Substation earthing is every important for safety of people in the substation and nearby. Proper design of substation earthing requires strong knowledge of earthing , fault current distribution .
Earthing of Substation
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)
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 Step Voltage , 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.