Earthing means connecting the dead part (it means the part which does not carries current under normal condition) to the earth for example electrical equipment’s frames, enclosures, supports etc. The purpose of earthing is to minimize the risk of receiving an electric shock if touching metal parts when a fault is present. Generally green wire is used for this as a nomenclature. Under fault conditions the non-current carrying metal parts of an electrical installation such as frames, enclosures, supports, fencing etc. may attain high potential with respect to ground so that any person touching these or approaching these will be subjected to potential which may result in the flow of a current through the body of the person or the animal of such a value as may prove fatal.
To avoid this non-current carrying metal parts of the electrical system are connected to the general mass of earth by means of an earthing system comprising of earth conductors to conduct the fault currents safely to the ground.
Earthing has been accomplished through bonding of a metallic system to earth. It is normally achieved by inserting ground rods or other electrodes deep inside earth.
Grounding means connecting the live part (it means the part which carries current under normal condition) to the earth for example neutral of power transformer. It is done for the protections of power system equipment and to provide an effective return path from the machine to the power source. For example grounding of neutral point of a star connected transformer. Grounding refers the current carrying part of the system such as neutral (of the transformer or generator). Because of lightening, line surges or unintentional contact with other high voltage lines, dangerously high voltages can develop in the electrical distribution system wires. Grounding provides a safe, alternate path around the electrical system of your house thus minimizing damage from such occurrences. In a properly balanced system, the phase currents balance each other, so that the total neutral current is also zero. For individual systems, this is not completely possible, but we strive to come close in aggregate. This balancing allows maximum efficiency of the distribution transformer’s secondary winding.
Bonding is simply the act of joining two electrical conductors together. These may be two wires, a wire and a pipe, or these may be two Equipments. Bonding has to be done by connecting of all the metal parts that are not supposed to be carrying current during normal operations to bringing them to the same electrical potential.
Bonding ensures that these two things which are bonded will be at the same electrical potential. That means we would not get electricity building up in one equipment or between two different equipment. No current flow can take place between two bonded bodies because they have the same potential. Bonding itself, does not protect anything. However, if one of those boxes is earthed there can be no electrical energy build-up. If the grounded box is bonded to the other box, the other box is also at zero electrical potential.
Objectives of Earthing
- To ensure safety of life and property from hazards of electric shock and electric fires.
- To ensure that system voltages on healthy lines remain within reasonable limits under fault Conditions thereby preventing insulation breakdowns.
- To provide a low impedance path (safe path) to dissipate lightning and short circuit currents.
- To minimize arcing burn downs as in an earthed system arcing fault would produce a current in ground path thereby providing an easy means of detecting and tripping against phase to earth arcing fault breakdowns.
- To provide stable platform for operation of sensitive electronic equipments i.e. To maintain the voltage at any part of an electrical system at a known value so as to prevent over current or excessive voltage on the appliances or equipment.
- To provide an alternative path for induced current and minimize the electrical noise in cables.
- To provide protection against static electricity from friction.
Need of Earthing
- Besides human safety risks, sensitive computers and electronics devices fail to operate properly and/or sustain damage at lower voltages which usually cannot be felt by humans, resulting in loss of productivity and expensive troubleshooting & repairs.
- Current also passes through the least path. In case of fault current, short circuit there is two options available for its dissipation into the earth:
Current may take the path of earthing
Current may take the path of human body of the affected person
- This depends on the least resistance of path between the two. Hence the resistance of the earthing should be always be lesser than the resistance of the human body.
- Earthing is absolutely necessary to protect human life and equipment from electrical hazards in this electronic age.
Qualities of Good Earthing
- There are various formal definitions of a good Earthing, e.g., how many amp of current it can drain at a particular voltage, or resistance per surface meter.
- Good Earthing must have impedance low enough to ensure that sufficient current can flow through the safety device so that it disconnects the supply ( <0.4 sec ) and that voltage of earth doesn’t rise more than 50V.
In simple theory this will prevent a potential difference between earth and earthed parts, as well as permitting the flow of fault current which will cause the operation of the protective systems.
Requirement for a good grounding system:
In an industrial plant or other facility that requires a grounding system, following factors influences requirement for a good grounding system.
- Limiting to definite values the voltage to earth of the entire electrical system. Use of a suitable grounding system can do this by maintaining some point in the circuit at earth potential. Such a grounding system provides these advantaged.
- Limits voltage to which the system-to-ground Insulation is subjected, thereby more definitely fixing the insulation rating.
- Limits system-to-ground or system-to-frame voltage to vales safe for personnel.
- Provides relatively stable system with a minimum of transient over voltage.
- Permits any system fault to ground to be quickly isolated.