IEEE 80 - Which fault to be considered?

[NickParker]

Which fault current to be considered for ground grid calculation at the substation with respect to step and touch potential?

a) L-G fault at HV voltage of HV/MV transformer (fault at the HV side of the transformer - source side from Grid)
b) L-G fault at MV voltage of HV/MV transformer (fault at the MV side of the transformer - at the load side)
b) L-G fault at LV voltage of MV/LV transformer (fault at the LV side of the MV/LV transformer - this is the distribution transformer at the same substation as a & b)

 

[jghrist]

In most cases just a). L-G faults on the low side of transformers return to the transformer source through the ground grid wires, not through the earth. Only currents flowing through the earth produce step- and touch-voltages.

The exception might be faults on LV or MV feeders where the fault current has to flow through the earth to get to the ground grid.

 

[Kiribanda]

If the transformer has got a high side WYE and low side delta then the L-L-G fault current is higher than the single line to ground fault current at its high side.

 

[bacon4life]

I consider all LG and LLG fault locations. I find it far simpler just to evaluate all the location each time rather tying to remember some rule of thumb, plus all the special cases where the rule of thumb fails. Many of the fault locations can be dismissed just by determining whether the fault current will flow through the earth or through conductors.