Welcome to this module, on earthing, importance and types of earthing. This topic explains importance of earthing system, types of earthing. Are the terms grounding and earthing same or different? They are one and the same, grounding is the terminology used in US, earthing is the terminology used in Europe and India. We shall use the term earthing in this module. However, wherever IEEE 80 standard is referred, the term grounding may have been used. Hence, you will note that both terminologies earthing and grounding are used interchangeably in this course. When it is possible to operate the electrical system without earthing, then why earthing is important? To ensure that living beings are not exposed to unsafe potentials. To provide a sufficiently low impedance part to facilitate satisfactory operation of protective system under fault conditions. Worldwide industry adopts IEEE 80 for earthing design and installations. National standards on earthing are also by and large based IEEE 80. Earthing can be classified as a system neutral earthing, safety earthing or protective or equipment earthing. System neutral earthing provides a path for ground fault current to flow back into the system to ensure safe operation of protective devices and to limit the potential rise of substation equipment. Safety earthing is provided to carry electric currents into the ground under normal and fault conditions, without exceeding permissible limits of operations or without affecting continuity of service adversely. The earthing also assures that persons in the vicinity of grounded facilities are not exposed to the danger of electric shock. System neutral earthing depends on electrical system design and local design practices. For LV system of 415, 400 and 433 vaults, the neutral is solidly earthed due to regulations and safety of personnel. In 3.3, 6.6 and 11kV systems, neutral is resistance or reactance earthed to limit ground currents. 11kV neutral is solidly earthed in some places. For 22kV and above neutral is solidly earthed due to high cost of insulation in case of impotence earthing, and fast fault clearing due to increased fault current. For example in India the utility power distribution system neutral at 11kV and 33kV system are solidly grounded. Whereas in Middle East countries, DEWA utility, Dubai and Kahramaa utility, Qatar use low resistance grounding at 11kV level. In your academics methods of system neutral earthing, have been covered extensively. Need to prevent multiple neutral earthing of generators has also been covered. Hence such aspects are not covered in this module. Examples of system neutral earthing are shown on the screen. Safety comprises of earthing matt, equipment earthing and equipotential bonding. Main earth mat below ground, the bare conductor buried underground forms the main earth grid. For switchyard earth mat is provided in the form of mesh. For widespread industrial area, each individual plant area is provided with earthing and all individual plant earth mats are interconnected to form overall earth grid. Equipment earthing, metallic body of all the electrical equipment such as transformers. LV switchgear, distribution board, charger etcetera, shall be connected to buried earth mat. Metallic body of all these equipment are earthed at two distinct places. Armour and screen conductors of cable shall be connected to earth at one or both ends. Earthing of equipment body may be connected to buried or above ground earth mat. Earth mat is in turn shall be connected to below ground earth grid. Earthing of all non current carrying metallic parts, they shall include cable trays and support. Structural supports such as steel columns. Steel staircase. Metallic doors and gates etcetera, shall be connected at minimum one end. Equipotential bonding, earthing of equipment body and metallic structures is also called equipotential bonding. And the conductor used for this purpose is called equipotential conductor, as it maintains the same potential at all connected metallics during a fault. People who may be exposed to these metallics in the substation may experience the same safe potential. Here is a quick recap of what you've learned thus far. Earthing system is necessary to ensure that living beings are not exposed to unsafe potentials. To provide a sufficiently low impedance part to facilitate satisfactory operation of protective system under fault conditions. Earthing can be classified as system neutral earthing, safety earthing or protective or equipment earthing. In LV system, neutral is usually solidly earthed. The MV system upto 11kV is resistance earthed to limit fault currents. The MV system of 22kV and above are solidly earthed. Bare conductor buried underground forms the main earth grid and for switchyard, earth mat is provided in the form of mesh. Metallic body of all the electrical equipment such as transformer, LV switchgear, distribution board, charger et cetera, shall be connected to buried earth mat at two distinct places. Earthing of all non current carrying metallic parts( this shall include cable tray and supports, structural supports such as steel columns, steel staircase, metallic doors and gates et cetera ) shall be connected at minimum one end. All metallic (both current carrying equipment body and non current carrying structures) shall be connected to ground to facilitate equipotential bonding. Equipotential bonding ensures safe potential at all operating and fault conditions.
