Earthing Systems Notes

EARTHING SYSTEMS

2.1. Introduction

In any medium or low voltage three-phase system there are three single-phase voltages which are measured between each phase and a common point called the "neutral point". In balanced operating conditions these three voltages are phase shifted by 120° and have the value:

U / 3

U being the phase-to-phase voltage measured between phases (see fig. 2-1).

From a physical point of view, the neutral is the common point of three star-connected

windings. It may or may not be accessible, may or may not be distributed and may or may not be earthed, which is why we refer to the earthing system.

The neutral may be connected to earth either directly or via a resistor or reactor. In the first

case, we say that the neutral is solidly (or directly) earthed and, in the second case, we say that the neutral is impedance-earthed.

When there is no intentional connection between the neutral point and earth, we say that the neutral is isolated or unearthed.

The earthing system plays a very important role in a network. On occurrence of an insulation fault or a phase being accidentally earthed, the values taken by the fault currents, touch voltages and overvoltages are closely related to the type of neutral earthing connection.

A solidly earthed neutral helps to limit overvoltages; however, it generates very high fault currents. On the other hand, an isolated or unearthed neutral limits fault currents to very low values but encourages the occurrence of high overvoltages.

In any installation, service continuity in the presence of an insulation fault also depends on the earthing system. An unearthed neutral allows continuity of service in medium voltage, as long as the security of persons is respected. On the other hand, a solidly earthed neutral, or low impedance-earthed neutral, requires tripping to take place on occurrence of the first insulation fault.

The extent of the damage to some equipment, such as motors and generators having an

internal insulation fault, also depends on the earthing system.

In a network with a solidly earthed neutral, a machine affected by an insulation fault suffers

extensive damage due to the high fault currents.

On the other hand, in an unearthed network or high impedance-earthed network, the damage

is reduced, but the equipment must have an insulation level compatible with the level of

overvoltages able to develop in this type of network.

The earthing system also has a considerable amount of influence on the nature and level of

electromagnetic disturbances generated in an electrical installation.

Earthing systems which encourage high fault currents and their circulation in the metallic

structures of buildings are highly disturbing.

On the other hand, earthing systems which tend to reduce these currents and which

guarantee good equipotential bonding of exposed conductive parts and metallic structures are not very disturbing.

The choice of earthing system, as much in low voltage as in medium voltage, depends both on the type of installation and network. It is also influenced by the type of loads, the service continuity required and the limitation of the level of disturbance applied to sensitive equipment

1. Introduction to Earthing Systems

Earthing or grounding is a crucial aspect of electrical systems that provides a path for the safe dissipation of electric current to the ground. It plays a key role in enhancing safety by reducing the risk of electric shock, ensuring equipment protection, and stabilizing voltage levels.

Objectives of Earthing:

Safety: Protects people from electric shocks due to insulation failures or faults in the system.
Equipment Protection: Reduces the risk of damage to electrical devices caused by surges, lightning strikes, or fault currents.
Voltage Stabilization: Helps in maintaining the voltage levels by providing a common reference point in the electrical system.
Prevention of Fire Hazards: By allowing fault currents to safely dissipate, it minimizes the risk of fires caused by electrical faults.

2. Components of an Earthing System

Earth Electrode: A conductor buried in the ground to provide a direct physical connection to the earth. It can be a rod, plate, or mesh made of conductive materials like copper or galvanized steel.
Earthing Conductor: A conductor that connects the electrical installation or equipment to the earth electrode. It ensures a low-resistance path for fault currents.
Main Earthing Terminal: The point where the earthing system connects with the installation's earthing conductors.
Bonding Conductors: These conductors connect exposed conductive parts of the installation to the main earthing terminal to ensure they are at the same potential.

3. Types of Earthing Systems Earthing systems can be classified based on the configuration of connections between the power source (transformer or generator), the consumer installation, and the earthing arrangements.

TN System (Terra-Neutral)

The TN system is characterized by a direct connection of the neutral point to the earth. The exposed conductive parts are connected to the neutral or earth conductor provided by the supply system.

TN-S System: The neutral (N) and protective earth (PE) conductors are separate throughout the system. The PE is connected to the earth electrode at the transformer.
TN-C System: The neutral and earth conductors are combined into a single conductor called the PEN (Protective Earth and Neutral) conductor.
TN-C-S System (Protective Multiple Earthing - PME): This is a hybrid system where the PEN conductor is used in parts of the installation and then split into separate PE and N conductors.

Advantages of TN Systems:

Simple design and installation.
Fast and effective disconnection of fault currents.
High reliability in urban areas.

Disadvantages:

Risk of electrical shock if the PEN conductor is damaged.
The potential difference can exist due to the shared conductor.

TT System (Terra-Terra)

In the TT system, the neutral of the power supply is earthed at the transformer, but the consumer's installation has its own independent earth electrode.

Advantages of TT Systems:

Reduced risk of shock due to separate earthing.
Ideal for rural areas or where the TN system is not feasible.

Disadvantages:

Requires the use of RCDs (Residual Current Devices) for protection against electric shocks.
Higher maintenance cost due to separate earthing.

IT System (Isolated Terra)

In the IT system, the neutral point is either unearthed or connected to the earth through a high impedance. The exposed conductive parts are connected to their own earth electrode.

Advantages of IT Systems:

High reliability and continuity of supply because the system can continue to operate with a single fault.
Reduced risk of electric shock in case of insulation failure.

Disadvantages:

Complex fault detection due to isolated neutral.
Requires insulation monitoring devices for safety.

4. Earthing Arrangements

Protective Earthing (PE): Connects exposed conductive parts of electrical equipment to the ground to prevent electric shocks.
Functional Earthing (FE): Used to ensure the proper functioning of electronic devices by providing a stable reference point, not necessarily for safety.
Combined Earthing: A system where the protective and functional earthing are combined, commonly seen in IT equipment.

5. Design Considerations for Earthing Systems

Soil Resistivity: The resistance of the soil affects the efficiency of the earthing system. Lower resistivity is preferred as it provides better conductivity.
Earth Electrode Resistance: The resistance between the earth electrode and the ground should be as low as possible, typically less than 1 ohm for critical installations.
Material Selection: Copper, galvanized steel, and stainless steel are commonly used for their high conductivity and corrosion resistance.
Grounding Grid: A network of interconnected ground rods or plates to reduce the overall earth resistance.

6. Testing and Maintenance Regular testing and maintenance of earthing systems are vital to ensure their effectiveness. Key tests include:

Earth Continuity Test: Ensures the continuity of the earthing conductor throughout the installation.
Earth Resistance Test: Measures the resistance between the earth electrode and the ground using a grounding resistance tester.
Soil Resistivity Test: Determines the resistivity of the soil, which helps in designing the earthing system.

7. Safety Standards and Regulations Various standards and codes govern earthing systems to ensure safety and reliability. Common standards include:

IEC 60364 (International Electrotechnical Commission): Defines earthing requirements for electrical installations.
IEEE 80 (Institute of Electrical and Electronics Engineers): Provides guidelines for safety in AC substation grounding.
NFPA 70 (NEC) (National Electrical Code): Establishes grounding requirements for electrical installations in the U.S.

8. Key Considerations for Effective Earthing:

Ensure a low-impedance path for fault currents to flow safely.
Minimize potential differences to avoid electric shocks.
Regularly inspect and maintain earthing systems to address degradation or corrosion.

Conclusion: An effective earthing system is a fundamental aspect of electrical safety. It protects both people and equipment by providing a reliable path for fault currents, preventing electric shock, and reducing the risk of fires. Proper design, installation, and maintenance are essential to ensure the long-term functionality and safety of earthing systems.

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Earthing Systems Notes - Schneider Electric