The earth mats or grounding mats bring the connection to the earth indoors. They usually connect through a conductor inserted within the ground port of an electrical outlet. They help in limiting the ground potential & protect against the faulty current. They are used in a place where the large fault current is to be experienced.
Electrical power systems comprise a network of electrical elements that generate, distribute and transmit electrical energy through transmission lines. The substation grounding systems are made from a network of conductors interconnecting the metallic parts of equipment and structures and an arrangement of conductors buried underground. These systems provide an electrical connection to the earth.
In general, the earth is composed of several layers of soil at any location selected for substation construction. The soil is a part of the fault circuit; therefore, a protection circuit is needed. That is the reason why earth mats or ground mats are used to protect humans who may come in contact with this fault circuit.
How are the Earth Mats placed for substation protection?
In a Substation, Earth Mats are connected to the following:
-The neutral point is the system through its independent earth.
-Equipment framework and other non-current carrying parts of the electrical equipment in the sub-station.
-All extraneous metallic frameworks are not associated with equipment. Handle of the operating pipe.
-Fence if it is within 2 m from the earth mat.
In addition to the grids below the ground level, earthing spikes or electrodes are driven into the ground. They connect to the equipment bodies, earth grid, structures, neutrals, etc. All these together are connected to the Earthing system of the substation by several Earthing strips.
All these aspects offer a low earth resistance which is the primary purpose of Earthing. The inter-link is made through a flat or rod conductor called an Earth Mat or Grid. It offers the surface of substation equipment a near-absolute earth potential. As a result, they help with physical protection against faulty currents.
While designing an Earthing mat, the following steps are considered:
– If the switchyards have soil that is low in resistance, the earth resistance of the Earthing system would also be low. If the soil resistivity is high, the mesh rods are placed with closer spacing (with more electrodes inserted in the ground).
– The voltage between the ground and the ground surface is not dangerous to a person during a fault condition. A person who may have to come in contact with the non-current-carrying conducting surface of the electrical system is safe during a fault condition.
– Interrupt a fault current that may flow into the Earthing mat if large enough to operate the protective relay. The resistance of the ground is low to allow the fault current to flow through it. Therefore, the resistance of the mat will not permit the flow of fatal current into the person’s body.
Ideally, the ground around the substations should have zero ohms resistance. There is no established ground resistance standard threshold available or recognized by agencies. However, the NFPA and IEEE have recommended a ground resistance value of 5.0 ohms or less. To achieve this, the substations must:
1. Have a neutral connection to the earth for generators, transformers, capacitors, and reactors.
2. Offer a Low-resistance path to the earth for the currents resulting from ground faults, lightning rods, surge arresters, gaps, and other devices.
3. Limit the potential differences that appear between the substation metallic objects or structures and the Ground Potential Rise (GPR). Due to the flow of ground current, they may pose a danger to equipment and personnel.
4. Improve the operation of the protective relay scheme to clear ground faults.
5. Increase the reliability and availability of the electrical system.
6. Allow the grounding of de-energized equipment during maintenance
How do the Earth mats help in grounding?
Without the earth mat, the electricity or the fault current that occurred may directly flow into the electrical equipment causing the increase in temperature or the heating up of the electrical equipment. This further leads to damage or hazardous situations in the areas exposed to fault currents.
As an important part of the substation grounding system, earth mats should:
1) Ensure safety against electrical shocks by keeping the touch and earth potential in the switchyard under any earth fault condition within safe limits.
2) Provide neutral earthing or ground connection to connect the neutrals of stat connected to transformer’s winding.
3) Discharge the overvoltage from overhead lightning masts or ground wires to the earth for providing a ground path for surge arresters.
4) Arrange a path to discharge the charge between phase and ground using Earthing switches.
5) Provide equipment earthing or earth connections to the substation and other non-current carrying metallic parts in the sub-station.
6) Ensure the safety of personnel and the general public through touch and step potential.
Earthing mat is one of the essential parts of electrical grounding required for reducing the overall grounding resistance in substations.
Axis India Limited manufactures high conductivity copper tape, lattice earth mats to minimise the dangers of high step and touch voltages to operators in a circumstance like High Voltage switching. Click here to view all of Axis’s Lightning Protection, Lightning Arresters and Earthing products.
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