Manual on Earthing of AC Power Systems

The Manual on Earthing of AC Power Systems serves as a comprehensive guide for the design, installation, and maintenance of earthing systems in alternating current (AC) power networks. Earthing (also known as grounding) is a crucial aspect of electrical systems, as it ensures the safety of people, protects equipment, and enhances the reliability of power systems by providing a path for fault currents. The manual typically covers various standards, methodologies, and best practices related to the earthing of AC power systems.

Key Aspects Covered in the Manual

1. Purpose of Earthing

The main purposes of earthing in AC power systems include:

• Personnel Safety: Protects humans from electric shock by providing a low-resistance path for fault currents to flow into the ground.
• Equipment Protection: Prevents damage to electrical equipment by ensuring that fault currents do not build up to dangerous levels.
• System Stability: Ensures reliable operation of protective devices (such as circuit breakers and relays) by providing a reference point for the system voltage.
• Minimizing Overvoltages: Helps in reducing overvoltages caused by lightning strikes or switching surges, ensuring the longevity of insulation and equipment.

2. Types of Earthing Systems

The manual typically outlines the different methods of earthing and their applications:

• Solid Earthing (Solid Grounding): Direct connection of the neutral point of the system (generator or transformer) to the earth without any impedance. This method provides high fault current, ensuring fast operation of protective devices.
• Resistance Earthing: Involves inserting a resistor between the system neutral and ground to limit the fault current to a manageable level, preventing excessive damage while allowing for fault detection.
• Reactance Earthing: Similar to resistance earthing but uses a reactor (inductive impedance) instead of a resistor to limit fault currents.
• Isolated (Ungrounded) Earthing: No intentional connection between the system neutral and earth, allowing only capacitively coupled current to flow during a fault. This method minimizes fault current but makes fault detection and control more complex.
• Peterson Coil (Arc Suppression Coil) Earthing: Involves using a tuned reactor to neutralize the capacitive current in the event of a fault, reducing arcing faults in systems with high capacitance to earth.

3. Design Principles of Earthing Systems

Designing an earthing system involves several key considerations to ensure performance and safety:

• Fault Current Magnitude: Earthing systems must be designed to handle expected fault current levels without excessive temperature rise or damage to conductors and equipment.
• Touch and Step Voltage: The manual highlights the importance of limiting touch and step voltage to prevent dangerous voltage levels that could lead to electric shocks during a fault. Touch voltage is the voltage difference between a grounded structure and the earth, while step voltage is the potential difference experienced when stepping between two points on the ground during a fault.
• Ground Resistance: The effectiveness of an earthing system depends on the resistance between the earth electrode and the ground. The manual provides guidance on maintaining ground resistance at low levels (typically less than 1 to 5 ohms) to ensure proper fault dissipation.

4. Components of Earthing Systems

The manual explains the various components that make up an earthing system:

• Earth Electrode: Conductive elements (such as rods, plates, or meshes) buried in the ground to provide a path for fault current. Proper selection of electrode material (copper, galvanized steel, etc.) and design ensures long-term performance.
• Earthing Conductors: Wires or cables that connect the system’s neutral point to the earth electrode. They must be sized correctly to handle fault currents without overheating.
• Grounding Grid: For substations and high-voltage systems, grounding grids are used to spread the fault current over a large area to minimize ground potential rise and limit step and touch voltages.
• Bonding: Ensures that all metallic parts of the system are interconnected and have the same potential, reducing the risk of dangerous voltage differences.

5. Earthing System Calculations

To design an effective earthing system, the manual provides formulas and procedures to calculate:

• Ground Resistance: Using methods such as the Wenner or Fall-of-Potential method, ground resistance is measured or estimated based on soil resistivity and the arrangement of earth electrodes.
• Touch and Step Voltage: Calculations are provided to ensure these voltages remain within safe limits under fault conditions.
• Thermal Capacity of Conductors: Conductors must be able to carry fault current without exceeding their thermal limits. The manual outlines how to calculate the thermal withstand capability of earthing conductors.

6. Earthing for Different Installations

The manual covers specific requirements for various types of installations:

• Substation Earthing: Detailed requirements for substation earthing systems, including the use of ground grids, rods, and bonding of equipment to prevent dangerous voltage gradients.
• Transmission and Distribution Networks: Earthing requirements for high-voltage transmission lines and medium-voltage distribution networks, including the use of shield wires, lightning arresters, and neutral grounding methods.
• Industrial and Commercial Installations: Guidance on earthing systems for industrial plants, commercial buildings, and other facilities to ensure safety and system reliability.

7. Maintenance and Testing of Earthing Systems

Regular maintenance and testing are critical for ensuring the ongoing effectiveness of an earthing system. The manual provides procedures for:

• Periodic Testing: Testing ground resistance, continuity, and integrity of earthing conductors and electrodes to detect any degradation due to corrosion, damage, or environmental changes.
• Visual Inspection: Routine inspections to check for physical damage, corrosion, or improper connections.
• Measurement of Step and Touch Voltages: Ensuring that touch and step voltages remain within safe limits after installation or system modifications.

8. Standards and Regulations

The manual typically references relevant international and national standards governing earthing systems, such as:

• IEEE 80: Guide for Safety in AC Substation Grounding.
• IEC 60364: Electrical Installations of Buildings – Part 5-54: Earthing Arrangements and Protective Conductors.
• BS 7430: Code of Practice for Earthing.

These standards provide specific guidelines for design, installation, and testing of earthing systems to ensure they meet safety and performance requirements.

Conclusion

The Manual on Earthing of AC Power Systems is essential for engineers and electricians responsible for designing and maintaining safe, reliable earthing systems. By following the guidelines and standards presented, the manual helps prevent electrical hazards, minimize equipment damage, and improve the overall stability of power systems.

Calculation of Earthing for AC Power Systems

Earthing (or grounding) in AC power systems is crucial for both the safety of personnel and the protection of equipment. It ensures that any fault currents, such as those caused by insulation failures, flow safely to the ground without causing damage or injury. The process of designing an earthing system requires careful calculation to ensure that it can handle fault currents while maintaining voltage stability and keeping touch and step voltages within safe limits.

1. Types of Earthing in AC Power Systems

There are several methods of earthing depending on the system configuration:

• TN-S System (Separate Neutral and Earth): Neutral and earth are separated and run as individual conductors.
• TN-C System (Combined Neutral and Earth): Neutral and earth are combined in a single conductor.
• TT System: The earth connection is local to the installation, separate from the supply's neutral.
• IT System: The neutral is not connected to earth, and the system is insulated from ground.

For most practical purposes, we focus on TN-S and TT systems in low-voltage distribution.

2. Parameters for Earthing System Calculation

The design of an earthing system typically involves determining:

• Earth Fault Current: The magnitude of current that flows to the earth during a fault condition.
• Earth Electrode Resistance: The resistance between the earth and the earthing electrode(s), which must be low enough to ensure safety during faults.
• Touch and Step Voltages: The voltages that can be present between two points accessible to humans during a fault. These must be kept below certain limits to prevent electric shock hazards.
• Earthing Conductor Sizing: The size of conductors used to connect system components to the earthing point.

3. Calculation of Earth Electrode Resistance

The earth electrode resistance needs to be low enough to allow the fault current to flow to ground and for protective devices to trip. For standard installations, the acceptable resistance to earth is typically below 1 ohm for substations and below 10 ohms for general equipment.

The total earth resistance (R_e) is dependent on several factors, including:

• Soil Resistivity (ρ): The resistivity of the soil, which varies based on soil composition, moisture content, and temperature.
• Electrode Geometry: The shape, depth, and number of earthing electrodes.

Formula for a Single Earth Rod:

For a vertical rod, the earth resistance can be approximated by:

$$R_e = \frac{\rho}{2 \pi L} \ln \left(\frac{4L}{d} \right)$$

Where:

• $R_e$ is the earth electrode resistance (ohms),
• $\rho$ is the soil resistivity (ohm-meters),
• $L$ is the length of the electrode (meters),
• $d$ is the diameter of the electrode (meters).

For multiple rods, their combined resistance $R_c$ can be calculated using empirical formulas, but adding more rods generally reduces the total resistance.

Example:

For a 3-meter rod with a diameter of 0.015 meters installed in soil with a resistivity of 100 ohm-meters:

$$R_e = \frac{100}{2 \pi \times 3} \ln \left(\frac{4 \times 3}{0.015} \right)$$ $$R_e = \frac{100}{18.85} \ln (800) \approx 3.32 \, \text{ohms}$$

Thus, a single rod in this case would have a resistance of approximately 3.32 ohms.

4. Fault Current Calculation

The fault current $I_f$ that flows through the earthing system during a fault can be calculated using:

$$I_f = \frac{V_{L-N}}{Z_{total}}$$

Where:

• $V_{L-N}$ is the line-to-neutral voltage (in volts),
• $Z_{total}$ is the total impedance of the fault path, including the earth resistance, impedance of the cables, and system impedance.

For a well-designed system, $Z_{total}$ should be low enough to ensure the protective devices (e.g., fuses, breakers) trip within a specified time.

5. Touch and Step Voltage Calculation

When calculating the safety of an earthing system, touch voltage and step voltage are critical parameters to ensure the protection of humans in the vicinity during a fault.

• Touch Voltage (V_t): The voltage between the earth and an accessible conductive part (such as a metal enclosure) that can be touched during a fault.

  $$V_t = I_f \times R_e$$

• Step Voltage (V_s): The voltage between two points on the ground separated by a step (typically 1 meter).

  $$V_s = \frac{I_f \times \rho}{2 \pi L}$$

Touch and step voltages must be limited according to standards, such as IEEE 80 or IEC 60479, to ensure that they do not exceed values that could cause injury during fault conditions. Typical safe limits are:

• Touch Voltage: Less than 50V AC (in wet conditions) and 70V AC (in dry conditions).
• Step Voltage: Typically limited to less than 100V AC.

Example:

For a fault current of 500A and an earth resistance of 1 ohm:

• Touch Voltage: $$V_t = 500 \times 1 = 500 \, \text{V}$$ This is above safe limits, so measures such as reducing the earth resistance or using bonding must be employed to reduce this voltage.

6. Sizing of Earthing Conductors

The size of the earthing conductor depends on the magnitude of the fault current and the duration of the fault. The conductor must be able to carry the fault current safely without excessive temperature rise.

The required cross-sectional area $A$ of the earthing conductor is calculated using:

$$A = \frac{I_f}{k \times \sqrt{t}}$$

Where:

• $A$ is the cross-sectional area of the conductor (in mm²),
• $I_f$ is the fault current (in amperes),
• $t$ is the fault duration (in seconds),
• $k$ is a constant depending on the conductor material (e.g., for copper, $k = 226$).

Example:

For a fault current of 500A lasting 1 second in a copper conductor:

In this case, a 2.21 mm² conductor would suffice, but for safety margins, larger standard conductor sizes such as 6 mm² or 10 mm² are typically used.

Conclusion

Designing an earthing system for an AC power network involves calculating key parameters like earth resistance, fault current, and ensuring that touch and step voltages are within safe limits. Soil resistivity, the length and type of electrodes, and proper conductor sizing all play vital roles in achieving an effective earthing system. These calculations are crucial for maintaining system reliability, minimizing hazards, and ensuring compliance with safety standards.

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Manual on Earthing of AC Power Systems