Earthing In Electrical System
1.Earthing means making an electrical connection to general mass of earth.
2.Earth forms an intrinsic part of electricity System.
3.Earthing of electrical installation is primarily concerned with ensuring safety.
4.Soil conductivity is an important consideration. Lower the resistivity, easier to get good
earth.
5.The ground is more effective conductor than copper wire used in electrical circuit.
6.Earth provides the shortest path for fault current to travel.
7.Short circuits allows electrical current to by-pass its normal route. Under short circuit,
current follows the least resistance fault to ground.
Reason for Poor Earthing Practices:
1.Inadequate understanding about technical requirement of earthing & Standards.
2.Earthing has no role to play during normal conditions.
3.Lack of compatibility between earthing requirement of electronic equipments & power
system
4.Lack of motivation & importance to engineers for keeping earthing & connections in proper state.
Why Earthing Important?
Earthing in a substation is extremely important for the safety of operating personnel as well as for proper system operation.
1.It provides safe passage to earth fault current to flow. E/F current flow through the earth and may readily cause operation of fuse or E/F relay.
2. It provides low resistance path to fault current to ensure prompt & consistent operation of protective devices under fault current.
3.It protects the equipment other than live parts from attaining potential. Under fault
conditions, frames & enclosures etc may attain high potential w.r.t ground. So these
non-current carrying parts should be connected to ground for the safety of the operating personal & other human beings for discharging electrical energy to earth.
4.It protects the equipment from over voltages, HV surges resulting from lightning &
other causes.
5.It ensures safety of life from electric shocks by grounding all metallic structures and also
avoid hazardous voltage gradients by suitably designing the grounding system. The
potential of earthed body does not reach to dangerously high values during fault.
6.It ensure that ground potential rise does not have any dangerous effect on
communication system.
7.It plays an important role in increasing the reliability & continuity of supply service and
stabilizing the voltage conditions.
8.It has means of discharging feeders & equipments to ground before proceeding with
maintenance.
Attributes Of Good Earth Connection
1.Lowest soil resistivity.
2.Low electrical resistance to earth.
3.Good corrosion resistance.
4.Ability to carry high current repeatedly.
5.A reliable life of at least 30 years.
Low loop impedance path of earthing.
Limits of Earthing Resistance Following are values of earth resistance considered appropriate for different electrical
installation
a) Large HV and EHV s/stns
-0.5 ohms
b) Industrial and Distribution s/stns-
2.0 ohms
c) Medium voltage systems(415 V)
- 5.0 ohms