A Comprehensive Guide to Earthing Systems
An earthing system, also known as a grounding system, connects specific parts of an electric power system with the ground, typically the Earth’s conductive surface, for safety and functional purposes. The choice of earthing system can affect the safety and electromagnetic compatibility of the installation. Regulations for earthing systems vary among countries, though most follow the recommendations of the International Electrotechnical Commission (IEC). In this article, we will explain the different types of earthing systems, their advantages and disadvantages, and how to design and install them.
What is an Earthing System?
An earthing system is defined as a set of conductors and electrodes that provide a low-resistance path for electrical current to flow to the ground in the event of a fault or malfunction. This is important for several reasons:
Protection of equipment: An earthing system helps to protect electrical equipment from damage due to overvoltage or short-circuit conditions. It also prevents static buildup and power surges caused by nearby lightning strikes or switching operations.
Protection of people: An earthing system helps to prevent electric shock hazards by ensuring that the exposed metal parts of electrical installations are at the same potential as the earth. It also facilitates the operation of protective devices such as circuit breakers or residual current devices (RCDs) that can disconnect the supply in case of a fault.
Reference point: An earthing system provides a reference point for electrical circuits and equipment so that they can operate at a safe voltage level with respect to the Earth. This ensures that any electrical energy that is not used by the load is safely dissipated to the earth.
Types of Earthing Systems
BS 7671 lists five types of earthing systems: TN-S, TN-C-S, TT, TN-C, and IT. The letters T and N stand for:
T = Earth (from the French word Terre)
N = Neutral
The letters S, C, and I stand for:
S = Separate
C = Combined
I = Isolated
The type of earthing system is determined by how the source of energy (such as a transformer or a generator) is connected to the earth and how the consumer’s earthing terminal is connected to the source or to a local earth electrode.
TN-S System
A TN-S system, shown in Figure 1, has the neutral source of energy connected with earth at one point only, at or as near as is reasonably practicable to the source. The consumer’s earthing terminal is typically connected to the metallic sheath or armor of the distributor’s service cable into the premises.
Figure 1: TN-S System
The advantages of a TN-S system are:
It provides a low impedance path for fault currents, which ensures a fast operation of protective devices.
It avoids any potential difference between neutral and earth within the consumer’s premises.
It reduces the risk of electromagnetic interference due to common mode currents.
The disadvantages of a TN-S system are:
It requires a separate protective conductor (PE) along with the supply conductors, which increases the cost and complexity of wiring.
It may be affected by corrosion or damage to the metallic sheath or armor of the service cable, which can compromise its effectiveness.
TN-C-S System
A TN-C-S system, shown in Figure 2, has the supply neutral conductor of a distribution main connected with the earth at the source and at intervals along its run. This is usually referred to as protective multiple earthing (PME). With this arrangement, the distributor’s neutral conductor is also used to return earth fault currents arising in the consumer’s installation safely to the source. To achieve this, the distributor will provide a consumer’s earthing terminal, which is linked to the incoming neutral conductor.
Figure 2: TN-C-S System
The advantages of a TN-C-S system are:
It reduces the number of conductors required for supply, which lowers the cost and complexity of wiring.
It provides a low impedance path for fault currents, which ensures a fast operation of protective devices.
It avoids any potential difference between neutral and earth within the consumer’s premises.
The disadvantages of a TN-C-S system are:
It may create a risk of electric shock if there is a break in the neutral conductor between two earth points, which can cause an increase in touch voltage on exposed metal parts.
It may cause unwanted currents to flow in metal pipes or structures that are connected to the earth at different points, which can result in corrosion or interference.
TT System
A TT system, shown in Figure 3, has both the source and the consumer’s installation connected to the earth through separate electrodes. These electrodes do not have any direct connection between them. This type of earthing system is applicable for both three-phase and single-phase installations.
Figure 3: TT System
The advantages of a TT system are:
It eliminates any risk of electric shock due to a break in the neutral conductor or contact between live conductors and earthed metal parts.
It avoids any unwanted currents in metal pipes or structures that are connected to the earth at different points.
It allows for more flexibility in choosing the location and type of earth electrodes.
The disadvantages of a TT system are:
It requires an effective local earth electrode for each installation, which may be difficult or costly to achieve depending on soil conditions and the availability of space.
It requires additional protection devices such as RCDs or voltage-operated ELCBs to ensure a reliable disconnection in case of a fault.
It may result in higher touch voltages on exposed metal parts due to higher earth loop impedance.
TN-C System
A TN-C system, shown in Figure 4, has both the neutral and protective functions combined in a single conductor throughout the system. This conductor is called PEN (protective earth neutral). The consumer’s earthing terminal is directly connected to this conductor.
Figure 4: TN-C System
The advantages of a TN-C system are:
It reduces the number of conductors required for supply, which lowers the cost and complexity of wiring.
It provides a low impedance path for fault currents, which ensures a fast operation of protective devices.
The disadvantages of a TN-C system are:
It creates a risk of electric shock if there is a break in the PEN conductor or if it comes into contact with live parts due to insulation failure.
It causes unwanted currents to flow in metal pipes or structures that are connected to PEN at different points, which can result in corrosion or interference.
It requires special precautions for connecting appliances with exposed metal parts that may be accessible simultaneously with other earthed metal parts.
IT System
An IT system, shown in Figure 5, has its source either isolated from Earth or connected to Earth through an impedance (such as a resistor or an inductor). The consumer’s installation is connected to the earth through one or more local electrodes. These electrodes do not have any direct connection with the source.
Figure 5: IT System
The advantages of an IT system are:
It eliminates any risk of electric shock due to first faults on live conductors because there is no return path through Earth.
It allows for continuity of supply even in case of first faults because there is no automatic disconnection required.
It reduces interference and overvoltage problems due to capacitive coupling between live conductors and the earth.
The disadvantages of an IT system are:
It requires special monitoring devices such as insulation monitors or fault detectors to identify first faults and locate them before they become dangerous second faults.
It requires additional protection devices such as RCDs or voltage-operated ELCBs to ensure a reliable disconnection in case of second faults.
It may result in higher touch voltages on exposed metal parts due to higher capacitance between live conductors and the earth.
How to Design an Earthing System
The design of an earthing system depends on several factors, such as:
The type and size of the power supply
The type and location of the load
The soil resistivity and moisture content
The environmental conditions and regulations
The availability and cost of materials
Some general steps for designing an earthing system are:
Determine the type of earthing system suitable for your application based on safety and functional requirements. Refer to BS 7671 or other relevant standards for guidance.
Calculate the maximum fault current that can flow through the earth electrode and the fault location. Refer to BS 7671 or other relevant standards for guidance.
Select the type and size of earth electrode suitable for your application based on soil resistivity, fault current, installation method, and cost. Refer to BS 7430 or other relevant standards for guidance.
Install the earth electrode according to the manufacturer’s instructions and best practices. Ensure that the earth electrode is properly connected to the earthing conductor and that the earthing conductor is adequately sized and protected.
Measure the earth electrode resistance using an appropriate instrument, such as a fall-of-potential tester or a clamp-on tester. Compare the measured value with the design value and make adjustments if necessary. Refer to BS 7430 or other relevant standards for guidance.
Verify that the earthing system meets the safety requirements for touch and step voltages, earth fault loop impedance, and protective device operation. Refer to BS 7671 or other relevant standards for guidance.
How to Maintain an Earthing System
The earthing system should be periodically inspected and tested to ensure its effectiveness and reliability. Some factors that may affect the performance of the earthing system are:
Corrosion or damage of earth electrodes or earthing conductors
Changes in soil resistivity or moisture content due to weather or environmental conditions
Alterations or additions to the electrical installation or power supply
Faults or defects in electrical equipment or protective devices
Some steps for maintaining an earthing system are:
Inspect the physical condition of the earth electrodes and earthing conductors for any signs of corrosion, damage, or deterioration. Repair or replace any defective parts as soon as possible.
Test the earth electrode resistance using an appropriate instrument, such as a fall-of-potential tester or a clamp-on tester. Compare the measured value with the previous value and check for any significant changes. If the resistance has increased beyond an acceptable limit, investigate the cause and take corrective actions.
Test the touch and step voltages, earth fault loop impedance, and protective device operation using suitable instruments such as a voltage tester, an impedance tester, or a loop tester. Compare the measured values with the design values and check for any deviations. If the values are outside the safety limits, investigate the cause and take corrective actions.
Record the results of inspection and testing in a log book or a database. Keep track of any changes or trends in the performance of the earthing system over time.
Conclusion
An earthing system is an essential part of any electrical installation that provides safety and functionality for both equipment and people. The type of earthing system depends on various factors such as power supply, load, soil conditions, and regulations.
The design of an earthing system requires careful calculation and selection of earth electrodes, earthing conductors, and protective devices. The installation of an earthing system requires proper methods and materials to ensure a low-resistance connection to the earth. The maintenance of an earthing system requires regular inspection and testing to ensure its effectiveness and reliability.
By following these guidelines, you can design, install and maintain an earthing system that meets your needs and expectations.
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