We now discuss earthing, which is very important to ensure safety at your workplace. First and foremost, you must understand that electricity is always trying to get to the ground. If something that conducts electricity (like your body) gives electricity an easy path to the ground, it will take it. So if you touch an electric circuit and the ground at the same time, you will become
electricity’s easiest path. Electricity will flow through you, and you could be seriously hurt or killed. Earthing is carried out to provide a path to the electric current to flow to the ground. It thus ensures
a) safety of personnel;
b) prevents or at least minimizes damage to equipment as a result of flow of heavy fault currents; and
c) improves reliability of power supply.
The systemic requirements for providing safe earthing are:
1. The systems designed and constructed for earthing should be capable of carrying current under normal and fault conditions to ground.
2. The earth path should be capable of handling the magnitude and duration of current as per the over-current protection of the system without any fire or flash or explosion.
3. Persons in the vicinity of earthed structures and installations should not be exposed to the dangers of electrical shocks.
Earthing practices adopted at Generating Stations, Substations, Distribution structures and lines are of great importance. Before discussing the earthing practices, we would like to state some definitions.
Box : Definitions related to Earthing
An effective grounding system must:
• provide a low impedance path to ground for personnel and equipment.
• withstand and dissipate repeated faults and surge currents.
• provide ample corrosion allowance to various chemicals to ensure continuous service during life of the equipment being protected.
• provide rugged mechanical properties for easy driving of earth electrodes with minimum.
All non current carrying metal parts associated with installation should
− be effectively earthed to a grounding system or mat which will limit the touch and step potential to tolerable values.
− limit the ground potential rise to tolerable values so as to prevent danger due to transfer of potential through ground, earth wires, cable sheath fences, pipe lines etc.
− maintain the resistance of the earth connection to such a value as to make operation of the protective device effective
The statutory stipulations for earthing are given below:
• All medium voltage equipment should be earthed by two separate and distinct connections.
• As far as possible, all earth connections should be visible for inspection.
• Each earth system should be so designed that testing of individual earth electrode should be possible.
• Resistance of earth system should conform to degree of shock protection desired.
Safety Precautions for Earthing
The precautions mentioned below should be adopted to the extent applicable and possible.
•General Earthing Precautions
1. Examine earthing devices periodically and always prior to their use.
2. Use only earthing switches or any other special apparatus where provided for earthing.
3. Verify that the circuit is dead by means of a discharging rod. The indicator itself should first be tested on a live circuit or proving unit before and after the verification.
4. Earthing should be done in such a manner that the persons doing the job are protected by earth connections on both sides of their working zone.
5. All the three phases should be effectively earthed and short circuited though work may be proceeding on one phase only.
electricity’s easiest path. Electricity will flow through you, and you could be seriously hurt or killed. Earthing is carried out to provide a path to the electric current to flow to the ground. It thus ensures
a) safety of personnel;
b) prevents or at least minimizes damage to equipment as a result of flow of heavy fault currents; and
c) improves reliability of power supply.
The systemic requirements for providing safe earthing are:
1. The systems designed and constructed for earthing should be capable of carrying current under normal and fault conditions to ground.
2. The earth path should be capable of handling the magnitude and duration of current as per the over-current protection of the system without any fire or flash or explosion.
3. Persons in the vicinity of earthed structures and installations should not be exposed to the dangers of electrical shocks.
Earthing practices adopted at Generating Stations, Substations, Distribution structures and lines are of great importance. Before discussing the earthing practices, we would like to state some definitions.
Box : Definitions related to Earthing
An effective grounding system must:
• provide a low impedance path to ground for personnel and equipment.
• withstand and dissipate repeated faults and surge currents.
• provide ample corrosion allowance to various chemicals to ensure continuous service during life of the equipment being protected.
• provide rugged mechanical properties for easy driving of earth electrodes with minimum.
All non current carrying metal parts associated with installation should
− be effectively earthed to a grounding system or mat which will limit the touch and step potential to tolerable values.
− limit the ground potential rise to tolerable values so as to prevent danger due to transfer of potential through ground, earth wires, cable sheath fences, pipe lines etc.
− maintain the resistance of the earth connection to such a value as to make operation of the protective device effective
The statutory stipulations for earthing are given below:
• All medium voltage equipment should be earthed by two separate and distinct connections.
• As far as possible, all earth connections should be visible for inspection.
• Each earth system should be so designed that testing of individual earth electrode should be possible.
• Resistance of earth system should conform to degree of shock protection desired.
Safety Precautions for Earthing
The precautions mentioned below should be adopted to the extent applicable and possible.
•General Earthing Precautions
1. Examine earthing devices periodically and always prior to their use.
2. Use only earthing switches or any other special apparatus where provided for earthing.
3. Verify that the circuit is dead by means of a discharging rod. The indicator itself should first be tested on a live circuit or proving unit before and after the verification.
4. Earthing should be done in such a manner that the persons doing the job are protected by earth connections on both sides of their working zone.
5. All the three phases should be effectively earthed and short circuited though work may be proceeding on one phase only.
•Earthing of Overhead Lines
− All metal supports and all reinforced and pre-stressed cement concrete supports of overhead lines and metallic fittings attached thereto should be permanently and effectively earthed. For this purpose a continuous earth wire should be provided and securely fastened to each pole and connected with earth ordinarily at three points in every km, the spacing between the points being as nearly equidistance as possible.
− Alternatively, each support and the metallic fitting attached thereto should be efficiently earthed.
− Metallic bearer wire used for supporting insulated wire of low and medium voltage overhead service lines should be efficiently earthed or insulated.
− Each stay wire should be similarly earthed unless insulator has been placed in it at a height not less than 3.0 m from the ground.•
•Earthing and Short – Circuiting Mains
− High voltage mains should not be worked upon unless they are discharged to earth, after making them dead are earthed, short-
circuited with earthing. Short circuiting equipment is adequate to carry possible short circuit currents. All earthing switches wherever installed should be locked up.
− If a cable is required to be cut, a steel wedge should be carefully driven through it at the point where it is to be cut.
− After testing the cable with DC voltage the cable should be discharged through 2 MΩ (mega-ohms) resistance and not directly owing to
dielectric absorption, which is particularly prominent in the DC voltage testing of high voltage cables. The cable should be discharged for sufficiently long period to prevent rebuilding up of the voltage.
− The earthing device when used should be first connected to an effective earth. The other end of the device should then be connected
to the conductors to be earthed.
− Except for the purpose of testing, phasing etc. the earthing and the short-circuiting devices should remain connected for the duration of the work.
Removing the Earth Connections
On completion of work, removal of the earthing and short circuiting devices should be carried out in the reverse order to that adopted for placing, that is, the end of earthing device attached to the conductors of the earthed mains or apparatus should be removed first and the other end connected to earth should be removed last. The conductor should not be touched after the earthing device has been removed from it.
•Testing and Record
1. All earthing systems belonging to the utility should in addition, be tested for resistance on dry day during the dry season not less than once every two years.
2. A record of every earth test made and the result thereof should be kept by the utility for a period of not less than two years after the day of testing.
3. It should be available to the Electrical Inspector or any officer appointed to assist the Inspector and authorized.
Electrical repair and maintenance work should only be carried out with proper equipment provided for the purpose and after taking necessary precautions.We now describe the safety instructions for personnel working at the site. But before studying further, you may like to check your understanding.
− All metal supports and all reinforced and pre-stressed cement concrete supports of overhead lines and metallic fittings attached thereto should be permanently and effectively earthed. For this purpose a continuous earth wire should be provided and securely fastened to each pole and connected with earth ordinarily at three points in every km, the spacing between the points being as nearly equidistance as possible.
− Alternatively, each support and the metallic fitting attached thereto should be efficiently earthed.
− Metallic bearer wire used for supporting insulated wire of low and medium voltage overhead service lines should be efficiently earthed or insulated.
− Each stay wire should be similarly earthed unless insulator has been placed in it at a height not less than 3.0 m from the ground.•
•Earthing and Short – Circuiting Mains
− High voltage mains should not be worked upon unless they are discharged to earth, after making them dead are earthed, short-
circuited with earthing. Short circuiting equipment is adequate to carry possible short circuit currents. All earthing switches wherever installed should be locked up.
− If a cable is required to be cut, a steel wedge should be carefully driven through it at the point where it is to be cut.
− After testing the cable with DC voltage the cable should be discharged through 2 MΩ (mega-ohms) resistance and not directly owing to
dielectric absorption, which is particularly prominent in the DC voltage testing of high voltage cables. The cable should be discharged for sufficiently long period to prevent rebuilding up of the voltage.
− The earthing device when used should be first connected to an effective earth. The other end of the device should then be connected
to the conductors to be earthed.
− Except for the purpose of testing, phasing etc. the earthing and the short-circuiting devices should remain connected for the duration of the work.
Removing the Earth Connections
On completion of work, removal of the earthing and short circuiting devices should be carried out in the reverse order to that adopted for placing, that is, the end of earthing device attached to the conductors of the earthed mains or apparatus should be removed first and the other end connected to earth should be removed last. The conductor should not be touched after the earthing device has been removed from it.
•Testing and Record
1. All earthing systems belonging to the utility should in addition, be tested for resistance on dry day during the dry season not less than once every two years.
2. A record of every earth test made and the result thereof should be kept by the utility for a period of not less than two years after the day of testing.
3. It should be available to the Electrical Inspector or any officer appointed to assist the Inspector and authorized.
Electrical repair and maintenance work should only be carried out with proper equipment provided for the purpose and after taking necessary precautions.We now describe the safety instructions for personnel working at the site. But before studying further, you may like to check your understanding.
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