Electrical Earthing practices adopted at various stages of the power system are of great importance. The codes of practice, Technical Reference books, and Handbooks contain a chapter on this subject but they are often skipped considering them as elementary or even as unimportant. Reference books on this subject are referred to and such of the points that are most important are compiled below. These are of importance to every practicing engineer working in substations or electrical installations.
Table of Contents
OBJECTIVES
The main objective of earthing is to ensure a zero potential surface in and around the area where the electrical equipment is installed or erected. In the case of shielded or enclosed conductors, earthing of the shield isolates the conductor from external interference and prevents the interference due to the current in the conductor from spreading outside of the shield.
To achieve this objective, the non-current carrying but conductive part of the electrical equipment is connected to the general mass of the earth, which prevents the appearance of dangerous voltage levels on the enclosures and helps to provide safety to working staff and the public who may come in contact with the equipment.
The basis of the use of earthing, as described, is the fact that all the generating plants in a grid are connected through earthing. Under this system, earthing may be used as the return path for electric current, under abnormal conditions.
ELECTRICAL EARTHING REQUIREMENTS
Good electrical earthing should have low resistance.
Earthing must provide a more stablized circuit potential with respect to the ground along with limitation in overall potential rise.
It should prevent, or at least minimize, the damage to the equipment due to the flow of eventual heavy fault currents.
Electrical earthing should protect men and equipment from injury or damage due to over-voltage.
It should improve the reliability of the power supply.
It should provide a low impedance path to fault currents to ensure prompt and consistent operation of protective relays, surge arresters, etc.
The electrical earthing should be done in such a way that the maximum potential gradient along the surface of the sub-station remains within safe limits on occurrence of any earth fault.
The electrical resistance to earth shall be as low as possible and shall never exceed the following limits:
Power Stations – 0.5 Ohms
EHT Substations – 1.0 Ohms
33KV Stations – 2.0 Ohms
D/T Structures – 5.0 Ohms
Tower foot resistance – 10.0 Ohms
CLASSIFICATION OF ELECTRICAL EARTHING
Equipment Earthing
Electrical Earthing associated with non-current carrying parts of electrical equipment is called equipment earthing. The safety of the operator, the user, and the safety of their property are the main reasons for equipment earthing, e.g. outer metallic body of a transformer, and the metallic body of an electric motor.
As stated, for equipment earthing to be effective, it must first be done at the generating plant. Here the neutral of the star-connected three phases is grounded or earthed.
Discharge Earthing
When a live electrical equipment or system is isolated or disconnected from the electrical power supply, the electrostatic charge still remains in the system. Before carrying out any work on this isolated electrical equipment, these electrostatic charges are discharged to the earth through earth switches or earth rods. This method of electrical earthing is called Discharge Earthing.
System Earthing
Electrical earthing associated with current carrying parts of the equipment is called system earthing. The system security, reliability, performance, and voltage stabilization, all rely only on the system earthing, e.g. neutral earthing of a transformer and surge arrester earthing.
TERMINALS OF EQUIPMENT TO BE EARTHED AND PURPOSE
| SL. NO. | TERMINALS EARTHED | PURPOSE OF EARTHING |
| 1 | Transformer neutral | For holding neutral at zero potential |
| 2 | Generator neutral | To provide a path for out-of-balance current |
| 3 | Star point of a load | To discharge voltage surges |
| 4 | Neutral of a circuit | To prevent arching to the ground on overhead lines |
| 5 | To provide path for out-of-balance current | Simpler earth fault protection secondary |
| 6 | Equipment earthing | To hold non-current carrying metallic parts at zero potential (Body earthing). To hold the current carrying parts at zero potential for safety, even on earth fault |
| 7 | Reference earthing | To provide a reference zero potential in the conductor circuit |
| 8 | Discharge earthing | To discharge capacitive current charge through earth switch to earth |
| 9 | Surge arrester | To discharge the surge voltages |
