What is the resistance of the earth pit

Device grounding

  • Electrical equipment classification IS: 9409-1980
  • Important rules for safety and earthing practice are based on IE rules 1956
  • Guide to the Effects of Electricity Through the Human Body - IS: 8437-1997
  • Protection of buildings and structures from lightning strikes - IS: 2309-1969
  • Earth: The conductive mass of the earth, the electrical potential of which is usually assumed at any point, is assumed to be ZERO.
  • Earth electrode: A conductor or group of conductors that are in close contact with the earth and provide this as an electrical connection to the earth.
  • Ground Electrode Resistance: The electrical resistance of a ground electrode to ground.
  • Grounding conductor: A protective conductor that connects the main grounding terminal to a grounding electrode or other grounding means.
  • Equipotential bonding: Electrical connection brings various exposed conductive parts and external conductive parts to essentially the same potential.
    Example: If necessary, connect the protective conductor, protective conductor and riser of AC / HV systems.
  • Potential gradient: The potential difference measured in the direction per unit of length is max.
  • Touch voltage: The P.D. between an earthed metal structure and a point on the surface of the earth covered by a horizontal range of one meter
  • Step voltage: The P.D. between two points on the earth's surface that are one step (step) away from one meter.
  • Ground Grid: A system of ground electrodes made up of interconnected connectors buried in the ground to provide a common ground for electrical equipment and metallic structures.
  • Earth mat: An earthing system consisting of a grid of horizontally buried conductors - serves to divert the earth fault current to the earth and also as an equipotential bonding conductor system.
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    Need for equipment grounding


    • Personnel security
    • Equipment safety
      Avoid or minimize damage to equipment from flowing strong currents.
    • Improving the reliability of the power system.

    Classification of grounding

    The grounding is broadly divided as

    1. System grounding (connection between a system part in an operating system such as a neutral conductor of a transformer winding) and ground.
    2. Connect device grounding (safety grounding) device housings (such as electric motor housings, transformer tanks, control cabinets, switching rods of air circuit breakers, LV circuit breaker housings, high-voltage circuit breaker housings, single splitter housings, etc.) to ground.

    Permissible values ​​of the earthing resistance

    • Power plants - 0.5 ohms
    • EHT stations - 1.0 ohm
    • 33 kV SS - 2 ohms
    • DTR structures - 5 ohms
    • Tower base resistance - 10 ohms

    What are the basics for achieving permissible earth resistances?

    According to IE rules you have to have a certain basis for this, according to IE rules the contact potential must be kept below

    1. Recommended safe value 523 volts
    2. Ierror = maximum current in the error state,
    3. The maximum fault current is 100 KVA, the current in 100 KVA is around 100 A. The percentage impedance is 4%
    4. For a substation with a 100 KVA transformer

      Since 0.26 ohms is quite low, quality work must be done during construction to get such a value from the grounding system, and the cost of doing this will be very high.
      Hence the electrical inspectors insist on about 1.0 ohm. This seems to be justified for the urban areas. This value can be 2 ohms in rural areas, which is recommended by most government agencies.
    5. The resistance value of the earth electrode is also important with regard to complete protection against lightning by lightning rods.
      The resistance value of the earth electrode in this case is given by the formula

      Flash overvoltage of 11 KV = 75 KV
    6. Lightning rod displacement = 40 KA.

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    Type of grounding

    Plate type grounding

    In this cast iron plate of size 600 mm × 600 mm, a plate with a thickness of 6.3 mm is used as a ground plate. This is connected to an earth-earth-GI-earth strip with a width of 50 mm × 6 mm × 2.5 m length using nuts, screws and washers of the required size. Depending on the location, the main earthing strip is connected with a superheated steam GI strip measuring 40 mm × 3 mm up to the earthing connection of the device. The grounding plate is filled on the back and covered with grounding material (mixture of charcoal and salt) with a distance of 150 mm from all six sides. The remaining pit is filled with excavated earth again. In addition to the earthing plate, a 2.5 meter long rigid PVC pipe is also provided in the earth pit for irrigation purposes in order to keep the earthing resistance within a certain limit.

    Type of pipe earthing

    This method uses a hot dip GI pipe with a diameter of 40mm × 2.5 meters is used for equipment grounding. This tube is perforated at intervals of 100 mm and tapered at the lower end. A clamped pipe is welded to this pipe 100mm below the top to connect to the superheated steam GI strip of size 40mm x 3mm of the required length according to the location position up to the earth connection of the device. A funnel for watering is attached to its open end. The earthing pipe is located at a depth of 2700 mm. A 600 mm dia “farma” made of GI sheet metal or cement pipe in two halves is placed around the pipe. Then the angular space between this “farma” and the earth pipe is filled with an alternative layer 300 mm high with salt and charcoal. The remaining space outside of “farma” is filled with excavated earth. The “Farma” is gradually increased as the replenishment progresses. This means that the pit is filled up to 300 mm below the ground. This remaining part is covered by building a small chamber of bricks so that the upper open end of the pipe and the connection to the main earthing pipe are accessible if necessary. The chamber is closed with a wood / stone cover. Water is poured into the pipe through its open funnel to keep the earth resistance within a certain limit.

    Other types of grounding: When the capabilities of certain equipment are limited and cannot withstand certain fault currents, the following types of grounding are used to limit the fault current.

    1. Resistance grounding
    2. Reactance ground
    3. Peterson coil ground.
    4. Grounding through grounding transformer.