Case Study: Procedure and Design Calculation of a Rod Earthing System for 4000KVA Substation

  

Case Study:

Procedure of a Rod Type Earthing

The procedure of a Rod Earthing System as follows: 

• Make a borehole (Depth will be as per design calculation) of as per the approved design and drawing.
• For rod earthing, a copper rod of required diameter put into the borehole.
• Fill the annular space between the electrode and borehole walls with alternating layers of coke or charcoal and common salt.
• Inspection chamber: Construct brick chamber of size 600 x 600 x 6000 mm with 100 mm thick brick walls over a P.C.C. layer. Keep 100 mm of the chamber above ground level. Cover the top with a cast iron (CI) cover.
• Follow the approved design and drawing for fixing and laying of earth wires or GI/copper strips between the earth electrode and the electrical room.

Design Calculation of a Rod Type Earthing

Step 01: Find the short circuit current of the Transformer

Calculate the transformer short circuit current by using the below equation: 

 

Where, 

S= KVA Rating of the Transformer 

U20= Phase to Phase no load voltage (440V)

Usc= Short circuit impedence of xmer 

In= Rated current of the trasnformer (A)

Isc= Short circuit current (kA)

For 4000KVA Transformer, 

In= 5255A

Isc= 88kA

Step 02: Find the cross sectional area of the Copper Lead

Calculate the cross sectional area of the copper lead by using the below equation: 

Here, 

A= Nominal cross sectional area (mm2}

Isc= Short circuit current (kA)

t= Duration of fault current (sec)

k= Factor depending on cable type (For HDBC k=220)

For 4000KVA Transformer, 

A= 400mm2

Step 03: Select the Size and number of Copper Wire for Earthing Lead

Find the size and number of copper wire from the below table as per locally available size copper lead. 

For 4000KVA Transformer [7/0 SWG copper wire from Data sheet], 

Size, D= 7/0

Quantity, Q= 3 Nos

Step 04: Calculate the depth of the borehole/Length of the earth lead

Find the length of the earth lead from the below equation, 

Here, 

R= Target value earth resistance (Below 1 Ohm)

ρ = Resistivity of soil (Ω meter),

L = Length of electrode (meter),
D = Diameter of electrode (meter)

For 4000KVA Transformer [Targeted value of resistance is .6 Ohm], 

Depth, s= 30 meter