Importance of Earthing :
The earthing is done to eliminate the possibility of electrocution if the live parts is accidentally touched by human being. The earthing also protects the equipment.The earthing provides least impedance path to the fault current. The three terms related to earthing are as follows. 1. Grounding 2. Earthing
The term grounding is commonly used in the electrical industry to mean both “equipment grounding” and “system grounding”.
When the live part of the equipment is connected to the earth potential is known as the system grounding. Under normal operation, the potential at the neutral point is zero and its potential rises during a fault condition. The unbalance phase currents gets added and flows through the neutral point. Thus, the unbalance/ earth fault current develops the potential rise at the neutral point of the common point of star connection. The rise in the potential at the neutral point cause the current to flow from neutral to the earthing point.
The grounding is used for the following equipment.
- Dyn11 vector group transformers used for distribution purpose.
- The current transformers used for protection of the equipment.
The neutral of the transformer must be earthed to the ground. The solid earthing is used for the equipment for the voltage rating less than 600 volts. The large rating transformer is earthed through the neutral grounding resistor (NGR) to limit the fault current to its full load current.
The value of the NGR is ;
Vph – Phase Voltage
If – Full load Current of equipment.
A current transformer may be connected between the neutral and the earth point and the unbalance current flowing through the CT can be fed to protective relay for necessary circuit protection.
The electrical devices are housed in a panel called enclosure. The panel is an isolated enclosure which provides necessary insulation between the live parts and the enclosure. If the insulation fails, or any conductor gets disconnected and touched to the enclosure body, the enclosure body will attain the live potential.
If a person touches the enclosure he may get an electric shock. To protect the man and machine, the enclosure is connected to the earth potential so that the current will be passed to the ground. The magnitude of the current, in this case, will be very high and it will trip the circuit breaker or blow out the fuses.
The electrical panel, motor,generator,transformer and all the electrical equipment must be earthed from two different points on the equipment body through the two distinct earth pits. The difference between earthing and grounding is summarized in below given table.
|Earthing Point has zero potential.||If current is balanced in all three phases then the grounding point has zero potential, otherwise neutral point has some potential.|
|The earth wire used is of green colour.||The ground wire is of black colour.|
|Earthing is done to prevent an electric shock to human when live parts touches the enclosure body and at the same time the person touches the enclosure.||Grounding potential rise from zero to phase voltage when fault occurs in the system. The grounding allows flow of large current from live potential to earth and the protection relay senses the fault current and trips the upstream breaker and, thus safeguard the human and machine.|
|The non live parts of the electric enclosure is connected to earth point.||The neutral point(live part of supply-which has zero potential under normal condition) is connected to the earth potential.|
The metallic items which are not designed to carry electricity in the room or building are intentionally connected to earth to avoid possibility of electric shock.The process is known as bonding. All the equipment connected will be at same potential and if someone touches to two piece of equipment simultaneously will not get shock because no current flow if potential difference is zero.
If the phase conductor touched to metal part the equipment will get the live potential. This will cause large current to flow through the bonded piece of equipment to ground and circuit breaker will get tripped.