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As of May 2020, there are 19 metro projects currently under construction across India with another 9 new or expansion projects announced. The common feature between all these different metro projects across varied geographies, landscapes and soil types is that they will need to be properly earthed to ensure the proper functioning of all the equipment. Designing the Earth Mat system for a metro substation or any other project is a laborious process with many calculations as shown below. Axis has been helping clients construct earthing and lightning protection systems for over 25 years.
What is Earthing?
At the time of a fault, very high levels of current flow through the system. As a result, a potential difference is produced between the external metallic parts of the system and the ground and due to this change in potential the current will flow through a person if they come in contact with the system causing an electric shock. Thus, non-current carrying metallic parts are connected to the earth so that these kinds of fault currents can flow safely to the ground. Thus, Earthing is the connection of equipment or other metallic parts which do not carry current under normal conditions to the earth using Earth Electrodes. These metallic parts could include electrical equipment frames, enclosures, supports, etc.
Designing Earthing for a Metro Substation?
We use the following standards to design an Earth Mat system for a Metro Substation.
In case of a substation where the power is received & transmitted from one place to another, there are various faults that can occur such as short circuits, earth faults, lightning surges and many more. Generally, a substation is provided with an Earth Mat which forms a Mesh Network for Earthing. The main purpose of the Earth Mat is to protect the equipment & person by limiting the step & touch potential. The following Earth Mat design calculation is in accordance with the IEEE 80 standard which is a Guide for Safety in Substation Grounding.
Designing an Earth Mat System
The design of the Main Earth Mesh is generally done by the Contractor constructing the receiving substation (RSS). This design is dependent on the site conditions with soil resistivity being the most important factor. The soil resistivity measurements should be done in dry conditions, usually during the summer when soil resistivity would be the highest. The value of the soil resistivity should be lower the 10Ω-m – if it is higher than that value then we can achieve our targeted soil resistivity by adding more Earth Electrodes.
In order to measure the soil resistivity, we need to use an instrument called an Earth Tester. The most commonly used method is the Wenner 4-point method but other 2 pole and 3 pole methods can also be used. The Wenner method is preferred for accurate results.
Important Parameters for the Earth Mat Design
The following are a few parameters that affect the design of the Earth Mat for the Metro Substation.
Block Diagram Representation for Earth Mat Design
The flow chart describes each step that needs to be taken for the design of the Earth Mat for a Metro Substation. Each step and its corresponding formula is described in detail later in the article.
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