Earthing of Power Supply Installations

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Questions and Answers

What size of MS flat is used to connect one terminal of the secondary winding of a traction power transformer directly to the earthing grid?

75 mm x 8 mm (correct)

100 mm x 10 mm

25 mm x 4 mm

50 mm x 6 mm

Which connections are required for the designated terminal of the secondary of each potential, current, and auxiliary transformer?

One connection of 100 mm x 10 mm

One connection of 75 mm x 8 mm

One connection of 50 mm x 6 mm and one of 75 mm x 8 mm

Two separate connections of 50 mm x 6 mm (correct)

What is the minimum size of MS flat used to connect the metallic framework of outdoor equipment to the earthing grid for equipment on the primary side of traction power transformers rated between 66 kV up to 3000 MVA?

25 mm x 4 mm

75 mm x 8 mm

100 mm x 10 mm

50 mm x 6 mm (correct)

For equipment rated at 220 kV up to 10000 MVA, which ground conductor size must be used?

75 mm x 8 mm Signup and view all the answers

How many separate and distinct connections are required for all outdoor equipment's metallic framework?

Two connections Signup and view all the answers

What is the material of the flat used for earthing connections as specified in the guidelines?

Mild Steel Signup and view all the answers

What is the maximum voltage rating discussed for equipment connections on the primary side of traction power transformers?

220 kV Signup and view all the answers

What is the required size of the MS flat for connecting masts and structures to the earthing ring?

50 mm x 6 mm Signup and view all the answers

What type of wire is used to connect fencing panels to the supporting uprights?

6 SWG G.I. wire Signup and view all the answers

How should the ground terminal of the lightning arrester be connected?

Directly to the earth electrode with two connections Signup and view all the answers

Where should the earthing ring inside the remote control cubicle be positioned?

300 mm from the floor level Signup and view all the answers

How are connections of earth strips to each other made?

Using welding Signup and view all the answers

What type of electrical systems are excluded from the scope of this code of practice?

Low voltage (LT) electrical power distribution systems Signup and view all the answers

What does 'Earth' refer to in this context?

A conductive mass of the earth with zero electrical potential Signup and view all the answers

Which term describes the resistance of an earth electrode with respect to the earth?

Combined Earth Resistance Signup and view all the answers

What is meant by 'Equipment earthing'?

Earthing all metal work of electrical equipment not carrying current Signup and view all the answers

What components make up an 'Earthing grid'?

Interconnected, horizontal bare conductors buried in the earth Signup and view all the answers

What does 'Mesh Voltage (E mesh)' refer to?

The maximum touch voltage within a mesh of an earthing grid Signup and view all the answers

Which of the following is not included in the power supply installation as defined in the content?

Signal and telecommunication equipment Signup and view all the answers

What is the purpose of equipment earthing?

To ensure effective operation of protective gear during leakage Signup and view all the answers

Which voltage levels are specified in the code for traction sub-stations?

220/25 kV, 132/25 kV, 110/25 kV, or 66/25 kV Signup and view all the answers

What is a key drawback of using coke treated electrodes?

They lead to rapid corrosion of electrodes and steel frameworks. Signup and view all the answers

Where should coke treated electrodes not be located?

Within 6.0 m of other metal work. Signup and view all the answers

What is recommended in high embankments if the specified earth resistance cannot be achieved?

Use electrodes longer than 4 m to reach the parent soil. Signup and view all the answers

What is the minimum depth at which the earthing grid should be buried?

600 mm Signup and view all the answers

What type of material is recommended for making connections between the earth electrode and the grid?

75 mm x 8 mm MS flat Signup and view all the answers

What technique is advised for joining longitudinal and transverse conductors in the earthing grid?

Welding Signup and view all the answers

How should the spacing of earthing grid conductors be managed?

They should be spaced to maintain acceptable step and touch potentials. Signup and view all the answers

What requires consideration when determining the size of the earthing grid conductor?

Incoming system voltage and fault level Signup and view all the answers

Which of the following is NOT a characteristic of an effective earthing grid?

Uniform installation at ground level Signup and view all the answers

What is the main purpose of earthing electrodes in traction substations?

To ensure safety and minimize earth resistance. Signup and view all the answers

What is the minimum height at which the earth screen wires should be strung above the live conductors?

2.5 m Signup and view all the answers

How many separate and distinct connections are required for each metallic fencing upright to the main earthing grid?

Two Signup and view all the answers

What type of wire is used to connect the metallic fencing panels to the uprights?

6 SWG G.I. wire Signup and view all the answers

Which of the following statements about the earthing arrangement at a switching station is correct?

Three earth electrodes must be interconnected to form a closed loop. Signup and view all the answers

What is the function of the earth screen in outdoor substations?

To shield against direct lightning strikes. Signup and view all the answers

What should the earthing arrangement for the 240 V ac supply to the oil filtration plant consist of?

Two separate distinct connections Signup and view all the answers

Which type of flat is required for making distinct connections in an earthing system?

50 mm x 6 mm MS flat Signup and view all the answers

What is the purpose of connecting to the traction rail in earthing arrangements?

To limit potential gradient during faults. Signup and view all the answers

What material is specified for the earth screen wires used in substations?

Galvanized steel Signup and view all the answers

How are the secondary terminals of transformers connected in the earthing arrangement?

By two separate and distinct connections. Signup and view all the answers

Which connection method is specified for the metallic door panels to supporting uprights?

Two separate and distinct connections using 6 SWG G.I. wire. Signup and view all the answers

What is the purpose of the earthing ring inside the remote control cubicle?

To ensure safe grounding for all electrical equipment in the cubicle. Signup and view all the answers

Which material is specified for connecting the metal casing of transformers to the mast/structures?

50 mm x 6 mm MS flat. Signup and view all the answers

How should connections of earth strips to each other be made according to standard practice?

By welding. Signup and view all the answers

Where should the earthing leads from the lightning arrester be positioned in relation to the earth electrode?

They must be brought to the earth electrode as short and straight as possible. Signup and view all the answers

What is the primary aim of conducting equipment earthing in electrical installations?

To maintain an effective operation of protective gear Signup and view all the answers

Which term specifically refers to the interconnected, horizontal bare conductors buried in the earth?

Earthing grid Signup and view all the answers

In the context of this code, how is the term 'Earth' defined?

The conductive mass of the earth with a potential taken as zero Signup and view all the answers

What does the term 'Combined Earth Resistance' specifically exclude from its definition?

The traction rail Signup and view all the answers

Which aspect of equipment does the earthing code primarily cater to?

Earthing at specific traction substations and switching stations Signup and view all the answers

What is the significance of 'Mesh Voltage (E mesh)' in the earthing context?

It represents the maximum touch voltage within an earthing grid mesh Signup and view all the answers

What type of electrical system is specifically excluded from the purview of this code?

Low voltage (LT) electrical power distribution systems Signup and view all the answers

What is a critical reason for limiting the use of coke treated electrodes in earthing systems?

They can cause rapid corrosion of nearby metal structures. Signup and view all the answers

How deep should the earthing grid be buried according to the guidelines?

600 mm Signup and view all the answers

Which of the following is recommended for difficult locations such as high embankments?

Using electrodes longer than 4 m to reach the parent soil. Signup and view all the answers

What is the connection method used between the MS flat and the MS rod in the earthing grid?

Welding Signup and view all the answers

What is the primary consideration when determining the size of the earthing grid conductor?

The incoming system voltage and fault level Signup and view all the answers

How should the connections between longitudinal and transverse conductors of the earthing grid be handled?

By welding Signup and view all the answers

What is essential for ensuring a low resistance and adequate current-carrying capacity in large sites?

A reasonable distribution of electrodes Signup and view all the answers

What joint method is specified for connecting an earth electrode to the grid?

Through MS links by bolted joints Signup and view all the answers

What spacing strategy is emphasized for earthing grid conductors?

Conductors should be spaced to maintain safe step and touch potentials Signup and view all the answers

Where should earth electrodes ideally be located in a traction substation?

Within and adjacent to the perimeter fence Signup and view all the answers

What is the main objective of an earthing system in a traction environment?

To provide a uniform potential around the station Signup and view all the answers

Which of the following does NOT represent a requirement of a good earthing system?

Provide high resistance path for fault currents Signup and view all the answers

In the context of earthing, what does 'Step Voltage' refer to?

Potential difference over one meter on earth's surface Signup and view all the answers

What is meant by 'Touch Voltage' in earthing terminology?

Voltage difference experienced by a person reaching towards a grounded structure Signup and view all the answers

Which of the following characteristics is crucial for an earthing system to ensure safety during faults?

Uniform potential surface around the station Signup and view all the answers

How does an effective earthing system contribute to protecting personnel?

By maintaining low impedance paths that minimize voltage differences Signup and view all the answers

What would likely happen if the maximum voltage gradient during earth faults exceeds safe limits?

Increased risk of shock or injury to personnel Signup and view all the answers

Which term describes the potential difference resulting from electrical potential variances on the earth's surface?

Step Voltage Signup and view all the answers

What is the assumed distance between two points when measuring Step Voltage?

One meter Signup and view all the answers

What is the minimum number of earth electrodes required at each switching station?

Three Signup and view all the answers

Which connection method is used for earthing the fencing panels?

Two separate connections with 6 SWG G.I. wire Signup and view all the answers

What is the purpose of the overhead earth screen in outdoor substations?

To limit lightning strikes on the equipment Signup and view all the answers

Which of the following materials is used as an earthing conductor for the main earthing grid connections?

Galvanized steel Signup and view all the answers

How should the earth electrodes at a switching station be interconnected?

Using 50 mm x 6 mm MS flat to form a closed loop Signup and view all the answers

What type of steel is specified for the earth screen wires in outdoor substations?

Galvanized steel Signup and view all the answers

What is the height requirement for the overhead earth screen above the live conductors?

At least 2.5 m Signup and view all the answers

What is the minimum requirement for connections to the 240 V AC supply for the oil filtration plant?

Two separate and distinct connections with 50 mm x 6 mm MS flat Signup and view all the answers

Which configuration is used for the earthing connections of each potential and current transformer?

Two distinct connections to the main earthing ring Signup and view all the answers

What connection technique is used for earthing the metallic door panels at the substation?

Two distinct connections with 6 SWG G.I. wire Signup and view all the answers

Study Notes

Appendix III: Code of Practice for Earthing of Power Supply Installations

Scope: This code covers earthing arrangements for 220/25 kV, 132/25 kV, 110/25 kV, or 66/25kV traction substations, 25 kV switching stations, booster transformer stations, and auxiliary transformer stations. It does not apply to low-voltage (LT) electrical power distribution systems, 25 kV overhead equipment, or signal and telecommunication equipment.

Terminology

Combined Earth Resistance: The resistance of an earth electrode(s) related to earth, with the earth electrode connected to the metal work (excluding live parts and the traction rails) of electrical equipment.
Earth: The conductive mass of Earth, with a conventionally assigned zero electrical potential.
Earth Electrode: A conductor (e.g., mild steel pipe, or a group of conductors) connected to Earth.
Earthing Grid: A network of interconnected horizontal conductors buried in the Earth, acting as a common ground.
Equipment Earthing: Earthing of the metalwork of electrical equipment not carrying live current or current. This protects against shock and fire hazards.
Mesh Voltage (Emesh): The maximum voltage that can be found within a mesh of an earthing grid.
Power Supply Installation: Electrical equipment and related structures at a Railway Traction Substation, Switching Station, or Booster/Auxiliary transformer station on the 25 kV overhead equipment.
System Earthing: Limiting the potential of live conductors with respect to earth to values that the insulation can withstand, thereby ensuring system safety.
Step Voltage (Estep): The potential difference between two points on the earth's surface separated by one meter, in the direction of maximum potential gradient.
Touch Voltage (Etouch): The potential difference between a grounded metallic structure and a point on the earth's surface, separated by one metre.
Traction Rail: The non-track-circuited rail of a wired track, not required for signalling and may be earthed.

Object of Earthing

Earthing aims to create a uniform zero potential surface around a station. This ensures personnel and equipment are at earth potential, preventing shock hazards during short circuits or other abnormal occurrences.

Governing Specifications

Earthing practices are based on standards such as IS: 3043-1987 (Indian Standard) and relevant Indian Electricity Rules.

Earth Resistance

Combined earthing resistance limits are specified for various station types (Traction substation, Switching Station, Booster transformer station, and Auxiliary transformer station)
Earthing electrode materials and placement are specified. Chemical treatment of soil may be required in high resistivity areas to further reduce resistance.
Installation and spacing of electrodes (e.g., rods, pipes) is essential to minimize touch and step voltages

System Earthing Details

Various substations are described with potential differences.
Earthing grid conductor sizes for different system voltage/fault levels are detailed.
Details on the construction of earthing grids are given.
Earthing of equipment such as transformers and circuit breakers is specified for both primary and secondary voltages, including those associated with lightning arrestors.

Earthing Inside Control Rooms

Earthing ring construction is detailed within control rooms.

Earthing Arrangement Diagrams

Multiple diagrams (FIG A 3.01, FIG A 3.02) visually represent earthing arrangements, illustrating typical electrode positioning, dimensions, connections etc.
Diagrams show details for a variety of components and placements.

Painting of MS Flats

All exposed MS flat surfaces above ground level must receive two coats of grass-green paint.

Crushed Rock Surface Layer

Crushed rock layers are required at traction substations and switching station site to reduce step and touch voltages. Thickness can vary based on site conditions.

Step/Touch Voltage Calculations

Standard formulas are given to calculate tolerable touch and step voltages, considering environmental factors.

Drawings

Detailed drawings (ETI/PSI/... numbers) are provided for different parts of the earthing system, including traction substations, switching stations, etc.

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Description

This quiz covers the key concepts and terminology related to the earthing of high voltage power supply installations, including traction substations and switching stations. Understand definitions like combined earth resistance and earth electrode as they pertain to electrical safety and functionality.