device is connected to earth through this ground point on the source side.

In the figure above: PE - The acronym for Protective Earth is a conductor that connects the exposed metal parts of a consumer’s electrical installation to earth. N is called neutral. This is a conductor connecting a star in a three-phase system to earth. From these designations in the diagram, it is immediately clear which grounding system belongs to the TN system.

Neutral line TN-S

This is a system that has separate neutral and protective conductors throughout the electrical installation.

The protective conductor (PE) is a metal coating of the cable, power supply unit or a separate conductor.

All exposed conductive parts with the installation are connected to this protective conductor through the main terminal of the installation.

TN-CS system

These are types of grounding system systems in which neutral and protective functions are combined into one system conductor.

In the TN-CS neutral grounding system, also known as Protective Multiple Earthing, the PEN conductor is called the combined neutral and ground conductor.

The PEN conductor of the power system is grounded at several points, and the ground electrode is located at or near the consumer’s installation site.

All exposed conductive parts with the installation are connected by a PEN conductor using the main ground terminal and the neutral terminal and are connected to each other.

TT protection circuit

This is a protective grounding system having one point of the energy source.

All exposed conductive parts with installation, which are connected to a grounded electrode, are not electrically dependent on the source of the earth.

IT isolation system

A protective grounding system that does not have a direct connection between live parts and ground.

All exposed conductive parts with installation that are connected to a grounded electrode.

The source is either connected to earth through a deliberately imposed system impedance, or isolated from earth.

Designs of protective systems

The connection between electrical appliances and devices with a grounding plate or electrode through a thick wire with low resistance for safety is called grounding or grounding.

The grounding or grounding system in the electrical network works as a safety measure to protect the lives of people and equipment. The main goal is to provide an alternative route for the passage of hazardous flows so that accidents due to electric shock and equipment damage can be avoided.

The metal parts of the equipment are grounded or connected to the ground, and if for some reason the insulation of the equipment does not work, then the high voltages that may be present in the outer coating of the equipment will have a discharge path to the ground. If the equipment is not grounded, this hazardous voltage can be transferred to anyone who touches it, which will result in electric shock. The circuit closes and the fuse blows immediately if the live wire touches the grounded enclosure.

There are several ways to implement the grounding system of electrical installations, such as grounding a wire or strip, plate or rod, grounding by grounding or through a water supply system. The most common methods are grounding and plate construction.

Grounding Mat

Grounding mat is made by connecting the number of rods through copper wires. This reduces the overall resistance of the circuit. These electrical grounding systems help limit the potential of the earth. The grounding mat is mainly used in the place where a large fault current is to be tested.

When designing a grounding mat, the following requirements are taken into account:

1. In the event of a malfunction, the voltage should not be hazardous to humans by touching the conductive surface of the equipment of the electrical system.
2. The direct current short circuit, which can flow into the grounding mat, must be quite large for the protective relay to work.
3. Soil resistance is low so that leakage current flows through it.
4. The design of the grounding mat should be such that the step voltage is less than the permissible value, which will depend on the specific resistance of the soil necessary to isolate the faulty installation from humans and animals.

Electrode overcurrent protection

With such a building grounding system, any wire, rod, pipe or bundle of conductors is placed horizontally or vertically in the ground next to the protective object. In distribution systems, the ground electrode may consist of a rod about 1 meter long and be placed vertically in the ground. In the manufacture of substations, a grounding mat is used, and not individual rods.

Current protection pipe circuit

This is the most common and best grounding system for electrical installations compared to other systems suitable for the same earth and moisture conditions. In this method, galvanized steel and a perforated pipe with an estimated length and diameter are arranged vertically on permanently moist soil, as shown below. Pipe size depends on current flow and soil type.

Typically, the pipe size for a home grounding system is 40 mm in diameter and 2.5 meters in length for ordinary soil or longer in the case of dry and rocky soil. The depth at which the pipe should be buried depends on soil moisture. Typically, the pipe is located 3.75 meters deep. The bottom of the pipe is surrounded by small pieces of coke or charcoal at a distance of about 15 cm.

Alternative levels of coal and salt are used to increase the effective land area and, accordingly, to reduce resistance. Another pipe with a diameter of 19 mm and a minimum length of 1.25 meters is connected at the top of the GI pipe through a reducer. In summer, soil moisture decreases, which leads to an increase in earth resistance.

Thus, work is carried out on a cement concrete base to maintain water availability in the summer and have land with the necessary protective parameters. Through a funnel connected to a pipe with a diameter of 19 mm, you can add 3 or 4 buckets of water. The ground wire is either GI or a strip of GI wire with sufficient cross-section for safe current removal is transferred to a GI pipe with a diameter of 12 mm at a depth of about 60 cm from the ground.

Plate ground

In this device of the grounding system, a ground plate of copper 60 cm × 60 cm × 3 m in size and galvanized iron 60 cm × 60 cm × 6 mm in size is immersed in the ground with a vertical surface at a depth of at least 3 m from the ground

The protective plate is inserted into the auxiliary layers of charcoal and salt with a minimum thickness of 15 cm. The ground wire (GI or copper wire) is tightly bolted to the ground plate.

Copper plate and copper wire are usually not used in protective circuits because of their higher cost.

Grounding connection through a water supply system

In this type, the GI or copper wire is connected to the water supply network using a steel bonding wire that is fixed to the copper lead, as shown below.

The water supply system consists of metal and is located below the surface of the earth, i.e. it is directly connected to the earth. The current flow through the GI or copper wire is directly grounded through the water supply.

Ground loop resistance calculation

The resistance of a single strip of the rod buried in the ground is:

R = 100xρ / 2 × 3.14 × L (loge (2 x L x L / W xt)), where:

ρ - soil stability (Ω ohm),

L is the length of the strip or conductor (cm),

w is the width of the strip or diameter of the conductor (cm),

t is the burial depth (cm).

Example: Calculate the resistance of the ground strip. A wire with a diameter of 36 mm and a length of 262 meters at a depth of 500 mm in the ground, the earth resistance is 65 Ohms.

R is the resistance of the ground rod in watts.

r - Soil resistance (Ohmmeter) = 65 Ohms.

Measuring instrument l - rod length (cm) = 262 m = 26200 cm.

d - inner diameter of the rod (cm) = 36 mm = 3.6 cm.

h - depth of the hidden strip / rod (cm) = 500 mm = 50 cm.

Ground / conductor resistance (R) = ρ / 2 × 3.14 x L (loge (2 x L x L / Wt))

Ground / conductor resistance (R) = 65/2 × 3.14 x 26200 x ln (2 x 26200 x 26200 / 3.6 × 50)

Ground / conductor resistance (R) = 1.7 ohms.

To calculate the amount of ground rod, you can apply the rule of thumb.

The approximate resistance of the Rod / Pipe electrodes can be calculated using the resistance of the rod / tube electrodes:

R = K x ρ / L, where:

ρ is the resistance of the earth in an ohmmeter,

L is the length of the electrode in the meter,

d is the diameter of the electrode in the meter,

K = 0.75 if 25 <L / d <100.

K = 1 if 100 <L / d <600.

K = 1.2 o / L if 600 <L / d <300.

The number of electrodes, if you find the formula R (d) = (1,5 / N) x R, where:

R (d) is the required resistance.

R is the resistance of a single electrode

N is the number of electrodes mounted in parallel at a distance of 3 to 4 meters.

Example: calculate the resistance of the grounding pipe and the number of electrodes to obtain a resistance of 1 Ohm, soil resistance from ρ = 40, length = 2.5 meters, pipe diameter = 38 mm.

L / d = 2.5 / 0.038 = 65.78, so K = 0.75.

The resistance of the pipe electrodes R = K x ρ / L = 0.75 × 65.78 = 12 Ω

One electrode - resistance - 12 Ohms.

To obtain a resistance of 1 Ohm, the total number of required electrodes = (1.5 × 12) / 1 = 18

Factors Affecting Earth Resistance

The NEC code requires a minimum grounding electrode length of 2.5 meters for contact with soil. But there are some factors that affect the earth's resistance of the protective system:

1. The length / depth of the ground electrode. By doubling the length, the surface resistance is reduced to 40%.
2. The diameter of the ground electrode. A double increase in the diameter of the ground electrode reduces the resistance to soil by only 10%.
3. The number of grounding electrodes. To increase efficiency, additional electrodes are installed to the depth of the main grounding electrodes.

Construction of protective electrical systems of a residential building

Currently, earth structures are the preferred method of grounding, especially for electrical networks. Electricity always follows the path of least resistance and diverts the maximum current from the circuit to ground pits designed to reduce resistance, ideally up to 1 ohm.

To achieve this goal:

1. An area of ​​1.5 mx 1.5 m is dug to a depth of 3 m. The pit is half filled with a mixture of charcoal powder, sand and salt.
2. A 500 mm x 500 mm x 10 mm GI plate fits in the middle.
3. Establish connections between the grounding plate for the grounding system of a private house.
4. The rest of the pit is filled with a mixture of coal, sand, salt.
5. Two GI strips with a cross-section of 30 mm x 10 mm can be used to connect the ground plate to the surface, but a 2.5-inch GI pipe with a flange at the top is preferred.
6. In addition, the upper part of the pipe can be covered with a special device to prevent the ingress of dirt and dust and clogging of the grounding pipe.

Grounding system installation and advantages:

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In order to determine how vulnerable a residential building is to lightning strikes, you can use the following data:

1. Low risk Single-level private residential buildings in the close surroundings of other houses of the same height.
2. Medium risk. Two-level private house surrounded by houses with similar heights or surrounded by houses of lower heights.
3. High risk. Isolated houses that are not surrounded by other structures, two-story houses or houses with a lower height.

Regardless of the likelihood of a lightning strike, the proper use of important components of lightning protection will help protect any residential building from such damage. Lightning protection and grounding systems are required in a residential building so that a lightning strike is diverted to the ground. The system usually includes a grounded rod with a copper connection, which is installed in the ground.

When installing a lightning protection circuit in a house, follow these requirements:

1. Ground electrodes should have a length of at least half 12 mm and 2.5 m in length.
2. Copper connections are recommended.
3. If there is stony soil on the site of the system or engineering underground lines are located, the use of a vertical electrode is prohibited, only a horizontal conductor is required.
4. It should be deepened at a distance of at least 50 cm from the ground and extend at least 2.5 m from the house.
5. The grounding systems of a private home should be interconnected using a conductor of the same size.
6. Connecting elements for all underground metal piping systems, such as water or gas pipes, should be located within 8 meters of the house.
7. If all systems have already been connected before the installation of lightning protection, you only need to bind the nearest electrode to the water supply system.

All people living or working in residential, public buildings are constantly in close contact with electrical systems and equipment and must be reliably protected from dangerous phenomena that can occur due to short circuits or very high voltages from lightning discharges.

To achieve this protection, the grounding systems of electric networks must be designed and installed in accordance with standard state requirements. With the development of electrical materials, the reliability requirements of protective devices increase.