Connecting above ground non conducting metal part to below ground buried metal is earthing or grounding. It is done to protect humans against electric shock, protect equipment from over voltage, provide safe path for lightning currents and prevent accumulation of static charges. When there is a fault large amount of energy is released. It should be diverted to some safe place i.e. earth. While this energy flows, it should not harm anyone nearby. Hence we earth the metallic parts.
During a fault the fault current will flow from live conductor to non conducting metal parts. With help of earthing, the fault current flows down to the earth grid. When current flows from non conducting metallic part to earth grid, the potential of earth grid and the non conducting metallic part rises. When someone touches that, they receive a shock.
Fault current does not flow directly through the human body, instead fault current develops some potential at the non conducting metallic surface. When a person touches this surface, some amount of current flow through their body. When the current passes through the body it has different effect at different magnitude.
The main purpose of earthing grid design is to limit the current through the human body below the value which leads to fibrillation or death. We can allow 116mA for 1sec, 164mA for 0.5sec, 211mA for 0.3sec currents through 50kg human body. The duration of current flowing through the body depends on operation of the main protection and backup protection.
The paths in which current can flow into human body
- Hand to foot
- Foot to foot (rare)
- Hand to Hand ( very rare)
Permissible touch and step voltage are the voltages which will keep the current through the body below the danger level. Etouch = IB (RB+1.5 ) Estep = IB (RB+6 )
Earthing Grid Consist of mesh of horizontal conductors and earth roads penetrating below the ground. The grid material is either copper or Galvanised iron rods or flats. The size depends on the fault current.
Mesh Voltage Em=ρ Km Ki IG Lm Step Voltage Es=ρ Ks Ki IG Ls , these are actual voltage developed during a fault. The Objective of earthing design is to keep the mesh and step voltage developed below the Permissible values. Developed voltages depends on the geometry of the earth grid. By proper design the voltage developed can be brought down below tolerable values.
Detailed guidelines on Guideline for Earthing of AC substation consist of 31 pages with few sample calculations. This detailed guideline gives many practical examples, calculations, illustrations which will help an application engineer to actually design a plant. This full guidelines can be accessed in below link.
Contents in the detailed guidelines are
1. What is earthing
2. Why do we do earthing
3. What is the Scope of this guideline
4. How to protect a person in a substation against electrical shock
5. What happens during a fault
6. Why fault take place
7. What happens when the current flows to the ground or earth
8. How is earthing design of domestic and industrial system different from substation
9. What happens during a shock10. At what current does a person feel an electric shock and when does a person die
11. What is the current allowed through human body
12. What should be the value of ts
13. What if weight of person is more or less than 50kg
14. What is the value of Resistance for the current through the body
15. What are the paths in which current can flow into human body
16. Permissible touch and step voltage
17. Correction factor for foot resistance
18. Permissible touch and step voltage with Cs
19. Earthing Grid
20. What needs to be connected to the earthing Grid
21. Sizing of earthing grid
22. Fault current distribution (Sf)
23. Decrement Factor
24. Grid Geometry
25. Mesh Voltage calculation
26. Step Voltage calculation
27. GPR- Ground potential rise
28. Transferred potential
29. What Causes GPR
30. Grid Resistance
31. Sample Calculations
32. Case 1: Fault inside substation with design fault current and duration 1sec
33. Case 2: Fault inside a substation with fault current 20.5kA and duration 100ms
34. Fault current distribution