Earth Mat Design for a Sub Station

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sanjay mishra

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Chennai, India
Gentlemen,

I have to design an earth mat for a substation whose area is very small, i.e. 20 mtr x 9.5 mtr, soil resistivity is 365.1 Ohm Mtr, Fault Current - 26.2 KA, Duration of Fault Current - 3 Seconds

I tried to design the earth mat system as per IEEE80 but area being very small, the attainable touch & step potential is much more than calculated value, the grid resistance is also coming about 10 Ohms and that make the system totally unsafe.:mad:

Can somebody guide me:roll: how to go about designing the system and how to reduce the soil resistivity value to bring it down to, say 30 Ohm Mtr.

regards,
 
The simple answer... more metal in the ground.

When you have unfavorable soil resistivity, the simplest solution is to provide a more extensive grounding electrode arrangement. You must create a larger surface area of soil contact by using more substantial grounding materials, a greater amount of grounding materials, or deeper grounding materials.

Another solution is attempting to change the soil characteristics with enhancement materials and/or moisture. Both of which requires longterm maintenance and constant monitoring, not to mention greater cost and less effectiveness.
 
Another solution is to use the overhead ground wire that runs with the power lines into the substation. Tie that ground wire to the grid. (It may also be called an earthed neutral, static wire, protective earth wire or other names.)

If the ground wire is connected to earth at each transmission line pole and then connected to earth at the remote station, it will provide an alternate path to the earth for fault current flow. That will reduce the current in the earth during a fault.

IEEE 80 accounts for this through the Sf factor or split factor that reduces the fault current.
 
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